Tag Archives: firefox

DocShell in a Nutshell – Part 3: Maturation (2005 – 2010)

Whoops

First off, an apology. I’ve fallen behind on these posts, and that’s not good – the iron has cooled, and I was taught to strike it while it was hot. I was hit with classic blogcrastination.

Secondly, another apology – I made a few errors in my last post, and I’d like to correct them:

  1. It’s come to my attention that I played a little fast and loose with the notions of “global history” and “session history”. They’re really two completely different things. Specifically, global history is what populates the AwesomeBar. Global history’s job is to remember every site you visit, regardless of browser window or tab. Session history is a different beast altogether – session history is the history inside the back-forward buttons. Every time you click on a link from one page, and travel to the next, you create a little nugget of session history. And when you click the back button, you move backwards in that session history. That’s the difference between the two – “like chalk and cheese”, as NeilAway said when he brought this to my attention.
  2. I also said that the docshell/ folder was created on Travis’s first landing on October 15th, 1998. This is not true – the docshell/ folder was created several months earlier, in this commit by “kipp”, dated July 18, 1998.

I’ve altered my last post to contain the above information, along with details on what I found in the time of that commit to Travis’s first landing. Maybe go back and give that a quick skim while I wait. Look for the string “correction” to see what I’ve changed.

I also got some confirmation from Travis himself over Twitter regarding my last post:

@mike_conley Looks like general right flow as far as 14 years ago memory can aid. :) Many context points surround…
@mike_conley 1) At that time, Mozilla was still largely in walls of Netscape, so many reviews/ alignment happened in person vs public docs.
@mike_conley 2) XPCOM ideas were new and many parts of system were straddling C++ objects and Interface models.
@mike_conley 3) XUL was also new and boundaries of what rendering belonged in browser shell vs. general rendering we’re [sic] being defined.
@mike_conley 4) JS access to XPCOM was also new driving rethinking of JS control vs embedding control.
@mike_conley There was a massive unwinding of the monolith and (re)defining of what it meant to build a browser inside a rendered chrome.

It’s cool to hear from the guy who really got the ball rolling here. The web is wonderful!

Finally, one last apology – this is a long-ass blog post. I’ve been working on it off and on for about 3 months, and it’s gotten pretty massive. Strap yourself into whatever chair you’re near, grab a thermos, cancel any appointments, and flip on your answering machine. This is going to be a long ride.

Oh come on, it’s not that bad, right? … right?

OK, let’s get back down to it. Where were we?

2005

A frame spoofing bug

Ah, yes – 2005 had just started. This was just a few weeks after a community driven effort put a full-page ad for Firefox in the New York Times. Only a month earlier, a New York Times article highlighted Firefox, and how it was starting to eat into Internet Explorer’s market share.

So what was going on in DocShell? Here are the bits I found most interesting. They’re kinda few and far between, since DocShell appears to have stabilized quite a bit by now. Mostly tiny bugfixes are landed, with the occasional interesting blip showing up. I guess this is a sign of a “mature” section of the codebase.

I found this commit on January 11th, 2005 pretty interesting. This patch fixes bug 103638 (and bug 273699 while it’s at it). What was going on was that if you had two Firefox windows open, both with <frameset>’s, where two <frames> had the same name attribute, it was possible for links clicked in one to sometimes open in the other. Youch! That’s a pretty serious security vulnerability. jst’s patch added a bunch of checks and smarter selection for link targets.

One of those new checks involved adding a new static function called CanAccessItem to nsDocShell.cpp, and having FindItemWithName (an nsDocShell instance method used to find some child nsIDocShellTreeItem with a particular name) take a new parameter of the “original requestor”, and ensuring that whichever nsIDocShellTreeItem we eventually landed on with the name that was requested passes the CanAccessItem test with the original requestor.

DocShell and session history

There are two commits, one on January 20th, 2005, and one on January 30th, 2005, both of which fix different bugs, but are interrelated and I want to talk about them for a second.

The first commit, for bug 277224, fixes a problem where if we change location to an anchor located within a document within a <script> tag, we stop loading the page content because the browser thinks we’re about to start loading a document at a different location. bz fixed the more common case of location change via setting document.location.href in bug 233963. Bug 277224 is interested in the case where document.location.href is modified with the .replace() method.

The solution that bz uses is to add new flags for nsIDocShellLoadInfo, which gives more power in how to stop loading a page. Specifically, it adds a LOAD_FLAGS_STOP_CONTENT flag which allows the caller to stop the rendering of content and all network activity (where the default was just to stop network activity). I believe what happens is that replace() causes an InternalLoad to kick off, and we need content rendering to be stopped in order for this new load to take over properly. That’s my reading on the situation, anyhow. If bz or anybody else examining that patch has another interpretation, please let me know!

So what about the commit on January 30th? Well that one also involves anchors. What was happening was that if we browsed to some page, and then clicked a link that scrolled us to an anchor in that page, clicking back would reload the entire document off the cache again, when we really just need to restore the old scroll position.

The patch to fix this basically detected the case where we were going back from an anchor to a non-anchor but had the same URL, and allowed a scroll in that case.

So how is this related to the commit for bug 277224? Well, what it shows is that at this time, DocShell was responsible for not just knowing how to load a document and subdocuments, but also about the user’s state in that document – specifically, their scroll position. It also more firmly establishes the link between DocShell and Session History – as DocShell traverses pages, it communicates with Session History to let it know about those transitions, and refers to it when traveling backwards and forwards, and when restoring state for those session history entries.

I just thought that was kinda neat to know.

Window pains

On February 8th, 2005, danm landed a patch to fix bug 278143, which was a bug that caused windows opened with window.open to open in a new window if they had no target specified. This wouldn’t normally be a problem, except that this could override a user preference to open those new windows in new tabs instead. So that was bad.

This was simply a matter of adding a check for the null case for the target window name in nsWindowWatcher. No big deal.

The reason I bring this code up, is because I find it familiar – I brushed by it somewhat when I was working on making it possible to open new windows for multi-process Firefox.

Semi-related (because of the “popup” nature of things), is a commit on February 23rd, 2005. This one is for bug 277574, which makes it so that modal HTTP auth prompts focus the tabs that spawn them. This patch works by making sure HTTP auth prompts fire the same DOMWillOpenModalDialog and DOMModalDialogClosed events that tabbrowser listens for to focus tabs.

The copy and the cache

On March 11, 2005, NeilAway landed a commit to add the “Copy Image” command item to the context menu. This was for bug 135300.

What’s interesting here is that “Copy Image Location” was already in the context menu, and in the bug, it looks like there’s some contention over whether or not to keep it. It seems that right around here, the solution they go with is to copy both the image and the image location to the clipboard, and mark each copy with the right “flavours”, so that if you were to paste to a program or field that accepted an “image” flavour, like Photoshop, you’d get the image. If you pasted to a program or field that accepted a “text” flavour, like Notepad, you’d get the image URL.

That’s the solution that was landed, anyhow. Notice that nowadays, Firefox has context menu items that allow users to copy just the image, and just the URL – so at some point, this approach was deemed wanting. I’ll keep my eye out to see if I can find out where that happened, and why. If anybody knows, please comment!

On April 28, 2005, roc landed a commit for bug 240276, which splits up something called “nsGfxScrollFrame” into two things – nsHTMLScrollFrame and nsXULScrollFrame. It seems like, up until this point, layout for both XUL and HTML scrollable frames were handled by the same code. This meant that we were using XUL box-model style layout for HTML, and XUL layout is… well… kind of tricky to work with. This patch helped to further distance our HTML rendering from our XUL rendering. As for how this affected DocShell, the patch removed some scroll calculations from DocShell, where they probably didn’t belong in the first place.

On May 4, 2005, Brian Ryner landed a patch which made it possible to move back and forward across web pages much more quickly. This was for bug 274784, and a key part of a project called “fastback”. When you view a web page, a DocShell is put in charge of requesting network activity to retrieve the document source, and then passing that source onto an appropriate nsIContentViewer. Up until Brian’s patch, it looks like every nsIContentViewer was just getting thrown away after browsing away from a page. His patch made it possible to store a certain number of these nsIContentViewers in the session history of the window, and then retrieve it when we browse back or forward to the associated page. This is a textbook trade-off between speed (the time to instantiate and initialize an nsIContentViewer) and space (stored nsIContentViewers consume memory). And it looks like the trade-off paid-off! We still cache nsIContentViewers to this day. What’s interesting about Brian’s patch is that it exposes an about:config preference1 for setting how many content viewers are allowed to be cached2. As DocShell seems to go hand in hand with session history, it’s not surprising that Brian’s patch touches DocShell code.

about:neterror arrives, Inner and Outer windows appear, and then Session History gets all snuggly with DocShell

On July 14th of 2005, bsmedberg landed a patch to add about:neterror pages, and close a privilege-escalation security vulnerability. Up until this point, network error pages were shown by browsing the DocShell to chrome URLs3, but this allowed certain types of attacks which load iframes resolving to network error pages to potentially gain chrome privileges4.

So instead of going to a chrome URL, the patch causes DocShell to internally load about:neterror5 The great news about this about:neterror page is that it has restricted permissions, so that security hole got plugged.

On July 30th, 2005, jst landed a patch to introduce the notion of inner and outer windows for bug 296639. Inner and outer windows has confused me for a while, but I think I’ve somewhat wrapped my head around it. This document helped.

The idea goes something like this:

The thing that is showing you web content can be considered the outer window – so that could be a browser tab, or an iframe, for example. The inner window is the content being displayed – it’s transitory, and goes away as you browse the web via the outer window.

The outer window then has a notion of all of the inner windows it might contain, and the inner window (via Javascript) gets a handle on the outer window via the window global.

So, for example, if you call window.open, the returned value is an outer window. Methods that you call on that outer window are then forwarded to the inner window.

I hope I got that right. I was originally trying to piece together the meaning of all of it by reading this WHATWG spec describing browsing context, and that was pretty slow going. The MDN page seemed much more clear.

Please comment with corrections if I got any of that wrong.

I’m not entirely sure, but based entirely on instinct and experience, I’m inclined to believe there are interesting security effects of this split. It seems to add a bit more of a membrane6 between web content and the physical window.

Anyhow, jst’s change was pretty monumental. It’s for bug 296639 if you want to read up more about it.

A semi-related change was landed on August 12th by mrbkab, where the entire inner window is stashed in the bfcache (as opposed to what we were doing before, which looks like serialization and deserialization of window state). That was for bug 303267, and sounds related to the fast back and forward caching work that Brian Ryner was working on back in May.

On August 18th of 2005, radha landed the first in the series of patches to session history. Unfortunately, the commit message for this patch doesn’t have a bug number, so I had some trouble tracking down what this work is for. I think this work is for bug 230363, and is actually a copy of interfaces from xpfe/components/shistory/public to docshell/shistory/public. Like I mentioned earlier, DocShell and session history are closely linked, so I suppose it makes sense to put the session history code under docshell/. Later that day, another patch copies the nsISHistoryListener interfaces over as well. Finally, a patch landed to build those interfaces from their new locations, and removes xpfe/components/shistory from Makefile.in. The bug for that last change is bug 305090.

Last bit of 2005

On August 22 of 2005, mrbkap landed a patch that changed how content viewer caching worked. There’s a special page in Firefox called about:blank – if you go to that page right now, you’re going to get a blank page. Some people like to set that as their home page or new tab page, as it is (or should be) very lightweight to load. That page is also special because, from what I can tell, when a new tab or window opens, it’s initially pointing at about:blank before it goes to the requested destination. Before this patch, we used to cache that about:blank content viewer in session history. We didn’t put an entry in the back-forward cache for about:blank though7, so that was a useless cache and a waste of memory. mrbkap’s patch made DocShell check to see if the page it was traveling to was going to re-use the current inner window, and if so, it’d skip caching it. Memory win!

That was the last thing I found interesting in 2005. On to 2006!

Preferences and threads…

On February 7th, 2006, bz landed a patch that made it possible for embedders to override where popup windows get opened.

There are preferences in Firefox that allow you to tweak how web content is able to open new windows8. Those preferences are browser.link.open_newwindow and browser.link.open_newwindow.restriction. If a page is attempting to open a new window, these preferences allow a user to control what actually occurs. The default (in most cases) is to open a new tab instead – but these preferences allow you to open that new window, or to open the content in the same window that the link is executed in. These are the kind of tweaks that power-users love.

Up until this point, only Firefox had these tweaking capabilities. bz’s patch moved that tweaking logic “up the chain”, so to speak, which means that applications that embedded Gecko could also be tweaked like this. Pretty handy.

For the Gecko hackers reading this, this patch also introduced the nsIWindowProvider interface9.

On May 10th, 2006, “darin” landed a patch for bug 326273 that put the nsIThreadManager interface and implementation into the tree. It’s a big commit, and affected many parts of the codebase. nsIThreadManager is, not surprisingly, used to implement multi-threading and thread manipulation in Gecko. From my look at the patch, it looks like it replaces something called nsIEventQueue / nsIEventQueueService. It looks like Gecko already had some facility for multi-threading10, but nsIThreadManager looks like a different model for multi-threading.

For DocShell, this change meant modifying the way that restoring PresShell’s from history would work. Before, DocShell had a RestorePresentationEvent that extended PLEvent, which allowed it to be posted to an nsIEventQueue. Now, instead, we define an inner-class that implements nsRunnable11, and also define a weak pointer to that runnable on a DocShell.

So the way things would go is this: DocShell::RestorePresentation would get called, and this would cancel any pending RestorePresentationRunnable that the DocShell is weak-pointing to. Next, we’d instantiate a new RestorePresentationRunnable that we’d then dispatch to the main thread. This isn’t really different to what we were doing before, but it makes use of the nsIThreadManager and nsRunnable class instead of nsIEventQueue and nsIEventQueueService.

What’s interesting about this patch, DocShell-wise, is that it shows the usage of FavorPerformanceHint, which looks like a way of trading-off UI interactivity with page-to-screen time. Basically, it looks like the FavorPerformanceHint is used when restoring PresShell’s to tell the nsIAppShell, “hey – we want you to favor native events over other events for a small pocket of time so we can get this stuff to the screen ASAP”. If I’m interpreting that right, it’s a tradeoff between total time to execute and responsiveness here. “Do you want it fast, or do you want it smooth?”.

I was probably wrong about the name

In one of my past posts, I made some guesses about why DocShell was called DocShell. I thought:

I think nsDocShell was given the “shell” monicker because it did the job of taking over most of nsWebShell’s duties. However, since nsWebBrowser was now the touch-point between the embedder and embedee… maybe shell makes less sense. I wonder if we missed an opportunity to name nsDocShell something better.

But now that I look at nsIAppShell, and nsIDocShell, and nsIPresShell… I think I’m starting to understand. A while back, when I first started planning these posts, I asked blassey why he thought nsIDocShell was named the way it was, and he said he thought it might be related to the notion of a command shell – like a terminal input. From my understanding, a shell is a command interface with which one can manipulate and control something pretty complex – like the file-system or processes of a computer. I think blassey is right – I think that’s the “Shell” in nsIDocShell. I think the idea is that this interface would be the one to control and manipulate the process of loading and displaying a document. It seems obvious now, but it sure wasn’t when I started looking into this stuff.

DOM Storage (session and global), KungFuDeathGrip, friendlier search…

On May 19th, 2006, jst picked up, finished and landed a patch originally be Enn that implemented DOM Storage for bug 335540.

This patch adds two new methods to nsIDocShell – getSessionStorageForDomain and addSessionStorage. The first method is accessed in a number of cases, but most importantly when some caller reads sessionStorage or globalStorage off of the window object12.

The relationship between nsGlobalWindow and nsDocShell is brought to my attention with this patch. Here’s a fragment from an old chat I had with Ms2ger, smaug and bz, which started when I asked Ms2ger what he’d rename DocShell to.

14:11 (Ms2ger) mconley, I would call it WindowProxy :)
14:12 (smaug) outer window? yes, WindowProxy please
14:12 (mconley) Ms2ger: wait, outer window = docshell currently?
14:12 (khuey) what are we doing with WindowProxy?
14:13 (Ms2ger) mconley, well, no, there’s nsDocShell and nsGlobalWindow (with IsOuterWindow() true)
14:13 (Ms2ger) mconley, those are pretty much isomorphic
14:13 (mconley) I see
14:13 (bz) nsDocShell and outer nsGlobalWindow are in a 1-1 relationship
14:14 (bz) The fact that they are two separate objects is sort of a historical accident that we may want to rectify sometime
14:14 (mconley) this sounds like another post to write – how nsDocShell and nsGlobalWindow are related…

So I think nsGlobalWindow (instances of which can either be “inner” or “outer”) when it has IsOuterWindow being true, works in tandem with nsDocShell to “be” the outer window. That’s really imprecise, hand-wavey language. I’ll probably need to tighten this up in a follow-up post once somebody reads this and gives me better words to describe things13.

On May 24, 2006, smaug landed a patch to fix bug 336978. Bug 336978 was a crash caused by loading the following code in an iframe:

<html>
<head></head>
<body>
  <script>
    window.addEventListener("pagehide", doe, true);
    function doe(e) {
      var x = parent.document.getElementsByTagName('iframe')[0];
      x.parentNode.removeChild(x);
    }
    setTimeout(doe2,500);
    function doe2() {
      window.location = 'about:blank';
    }
  </script>
</body>
</html>

What this code does is wait 500ms, and then change the location to about:blank. Changing the location causes the pagehide event to fire while we’re unloading the original page, and when we hear it, we get the host of the iframe to remove the iframe from itself.

smaug’s solution to this bug is for nsDocShell to hold a reference via an nsCOMPtr to the nsIContentViewer for the document while the pagehide event is fired. This ensures that the nsIContentViewer doesn’t get destructed before we’re truly done to it. The name we give this nsCOMPtr is “kungFuDeathGrip”. This isn’t the only place where some hold of an object is maintained with a variable called kungFuDeathGrip – check out dxr for some more uses.

I’d seen kungFuDeathGrip over the years, and I never looked closely at what it was doing. I always thought kungFuDeathGrip was some magical global function that destroyed things unequivocally, but on closer inspection, I’m pretty sure it’s really just a way of saying “this variable’s sole purpose is to hold a reference to this thing until I’m done with it.”

I think the phrase “kung fu” distracted me. I thought it did this:

Black Dynamite layin' the smack down.

Woooooo!

But it’s really more like this:

Spock taking out Kirk with the Vulcan nerve pinch thing.

Kkkg….*gurgle*…ngahh….

On June 15th, 2006, “brettw” landed a patch for bug 245597 to make it so that anything that gets put into the AwesomeBar that isn’t parse-able as a URI automatically turns into a keyword search. That’s great! This made both the search input and the AwesomeBar useful for more users. This change occurred in docshell/base/nsDefaultURIFixup.cpp, which is, as I understand it, the central location for code that turns erroneous URIs into what the user probably intended.

nsIMutationObserver, some new about: pages…

On July 2nd, 2006, Jonas Sicking added nsIMutationObserver to the tree for bug 342062, making it possible to observe changes to the DOM within a subtree. It’s a pretty big patch, but it looks like a good chunk of it is just swapping in usage of nsIMutationObserver to replace old usage of nsIDocumentObserver (which supplied the same observations, but for an entire document instead of a subtree). Note that it’d still be a few years before DOM3 Mutation Events would be exposed for web developers to use, and after a few more years, those events were deprecated in favour of the Mutation Observer API.

On September 15th, 2006, several new Gecko-wide about: pages landed, which means they got put into the redirection map in docshell/base/nsAboutRedirector.cpp. These pages were about:buildconfig (bug 140034), about:about (bug 56061), and about:license (bug 256945). That same day, bz landed a patch to make it so that new about: pages didn’t have to have special rules hardcoded into nsScriptSecurityManager::CanExecuteScripts to execute script even if the user has script disabled. Instead, the nsAboutRedirector mapping was extended to allow a boolean for indicating that an about: page required script execution.

Simplifying DocShell, and then some spoofing and malware protection

The next interesting thing (according to me, anyhow) didn’t occur until the following May 6th, 2007. That day, bz landed a patch for bug 377303 which simplified the structure of things inside the DocShell tree.

Up until that point, there had existed both nsIDocShellTreeItem and nsIDocShellTreeNode had both existed as interfaces for interacting with nodes within a DocShell tree. I’ll quote myself from my previous post:

The (somewhat nebulous) distinction of DocShell “treeItems” and “treeNodes” is made. At this point, the difference between the two is that nsIDocShellTreeItem must be implemented by anything that wishes to be a leaf or middle node of the DocShell tree. The interface itself provides accessors to various attributes on the tree item. nsIDocShellTreeNode, on the other hand, is for manipulating one of these items in the tree – for example, finding, adding or removing children. I’m not entirely sure this distinction is useful, but there you have it.

It looks like enough was finally enough. bz didn’t go so far as to fully merge the two interfaces (though he makes a note in his patch about doing so), but instead made the (arguably more complex) nsIDocShellTreeItem interface inherit from nsIDocShellTreeNode14.

Later, on May 17th, 2007, Mats Palmgren landed a patch for bug 376562 to remove a childOffset attribute from nsIDocShellTreeItem, and instead move a setter for the childOffset to the nsIDocShell interface instead.

Reading this bug comment as well as one of Mats’ comments in the patch, it sounds as if childOffset never really worked as advertised, and was a bit of a foot-gun.15

On June 14th, 2007, bz landed a patch for bug 371360, which prevents onUnload handlers from starting any page loads. Before this, it seems that it was possible for a page do to something like this:

<html>
  <body onunload="location.href = 'http://www.somesite.com';">
    <a href="http://slashdot.org/">http://slashdot.org/</a>
  </body>
</html>

With the result being that you could (potentially) phish a user. For example, suppose you’re a member of MySafeBank, which has a site at mysafebank.com. Suppose you’re at my seemingly innocent site totallyevil.com, and also suppose that I’ve registered a domain at rnysafebank.com (that’s an r and an n, which, if you’re not paying attention, look pretty close to a m). If you’re at my site, and I notice that you’re trying to head to mysafebank.com, I could redirect you to rnysafebank.com, which has a very similar user interface and favicon. Yadda yadda yadda, your bank info is now mine.

So bz stopped that one in its tracks by just preventing a DocShell from attempting any kind of load if we’re in the middle of an unload.

On July 3rd, 2007, johnath16 landed a patch for bug 380932 to add a new mode for about:neterror for pages suspected of serving up malware.

If you haven’t seen that page before, count yourself lucky – you’ve been surfing in safe places! This is what the page looks like (or, used to look like, anyhow):

Minefield reporting a Suspected Attack Site

Old version of Firefox showing a Suspected Attack Site

johnath’s patch allowed an about:neterror page to have a specific CSS class associated with it as part of its URL. This allowed for the dramatic styling in the image above17.

showModalDialog

showModalDialog was a non-standard function that Microsoft introduced in Internet Explorer 418. This allows a web page to create a modal dialog that contains web content. jst landed a patch on July 26th, 2007 to implement it in Firefox as part of bug 194404. This patch made it possible for a DocShell to have a modal dialog be its parent.

showModalDialog has since been marked as deprecated on MDN, and Google Chrome have announced they will no longer support it after May of 2015. Firefox will support it until sometime after Firefox 39 on the release channel19.

about:crashes, Larry, and tab tearing

There’s a long gap in time here where nothing really interesting happens under docshell/.

Finally, on January 24th, 2008, Mossop landed a patch for bug 411490 that exposes about:crashes as a handy way of getting at the list of crash reports that have been collected.

As about:crashes is a Gecko-wide about: page, this meant once again adding an entry to the docshell/base/nsAboutRedirector.cpp map, as had been done with about:buildconfig and about:about.

On April 28th, 2008, gavin landed a patch originally by ehsan that adds a friendlier set of icons for reporting SSL errors for bug 430904.

That icon is Larry. Have you met Larry? This is Larry:

Larry, the SSL dude.

This is Larry.

Larry is the name for a series of icons that were developed to describe how secure your communications are with a particular site. You might recognize him from the airport, because he looks a lot like a customs agent or border patrol.

You can read up on Larry here on johnath’s blog.

On August 7th, 2008, bz landed a patch for bug 113934 to lay the foundation for letting users drag tabs out from a window, or drag tabs between windows. This introduced a new method to nsIFrameLoader, “swapFrameLoaders”. This method is the real key to moving tabs between windows – each <xul:browser> implements nsIFrameLoaderOwner, and the nsIFrameLoader is (yet another) thing that can load web content. This is essentially a brain transplant between two <xul:browser>’s. You can see the real guts of the brain transplant in this method of the patch.

On January 13th, in a semi-related patch, bz landed code for bug 449780 to flush the bfcache20 when swapping frameloaders. Apparently, we were storing information in the bfcache that was simply incorrect after a frameloader swap. The best way to avoid internal confusion in such a case was to just invalidate the cache.

Big gaps…

Lots of big gaps between the next few changes.

On March 18th, 2009, Honza Bambas landed a patch for bug 422526 to implement window.localStorage. localStorage was a replacement for globalStorage21 that persisted across browser restarts (unlike sessionStorage).

Note that both localStorage and sessionStorage were synchronous storage APIs. It’d take until around June 24th of 2010 before an asynchronous storage mechanism became available.

On May 7th, 2009, bz landed a patch for bug 490957 to finally get rid of nsWebShell.cpp. If you recall from my earlier blog post, that was one of Travis Bogard’s goals at the start of this whole adventure. bz’s patch essentially folds the functionality of nsWebShell into nsDocShell. The webshell/ folder remained, but just contained interfaces.

Curiously, a good chunk of nsWebShell’s functionality seemed to revolve around anchor pings, a massively unpopular “feature” that allows a website to get your browser to send a request every time you click on a link. Thankfully, this “feature” is disabled by default in Firefox22. Here’s Jorge Villalobos’s post on anchor pings. Correction (Jan 4th, 2015) - I’ve since changed my tune about anchor pings. See these three comments.

On June 29th, 2009, dbolter landed a patch for bug 467144 so that nsIMutationObserver’s, when they observe an attribute being changed, also get a copy of the old attribute as well as the new one. Actually, to be specific, it adds a callback “AttributeWillChange” to include the old attribute. This fires before the “AttributeChanged” callback.

A day later, bsmedberg landed a massive patch to implement remote tabs. There’s no bug number in the commit message, but this is clearly part of the Electrolysis efforts that were just starting up around this time. Remote tabs means browsers that run in different processes, which is the overall goal of Electrolysis, and (possibly unbeknownst to bsmedberg at the time) a foundational piece for Fennec (Firefox for Android)23.

On October 3rd, 2009, vvuk landed about:memory for bug 515354, a key piece of the war against high-memory consumption in Firefox (a.k.a. MemShrink). This is very similar to the about:crashes page that Mossop landed back in 2008.

And finally…

On January 7th, 2010, smaug landed a patch for bug 534226 to remove support for multiple PresShell’s. PresShell stands for “Presentation Shell”, and as I understand it, is the primary interface to the “frame tree”24.

It looks like, up until this point, Gecko had the ability to have multiple frame trees per content tree. I’m not entirely sure what the point of that was, but the capability was there. smaug’s patch simplifies everything by making sure a document has only a single, primary PresShell. This removes a lot of iteration and management code for those multiple PresShell’s, which is nice.

And last, but not least, on June 30th, 2010, Benjamin Stover landed a patch for bug 556400 which made it so that visits to webpages are recorded asynchronously in Places. It looks like this patch takes I/O off the main-thread, so it gets a big thumbs-up from me.

Essentially, this patch adds new asynchronous write methods to the History service, and then makes nsDocShell use those methods on webpage visits. nsDocShell falls back to the synchronous methods of nsIGlobalHistory2 if, for some reason, it can’t get at the History service and its asynchronous methods.

Phew!

Did you make it? Are you still with me? I know it might feel like this:

qwop

everyone is a winner

but I think we’re making real progress here. I think we’re learning important stuff about the history of Firefox, and changes that have occurred in some of its core functionality over time.

So to sum, that, to me, was the most interesting stuff to happen in and around docshell/ from 2005-2010. There might have been other neat stuff in there, but it didn’t catch my eye when I was browsing commit messages.

There’s still much to do – I have to look at commits from 2011 to 2014. After that, I’m planning on doing a line-by-line code review / walkthrough of nsDocShell.cpp, and then I’d like to try to summarize my findings and any recommendations I’ve put together from my time studying this stuff.

Hold tight!

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  1. This pref was browser.sessionhistory.max_viewers, if you’re interested – though that preference appears to have been superseded by browser.sessionhistory.max_total_viewers. The default value for that pref is -1, meaning to adjust the number of allowed cached viewers based on how much memory is available. If you’re looking to reduce how much memory Firefox consumes, it’s possible setting this to some low integer will allow you reverse that trade-off between space and speed. 

  2. I assume per session history 

  3. You wouldn’t notice that you were at a chrome URL though, because DocShell loads this URL internally, while pretending to be at the URL that caused the error. The end result is the user going to http://www.sitethatcausesnetworkerror.com still sees that URL in their AwesomeBar, despite the fact that their web content shows the appropriate network error page hosted at a chrome URL. 

  4. “chrome privileges” means that a web page now essentially has the same permissions that Firefox, the program on your computer, has – meaning it can potentially read and write files, and communicate with anybody on your network. Yikes! 

  5. You can visit this page in Firefox right now and see a generic network error. It’s showing a generic error because it hasn’t the foggiest idea how you’ve arrived at about:neterror, since it wasn’t passed any error information. 

  6. Or the infrastructure to create such a membrane. 

  7. So you couldn’t go back to about:blank in cases I described, where a tab or window was initialized at about:blank before going to a new page. 

  8. I’m actually quite familiar with this stuff because I worked on opening new windows for Electrolysis not too long ago. 

  9. From the header of that interface:

    /**
     * The nsIWindowProvider interface exists so that the window watcher's default
     * behavior of opening a new window can be easly modified.  When the window
     * watcher needs to open a new window, it will first check with the
     * nsIWindowProvider it gets from the parent window.  If there is no provider
     * or the provider does not provide a window, the window watcher will proceed
     * to actually open a new window.
     */

     

  10. The nsIEventQueueService service mentions that it is used to manage event queues for a particular thread, and makes use of nsIThread – so multi-threading must have already been a thing. 

  11. Still called RestorePresentationEvent though… strange that the opportunity wasn’t taken to rename this to RestorePresentationRunnable. 

  12. Those two properties are part of a new nsIDOMStorageWindow interface that nsGlobalWindow implements after this patch. That interface is later removed in bug 670331, and the two accessors are moved directly into nsIDOMWindow instead. 

  13. I have a feeling the real answer lies somewhere in the comments in bug 296639

  14. It wasn’t immediately clear to me why the inheritance didn’t go the other way around – especially since bz himself had a comment in nsIDocShellTreeNode suggesting that arrangement. Look at his first comment in the bug though:

    This would allow consumers to start using just nsIDocShellTreeItem in their code, until we can just merge nsIDocShellTreeNode into nsIDocShellTreeItem.

    Basically, it sounds like nsIDocShellTreeNode was being deprecated, and that callers who used to use nsIDocShellTreeNode should migrate to use nsIDocShellTreeItem instead (which inherits nsIDocShellTreeNode’s methods). Then the two interfaces could be merged. 

  15. Mats’ warning was removed on August 17, 2010 as part of bug 462076. It looks like SetChildOffset is still only ever used when adding the child though, so it’s still probably valid. 

  16. The same johnath who is currently the VP of Firefox! 

  17. Later on in August, dcamp would land this patch as part of bug 384941 which prevents suspected malware sites from even loading, instead of just not displaying them. 

  18. To quote Douglas Adams:

    This has made a lot of people very angry and has been widely regarded as a bad move.

     

  19. And the 38 ESR will continue to be supported until mid-2016. If you maintain a site that uses showModalDialog, you’d best get rid of it. 

  20. The bfcache, or “back-forward cache” is a collection of “frozen” pages that are stored in memory for fast back/forward action – see this page for more detail. 

  21. globalStorage, I believed, allowed all web properties read and write access to the same storage – so clearly it was a good idea to replace it. globalStorage was removed on October 9th, 2011 by Honza as part of bug 687579

  22. But according to this, is enabled by default in both Chrome and Opera. Lovely

  23. Remote tabs are also hugely important for Boot2Gecko / Firefox OS

  24. From this document:

    …the frame tree…is the visual representation of the document. Each frame can be thought of as a rectangular area on the page. The content nodes for XML elements are usually associated with one or more frames which display the element — one frame if the element is rectangular, more if the element is something more complex (like a chunk of bolded text that happens to be word-wrapped)…

    And from the nsIPresShell.h header:

    /**
    * Presentation shell interface. Presentation shells are the
    * controlling point for managing the presentation of a document. The
    * presentation shell holds a live reference to the document, the
    * presentation context, the style manager, the style set and the root
    * frame. <p>
    *
    * When this object is Release’d, it will release the document, the
    * presentation context, the style manager, the style set and the root
    * frame.

     

Australis Performance Post-mortem Summary

Over the last few months, I’ve been talking about all of the work we put into making Australis feel fast when it shipped in Firefox 29.

I talked about where we started with our performance work, and how we grappled with the ts_paint and tpaint performance (“talos”) tests. After that, I talked a bit about the excellent tools we have (and ones we developed ourselves) to make finding our performance bottlenecks easier.

After a brief delay, I rounded out the series by talking about our tab animation performance work, and the customization transition performance work.

I think over the course of working on these things, I’ve learned quite a bit about performance work in general. If I had to distill it down to a few tidbits, it’d be:

  • Measure first to get a baseline, then try to improve. (Alternatively, “you can’t improve what you can’t measure”)
  • Finding the solutions to performance problems is usually the easy part. The hard part is finding and isolating the problems to begin with.
  • While performance work can be a bit of a grind, users do feel and appreciate the efforts. It’s totally worth it.

So that’s it on the series. Enjoy your zippy Firefox!

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Australis Performance Post-mortem Part 5: The Customize Mode Transition

Another new thing that came out with Firefox 29 is a sexy new customization interface. We wanted to make UI customization something that anybody would feel comfortable doing, instead of something only a few mighty power users might do.

The new customization mode is accessible by pressing the Menu Button (☰), and then clicking Customize.

Give it a shot now if you’ve never done it.

BAM, did you see that? What you just saw was a full-on mode switch in the browser chrome to indicate to you that you’ve put the browser into a different state – specifically, a state where much of the browser UI is malleable. You can drag and drop things in your toolbars and the menu, enable toolbars, etc.

Going in and out of customization mode is not something that most people will do frequently, but we still invested a bunch of time trying to make it smooth. I still think there is more we can do on that front, and I’ll get into that at the end of this post.

Anyhow, as with any performance-related project, we started with a way of measuring. In this case, our trusty performance team re-purposed the TART test that we used for tab animation, and pointed it at the customize mode transition. We called this new test CART (customization animation regression test, natch).

This is slightly different from the tab animation stuff, because we didn’t really have a baseline measurement to compare against – there was no old customization mode transition to try to match or beat. So we just had to do our best to do all of our processing under 16ms in order to draw the transition at 60fps.

That means that instead of comparing two sets of data over time, we’ll only be looking at a single series of points, and how they change over time.

So let’s see where we started, and where we finished, and how we got there.

CART results - pre-optimizations

CART results – pre-optimizations

So in an effort to get this blog post done, I’ve cut some corners and combined the data from a number of different platforms into a single graph. I apologize if this makes it difficult to interpret for some of you – but I’ll do my best to explain what the graph means.

I chose a representative sample of the platforms that we measured – we’ve got OS X (10.6, 10.8), Ubuntu 12.04 (32-bit), Windows XP, Windows 7, and Windows 8.

The X-axis is obviously plotting the time (we started gathering CART data right at the end of January 2014). The Y-axis plots the “final CART value”. CART, like TART, measures a number of things, and we “boil that number down” into a single value that we can plot. Each of the subtests are measured in milliseconds, so I guess you could say the Y-axis is also in milliseconds – but as it’s an aggregate, that’s not too meaningful. It’s not the greatest for detecting small shifts in the subtests (we use Datazilla for that), but it allows me to illustrate the changes over time in a pretty simple way.

So it looks like my sampled platforms are all floating around the 40’s and 50’s. Is that good or bad? Well, it’s neither really. It’s just our starting point, and we wanted to try to improve on that.

And so we set out trying to move those dots downwards (for this data, without going into grand detail, lower milliseconds = faster and smoother animation).

As with TART, we did a lot of profiling, and used a lot of the tools I mentioned in this post. Here are the bugs that seemed to move the needle the most.

The Rogue’s Gallery

As before, this is not a complete list, but captures some of the more interesting bugs we worked on.

Bug 932963 – Break customize mode transition into several phases

This change allowed us to break the transition in and out of customization mode into some phases:

  1. Not in customization mode (default)
  2. Entering customization mode
  3. Entered customization mode
  4. Exiting customization mode

Phases 2, 3 and 4 have attributes on the main browser window that we can target with CSS selectors. We were then able to “lighten” the CSS during phases 2 and 4, in order to optimize the frames during the transition. For example, we don’t display the semi-transparent grid texture on the main window unless we’re in phase 3.

Bug 972485 – Find out why we’re doing a bunch of synchronous file reading at the start of the customize mode transition

This was a rather surprising one – using the Gecko Profiler, it looked like we were doing sync file IO as the blank about:customizing document was loading.

For context, we have a page registered at about:customizing that’s a blank document. When we detect that about:customizing has been loaded or switched to, we enter customize mode.

So this sync file IO was causing some jank at the start of the customization mode transition.

Strangely, it turned out that the XHTML file we were loading for the blank about:customizing document was synchronously loading a bunch of MathML localization stuff as it loaded.

We switched the document from XHTML to XUL, and the sync IO load went away. We filed a follow-up bug to investigate why exactly we were doing sync file IO on loading XHTML files, because that’s a bad thing to do.

Anyhow, this was a small but significant win for almost all platforms.

Bug 975552 – Preload about:customizing like we do with about:newtab

I think this was the biggest win we achieved during the performance work. That browser that we load the blank about:customizing page in is not free – a bunch of stuff gets instantiated in order for a working browser to be created, and all of that expense is wasted on a blank document. That expense also causes enough main thread thinking that it reduces the smoothness of the customization entering transition.

So the solution was a hack that takes after the same strategy we use for about:newtab. Essentially, we preload the about:customizing browser and document in the background at what seems like a logical time (right after the user has opened the menu). This allows us to front-load all of the expense in creating the browser, and we end up with something much smoother.

Here’s a before video and an after video from my Windows 7 machine, for reference.

This was a pretty bodacious hack, and in the future, I’d like to remove it completely, and find some way of sidestepping all of the browser internal loading for the about:customizing document (or, even better, find a way of not using a browser element at all!). That’s filed here.

Bug 977796 – Disable subpixel anti-aliasing during customize mode transition.

A profile on Windows 8 showed that we were spending an inordinate amount of time rendering text during the customize mode transition, and this has to do with subpixel anti-aliasing in the menu that animates in.

So I landed a patch that temporarily disables subpixel anti-aliasing on that element during the customize mode transition, and that bought us a huge win for Window 8 (about 20%). Not much of a win for any of the other platforms.

So where did we end up?

CART results - post-optimizations

CART results – post-optimizations

Those numbers are indeed lower!

Now, before you lose your mind about that epic cross-platform win around Feburary 20th, I investigated that changeset range and didn’t find anything we worked on directly in there.

There are some platform patches in there (specifically in Graphics) that might account for this win, but I’m pretty certain it’s because of bug 974621, which updated our test runners to use a different version of Talos that included the patch in bug 967186. 967186 altered the CART test to be more accurate in its measurements, so actually a lot of our initial data was erroneous. That blows a bit because it adds some unnecessary noise to our graph, but it’s also good because accuracy is a thing we definitely want.

Future work

I still think there’s more wins to be made in the customize mode transition. Finding a way of getting rid of the about:customizing preloading hack and replace it with something smarter (like a thin browser that doesn’t need to instantiate much of its backend, or no browser at all) is probably the first step. More profiling might find more wins. I’m learning more and more about platform as I work on Electrolysis, so maybe I’ll come back to this problem with some new skills and information, and I can get it performing reliably on all platforms as it really should: 60fps, smooth as silk.

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Australis Performance Post-mortem Part 4: On Tab Animation

Whoops, forgot that I had a blog post series going on here where I talk about the stuff we did to make Australis blazing fast. In that time, we’ve shipped the thing (Firefox 29 represent!), so folks are actually feeling the results of this performance work, which is pretty excellent.

I ended the last post on an ominous note – something about how we were clear to land Australis on Nightly, or so we thought.

This next bit is all about timing.

There is a Performance Team at Mozilla, who are charged with making our products crazy-fast and crazy-smooth. These folks are geniuses at wringing out every last millisecond possible from a computation. It’s what they do all day long.

A pretty basic principle that I’ve learned over the years is that if you want to change something, you first have to develop a system for measuring the thing you’re trying to change. That way, you can determine if what you’re doing is actually changing things in the way you want them to.

If you’ve been reading this Australis Performance Post-mortem posts, you’ll know that we have some performance tests like this, and they’re called Talos tests.

Just about as we were finishing off the last of the t_paint and ts_paint regressions, the front-end team was suddenly made aware of a new Talos test that was being developed by the Performance team. This test was called TART, and stands for Tab Animation Regression Test. The purpose of this test is to exercise various tab animation scenarios and measure the time it takes to paint each frame and to proceed through the entire animation of a tab open and a tab close.

The good news was that this new test was almost ready for running on our Nightly builds!

The bad news was that the UX branch, which Australis was still on at the time, was regressing this test. And since we cannot land if we regress performance like this… it meant we couldn’t land.

Bad news indeed.

Or was it? At the time, the lot of us front-end engineers were groaning because we’d just slogged through a ton of other performance regressions. Investigating and fixing performance regressions is exhausting work, and we weren’t too jazzed that another regression had just shown up.

But thinking back, I’m somewhat glad this happened. The test showed that Australis was regressing tab animation performance, and tabs are opened every day by almost every Firefox user. Regressing tab performance is simply not a thing one does lightly. And this test caught us before we landed something that regressed those tabs mightily!

That was a good thing. We wouldn’t have known otherwise until people started complaining that their tabs were feeling sluggish when we released it (since most of us run pretty beefy development machines).

And so began the long process of investigating and fixing the TART regressions.

So how bad were things?

Let’s take a look at the UX branch in comparison with mozilla-central at the time that we heard about the TART regressions.

Here are the regressing platforms. I’ll start with Windows XP:

Where we started with TART on Windows XP

Where we started with TART on Windows XP

Forgive me, I couldn’t get the Graph Server to swap the colours of these two datasets, so my original silent pattern of “red is the regressor” has to be dropped. I could probably spend some time trying to swap the colours through various tricky methods, but I honestly don’t think anybody reading this will care too much.

So here we can see the TART scores for Windows XP, and the UX branch (green) is floating steadily over mozilla-central (red). Higher scores are bad. So here’s the regression.

Now let’s see OS X 10.6.

Where we started with TART on OS X 10.6

Where we started with TART on OS X 10.6

Same problem here – the UX nodes (tan) are clearly riding higher than mozilla-central. This was pretty similar to OS X 10.7 too, so I didn’t include the graph.

On OS X 10.8, things were a little bit better, but not too much:

Where we started with TART on OS X 10.8

Where we started with TART on OS X 10.8

Here, the regression was still easily visible, but not as large in magnitude.

Ubuntu was in the same boat as OS X 10.6/10.7 and Windows XP:

Where we started with TART on Ubuntu

Where we started with TART on Ubuntu

But what about Windows 7 and Windows 8? Well, interesting story – believe it or not, on those platforms, UX seemed to perform better than mozilla-central:

Windows 7 (the blue nodes are mozilla-central, the tan nodes are UX)

Where we started with TART on Windows 7

Where we started with TART on Windows 7

Windows 8 (the green nodes are mozilla-central, the red nodes are UX)

Where we started with TART on Windows 8

Where we started with TART on Windows 8

So what the hell was going on?

Well, we eventually figured it out. I’ll lay it out in the next few paragraphs. The following is my “rogue’s gallery” of regressions. This list does not include many false starts and red herrings that we followed during the months working on these regressions. Think of this as “getting to the good parts”.

Backfilling

The problem with having a new test, and having mozilla-central better than UX, is not knowing where UX got worse; there was no historical measurements that we could look at to see where the regression got introduced.

MattN, smart guy that he is, got us a few talos loaner machines, and wrote some scripts to download the Nightlies for both mozilla-central and UX going back to the point where UX split off. Then, he was able to run TART on these builds, and supply the results to his own custom graph server.

So basically, we were able to backfill our missing TART data, and that helped us find a few points of regression.

With that data, now it was time to focus in on each platform, and figure out what we could do with it.

Windows XP

We started with XP, since on the regressing platforms, that’s where most of our users are.

Here’s what we found and fixed:

Bug 916946 – Stop animating the back-button when enabling or disabling it.

During some of the TART tests, we start with single tab, open a new tab, and then close the new tab, and repeat. That first tab has some history, so the back button is enabled. The new tab has no history, so the back button is disabled.

Apparently, we had some CSS that was causing us to animate the back-button when we were flipping back and forth from the enabled / disabled states. That CSS got introduced in the patch that bound the back/forward/stop/reload buttons to the URL bar. It seemed to affect Windows only. Fixing that CSS gave us our first big win on Windows XP, and gave us more of a lead with Windows 7 and Windows 8!

Bug 907544 - Pass the D3DSurface9 down into Cairo so that it can release the DC and LockRect to get at the bits

I don’t really remember how this one went down (and I don’t want to really spend the time swapping it back in by reading the bug), but from my notes it looks like the Graphics team identified this possible performance bottleneck when I showed them some profiles I gathered when running TART.

The good news on this one, was that it definitely gave the UX branch a win on Windows XP. The bad news is that it gave the same win to mozilla-central. This meant that while overall performance got better on Windows XP, we still had the same regression preventing us from landing.

Bug 919541 - Consider not animating the opacity for Australis tabs

Jeff Muizelaar helped me figure this out while we were using paint flashing to analyze paint activity while opening and closing tabs. When we slowed down the transitions, we noticed that the closing tabs were causing paints even though they weren’t visible. Closing tabs aren’t visible because with Australis, we don’t show the tab shape around tabs when they’re not selected – and closing a tab automatically unselects it for something else.

For some reason, our layout and graphics code still wanted to paint this transition even though the element was not visible. We quickly nipped that in the bud, and got ourselves a nice win on tab close measurements for all platforms!

Bug 921038 – Move selected tab curve clip-paths into SVG-as-an-image so it is cached.

This was the final nail in the coffin for the TART regression on Windows XP. Before this bug, we were drawing the linear-gradients in the tab shape using CSS, and the clipping for the curve background colour was being pulled off using clip-path and an SVG curve defined in the browser.xul document.

In this bug, we moved from clipping a background to create the curve, to simply drawing a filled curve using SVG, and putting the linear-gradient for the texture in the “stroke” image (the image that overlays the border on the tab curve).

That by itself was not enough to win back the regression – but thankfully, Seth Fowler had been working on SVG caching, and with that cache backend, our patch here knocked the XP and Ubuntu regressions out! It also took out a chunk from OS X. Things were looking good!

OS X

Bug 924415 - Find out why setting chromemargin to 0,-1,-1,-1 is so expensive for TART on UX branch on OS X.

I don’t think I’ll ever forget this bug.

I had gotten my hands on a Mac Mini that (after some hardware modifications) matched the specs of our 10.6 Talos test machines. That would prove to be super useful, as I was easily able to reproduce the regression that machine, and we could debug and investigate locally, without having to remote in to some loaner device.

With this machine, it didn’t take us long to identify the drawing of the tabs in the titlebar as the main culprit in the OS X regression. But the “why” eluded us for weeks.

It was clear I wasn’t going to be able to solve it on my own, so Jeff Muizelaar from the Graphics team joined in to help me.

We looked at OpenGL profiles, we looked at apitraces, we looked at profiles using the Gecko Profiler, and we looked at profiles from Instruments – the profiler that comes included with XCode.

It seemed like the performance bottleneck was coming from the operating system, but we needed to prove it.

Jeff and I dug and dug. I remember going home one day, feeling pretty deflated by another day of getting nowhere with this bug, when as I was walking into my apartment, I got a phone call.

It was Jeff. He told me he’d found something rather interesting – when the titlebar of the browser overlapped the titlebar of another window, he was able to reproduce the regression. When it did not overlap, the regression went away.

Talos tests open a small window before they open any test browser windows. That little test window stays in the background, and is (from my understanding) a dispatch point for making talos tests occur. That little window has a titlebar, and when we opened new browser windows, the titlebars would overlap.

Jeff suggested I try modifying the TART test to move the browser approximately 22px (the height of a standard OS X titlebar) so that they would no longer overlap. I set that up, triggered a bunch of test runs, and went to bed.

I wasn’t able to sleep. Around 4AM, I got out of bed to look at the results – SUCCESS! The regression had gone away! Jeff was right!

I slept like a baby the rest of the night.

We closed this bug as a WONTFIX due to it being way outside our control.

Comment 31 and onward in that bug are the ones that describe our findings.

Eat it TART, your tears are delicious

Those were the big regressions we fixed for TART. It was a long haul, but we got there – and in the end, it means faster and smoother tab animation for our users, which means a better experience – and it’s totally worth it.

I’m particularly proud of the work we did here, and I’m also really happy with the cross-team support and collaboration we had – from Performance, to Layout, to Graphics, to Front-end – it was textbook teamwork.

Here’s an e-mail I wrote about us beating TART.

Where did we end up?

After the TART regression was fixed, we were set to land on mozilla-central! We didn’t just land a more beautiful browser, we also landed a more performant one.

Noice.

Stay tuned for Part 5 where I talk about CART.

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Electrolysis Code Spelunking: How links open new windows in Firefox

Hey. I’ve started hacking on Electrolysis bugs. I’m normally a front-end engineer working on Firefox desktop, but I’ve been temporarily loaned out to help get Electrolysis ready to be enabled by default on Nightly.

I’m working on bug 989501. Basically, when you click on a link that targets “_blank” or uses window.open, we open a new tab instead. That’s no good – assuming the user’s profile is set to allow it, we should open the link in a new window.

In order to fix this, I need a clearer picture on what happens in the Firefox platform when we click on one of these links.

This isn’t really a tutorial – I’m not going to go out of my way to explain much here. Think of this more as a public posting of my notes during my exploration.

So, here goes.

(Note that the code in this post was current as of revision 400a31da59a9 of mozilla-central, so if you’re reading this in the future, it’s possible that some stuff has greatly changed).

I know for a fact that once the link is clicked, we eventually call mozilla::dom::TabChild::ProvideWindow. I know this because of conversations I’ve had with smaug, billm and jdm in and out of Bugzilla, IRC, and meatspace.

Because I know this, I can hook up gdb to see how I get to that call. I have some notes here on how to hook up gdb to the content process of an e10s window.

Once that’s hooked up, I set a breakpoint on mozilla::dom::TabChild::ProvideWindow, and click on a link somewhere with target=”_blank”.

I hit my breakpoint, and I get a backtrace. Ready for it? Here we go:

#0  mozilla::dom::TabChild::ProvideWindow (this=0x109afb400, aParent=0x10b098820, aChromeFlags=4094, aCalledFromJS=false, aPositionSpecified=false, aSizeSpecified=false, aURI=0xffe, aName=@0x0, aFeatures=@0x0, aWindowIsNew=0x10b098820, aReturn=0x7fff5fbfb648) at TabChild.cpp:1201
#1  0x00000001018682e4 in nsWindowWatcher::OpenWindowInternal (this=0x10b05b540, aParent=0x10b098820, aUrl=<value temporarily unavailable, due to optimizations>, aName=<value temporarily unavailable, due to optimizations>, aFeatures=<value temporarily unavailable, due to optimizations>, aCalledFromJS=false, aDialog=<value temporarily unavailable, due to optimizations>, aNavigate=<value temporarily unavailable, due to optimizations>, _retval=<value temporarily unavailable, due to optimizations>) at nsWindowWatcher.cpp:601
#2  0x0000000101869544 in non-virtual thunk to nsWindowWatcher::OpenWindow2(nsIDOMWindow*, char const*, char const*, char const*, bool, bool, bool, nsISupports*, nsIDOMWindow**) () at nsWindowWatcher.cpp:417
#3  0x0000000100e5dc63 in nsGlobalWindow::OpenInternal (this=0x10b098800, aUrl=@0x7fff5fbfbf90, aName=@0x7fff5fbfc038, aOptions=@0x103d77320, aDialog=false, aContentModal=false, aCalleePrincipal=<value temporarily unavailable, due to optimizations>, aJSCallerContext=<value temporarily unavailable, due to optimizations>, aReturn=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/dom/base/nsGlobalWindow.cpp:11498
#4  0x0000000100e5e3a4 in non-virtual thunk to nsGlobalWindow::OpenNoNavigate(nsAString_internal const&, nsAString_internal const&, nsAString_internal const&, nsIDOMWindow**) () at /Users/mikeconley/Projects/mozilla-central/dom/base/nsGlobalWindow.cpp:7463
#5  0x000000010184d99d in nsDocShell::InternalLoad (this=<value temporarily unavailable, due to optimizations>, aURI=0x113eed200, aReferrer=0x1134c0fe0, aOwner=0x114a69070, aFlags=0, aWindowTarget=0x10b098820, aLoadType=<value temporarily unavailable, due to optimizations>, aSHEntry=<value temporarily unavailable, due to optimizations>, aSourceDocShell=<value temporarily unavailable, due to optimizations>, aDocShell=<value temporarily unavailable, due to optimizations>, aRequest=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/docshell/base/nsDocShell.cpp:9079
#6  0x0000000101855758 in nsDocShell::OnLinkClickSync (this=0x10b075000, aContent=0x112865eb0, aURI=0x113eed3c0, aTargetSpec=<value temporarily unavailable, due to optimizations>, aFileName=@0x106f27f10, aPostDataStream=0x0, aDocShell=<value temporarily unavailable, due to optimizations>, aRequest=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/docshell/base/nsDocShell.cpp:12699
#7  0x0000000101857f85 in mozilla::Maybe<mozilla::AutoCxPusher>::~Maybe () at /Users/mikeconley/Projects/mozilla-central/obj-x86_64-apple-darwin12.5.0/dist/include/nsCxPusher.h:12499
#8  0x0000000101857f85 in nsCxPusher::~nsCxPusher () at /Users/mikeconley/Projects/mozilla-central/docshell/base/nsDocShell.cpp:41
#9  0x0000000101857f85 in nsCxPusher::~nsCxPusher () at /Users/mikeconley/Projects/mozilla-central/obj-x86_64-apple-darwin12.5.0/dist/include/nsCxPusher.h:66
#10 0x0000000101857f85 in OnLinkClickEvent::Run (this=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/docshell/base/nsDocShell.cpp:12502
#11 0x0000000100084f60 in nsThread::ProcessNextEvent (this=0x106f245e0, mayWait=false, result=0x7fff5fbfc947) at nsThread.cpp:715
#12 0x0000000100023241 in NS_ProcessPendingEvents (thread=<value temporarily unavailable, due to optimizations>, timeout=20) at nsThreadUtils.cpp:210
#13 0x0000000100d41c47 in nsBaseAppShell::NativeEventCallback (this=0x1096e8660) at nsBaseAppShell.cpp:98
#14 0x0000000100cfdba1 in nsAppShell::ProcessGeckoEvents (aInfo=0x1096e8660) at nsAppShell.mm:388
#15 0x00007fff86adeb31 in __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ ()
#16 0x00007fff86ade455 in __CFRunLoopDoSources0 ()
#17 0x00007fff86b017f5 in __CFRunLoopRun ()
#18 0x00007fff86b010e2 in CFRunLoopRunSpecific ()
#19 0x00007fff8ad65eb4 in RunCurrentEventLoopInMode ()
#20 0x00007fff8ad65c52 in ReceiveNextEventCommon ()
#21 0x00007fff8ad65ae3 in BlockUntilNextEventMatchingListInMode ()
#22 0x00007fff8cce1533 in _DPSNextEvent ()
#23 0x00007fff8cce0df2 in -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] ()
#24 0x0000000100cfd266 in -[GeckoNSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] (self=0x106f801a0, _cmd=<value temporarily unavailable, due to optimizations>, mask=18446744073709551615, expiration=0x422d63c37f00000d, mode=0x7fff7205e1c0, flag=1 '\001') at nsAppShell.mm:165
#25 0x00007fff8ccd81a3 in -[NSApplication run] ()
#26 0x0000000100cfe32b in nsAppShell::Run (this=<value temporarily unavailable, due to optimizations>) at nsAppShell.mm:746
#27 0x000000010199b3dc in XRE_RunAppShell () at /Users/mikeconley/Projects/mozilla-central/toolkit/xre/nsEmbedFunctions.cpp:679
#28 0x00000001002a0dae in MessageLoop::AutoRunState::~AutoRunState () at message_loop.cc:229
#29 0x00000001002a0dae in MessageLoop::AutoRunState::~AutoRunState () at /Users/mikeconley/Projects/mozilla-central/ipc/chromium/src/base/message_loop.h:197
#30 0x00000001002a0dae in MessageLoop::Run (this=0x0) at message_loop.cc:503
#31 0x000000010199b0cd in XRE_InitChildProcess (aArgc=<value temporarily unavailable, due to optimizations>, aArgv=<value temporarily unavailable, due to optimizations>, aProcess=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/toolkit/xre/nsEmbedFunctions.cpp:516
#32 0x0000000100000f1d in main (argc=<value temporarily unavailable, due to optimizations>, argv=0x7fff5fbff4d8) at /Users/mikeconley/Projects/mozilla-central/ipc/app/MozillaRuntimeMain.cpp:149

Oh my. Well, the good news is, we can chop off a good chunk of the lower half because that’s all message / event loop stuff. That’s going to be in every single backtrace ever, pretty much, so I can just ignore it. Here’s the more important stuff:

#0  mozilla::dom::TabChild::ProvideWindow (this=0x109afb400, aParent=0x10b098820, aChromeFlags=4094, aCalledFromJS=false, aPositionSpecified=false, aSizeSpecified=false, aURI=0xffe, aName=@0x0, aFeatures=@0x0, aWindowIsNew=0x10b098820, aReturn=0x7fff5fbfb648) at TabChild.cpp:1201
#1  0x00000001018682e4 in nsWindowWatcher::OpenWindowInternal (this=0x10b05b540, aParent=0x10b098820, aUrl=<value temporarily unavailable, due to optimizations>, aName=<value temporarily unavailable, due to optimizations>, aFeatures=<value temporarily unavailable, due to optimizations>, aCalledFromJS=false, aDialog=<value temporarily unavailable, due to optimizations>, aNavigate=<value temporarily unavailable, due to optimizations>, _retval=<value temporarily unavailable, due to optimizations>) at nsWindowWatcher.cpp:601
#2  0x0000000101869544 in non-virtual thunk to nsWindowWatcher::OpenWindow2(nsIDOMWindow*, char const*, char const*, char const*, bool, bool, bool, nsISupports*, nsIDOMWindow**) () at nsWindowWatcher.cpp:417
#3  0x0000000100e5dc63 in nsGlobalWindow::OpenInternal (this=0x10b098800, aUrl=@0x7fff5fbfbf90, aName=@0x7fff5fbfc038, aOptions=@0x103d77320, aDialog=false, aContentModal=false, aCalleePrincipal=<value temporarily unavailable, due to optimizations>, aJSCallerContext=<value temporarily unavailable, due to optimizations>, aReturn=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/dom/base/nsGlobalWindow.cpp:11498
#4  0x0000000100e5e3a4 in non-virtual thunk to nsGlobalWindow::OpenNoNavigate(nsAString_internal const&, nsAString_internal const&, nsAString_internal const&, nsIDOMWindow**) () at /Users/mikeconley/Projects/mozilla-central/dom/base/nsGlobalWindow.cpp:7463
#5  0x000000010184d99d in nsDocShell::InternalLoad (this=<value temporarily unavailable, due to optimizations>, aURI=0x113eed200, aReferrer=0x1134c0fe0, aOwner=0x114a69070, aFlags=0, aWindowTarget=0x10b098820, aLoadType=<value temporarily unavailable, due to optimizations>, aSHEntry=<value temporarily unavailable, due to optimizations>, aSourceDocShell=<value temporarily unavailable, due to optimizations>, aDocShell=<value temporarily unavailable, due to optimizations>, aRequest=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/docshell/base/nsDocShell.cpp:9079
#6  0x0000000101855758 in nsDocShell::OnLinkClickSync (this=0x10b075000, aContent=0x112865eb0, aURI=0x113eed3c0, aTargetSpec=<value temporarily unavailable, due to optimizations>, aFileName=@0x106f27f10, aPostDataStream=0x0, aDocShell=<value temporarily unavailable, due to optimizations>, aRequest=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/docshell/base/nsDocShell.cpp:12699
#7  0x0000000101857f85 in mozilla::Maybe<mozilla::AutoCxPusher>::~Maybe () at /Users/mikeconley/Projects/mozilla-central/obj-x86_64-apple-darwin12.5.0/dist/include/nsCxPusher.h:12499
#8  0x0000000101857f85 in nsCxPusher::~nsCxPusher () at /Users/mikeconley/Projects/mozilla-central/docshell/base/nsDocShell.cpp:41
#9  0x0000000101857f85 in nsCxPusher::~nsCxPusher () at /Users/mikeconley/Projects/mozilla-central/obj-x86_64-apple-darwin12.5.0/dist/include/nsCxPusher.h:66
#10 0x0000000101857f85 in OnLinkClickEvent::Run (this=<value temporarily unavailable, due to optimizations>) at /Users/mikeconley/Projects/mozilla-central/docshell/base/nsDocShell.cpp:12502

That’s a bit more manageable.

So we start inside something called a docshell. I’ve heard that term bandied about a lot, and I can’t say I’ve ever been too sure what it means, or what a docshell does, or why I should care.

I found some documents that make things a little bit clearer.

Basically, my understanding is that a docshell is the thing that connects incoming stuff from some URI (this could be web content, or it might be a XUL document that’s loading the browser UI…), and connects it to the things that make stuff show up on your screen.

So, pretty important.

It seems to be a place where some utility methods and functions go as well, so it’s kind of this abstract thing that seems to have multiple purposes.

But the most important thing for the purposes of this post is this: every time you load a document, you have a docshell taking care of it. All of these docshells are structured in a tree which is rooted with a docshell owner. This will come into play later.

So one thing that a docshell does, is that it notices when a link was clicked inside of its content. That’s nsDocShell.cpp’d OnLinkClickEvent::Run, and that eventually makes its way over to nsDocShell::OnLinkClickSync.

After some initial checks and balances to ensure that this thing really is a link we want to travel to, we get sent off to nsDocShell::InternalLoad.

Inside there, there’s some more checking… there’s a policy check to make sure we’re allowed to open a link. Lots of security going on. Eventually I see this:

if (aWindowTarget && *aWindowTarget)

That’s good. aWindowTarget maps to the target=”_blank” attribute in the anchor. So we’ll be entering this block.

    if (aWindowTarget && *aWindowTarget) {
        // Locate the target DocShell.
        nsCOMPtr<nsIDocShellTreeItem> targetItem;
        rv = FindItemWithName(aWindowTarget, nullptr, this,
                              getter_AddRefs(targetItem));

So now we’re looking for the right docshell to load this new document in. That makes sense – if you have a link where target=”foo”, subsequent links from the same origin targeted at “foo” will open in the same window or tab or what have you. So we’re checking to see if we’ve opened something with the name inside aWindowTarget already.

So now we’re in nsDocShell::FindItemWithName, and I see this:

        else if (name.LowerCaseEqualsLiteral("_blank"))
        {
            // Just return null.  Caller must handle creating a new window with
            // a blank name himself.
            return NS_OK;
        }

Ah hah, so target=”_blank”, as we already knew, is special-cased – and this is where it happens. There’s no existing docshell for _blank because we know we’re going to be opening a new window (or tab if the user has preffed it that way). So we don’t return a pre-existing docshell.

So we’re back in nsDocShell::InternalLoad.

        rv = FindItemWithName(aWindowTarget, nullptr, this,
                              getter_AddRefs(targetItem));
        NS_ENSURE_SUCCESS(rv, rv);

        targetDocShell = do_QueryInterface(targetItem);
        // If the targetDocShell doesn't exist, then this is a new docShell
        // and we should consider this a TYPE_DOCUMENT load
        isNewDocShell = !targetDocShell;

Ok, so now targetItem is nullptr, targetDocShell is also nullptr, and so isNewDocShell is true.

There seems to be more policy checking going on in InternalLoad after this… but eventually, I see this:

   if (aWindowTarget && *aWindowTarget) {
        // We've already done our owner-inheriting.  Mask out that bit, so we
        // don't try inheriting an owner from the target window if we came up
        // with a null owner above.
        aFlags = aFlags & ~INTERNAL_LOAD_FLAGS_INHERIT_OWNER;
        
        bool isNewWindow = false;
        if (!targetDocShell) {
            // If the docshell's document is sandboxed, only open a new window
            // if the document's SANDBOXED_AUXILLARY_NAVIGATION flag is not set.
            // (i.e. if allow-popups is specified)
            NS_ENSURE_TRUE(mContentViewer, NS_ERROR_FAILURE);
            nsIDocument* doc = mContentViewer->GetDocument();
            uint32_t sandboxFlags = 0;

            if (doc) {
                sandboxFlags = doc->GetSandboxFlags();
                if (sandboxFlags & SANDBOXED_AUXILIARY_NAVIGATION) {
                    return NS_ERROR_DOM_INVALID_ACCESS_ERR;
                }
            }

            nsCOMPtr<nsPIDOMWindow> win =
                do_GetInterface(GetAsSupports(this));
            NS_ENSURE_TRUE(win, NS_ERROR_NOT_AVAILABLE);

            nsDependentString name(aWindowTarget);
            nsCOMPtr<nsIDOMWindow> newWin;
            nsAutoCString spec;
            if (aURI)
                aURI->GetSpec(spec);
            rv = win->OpenNoNavigate(NS_ConvertUTF8toUTF16(spec),
                                     name,          // window name
                                     EmptyString(), // Features
                                     getter_AddRefs(newWin));

So we check again to see if we’re targeted at something, and check if we’ve found a target docshell for it. We hadn’t, so we do some security checks, and then … what the hell is nsPIDOMWindow? I’m used to things being called nsIBlahBlah, but now nsPIBlahBlah… what does the P mean?

It took some asking around, but I eventually found out that the P is supposed to be for Private – as in, this is a private XPIDL interface, and non-core embedders should stay away from it.

Ok, and we also see do_GetInterface. This is not the same as QueryInterface, believe it or not. The difference is subtle, but basically it’s this: QueryInterface says “you implement X, but I think you also implement Y. If you do, please return a pointer to yourself that makes you seem like a Y.” GetInterface is different – GetInterface says “I know you know about something that implements Y. It might be you, or more likely, it’s something you’re holding a reference to. Can I get a reference to that please?”. And if successful, it returns it. Here’s more documentation about GetInterface.

It’s a subtle but important difference.

So this docshell knows about a window, and we’ve now got a handle on that window using the private interface nsPIDOMWindow. Neat.

So eventually, we call OpenNoNavigate on that nsPIDOMWindow. That method is pretty much like nsIDOMWindow::Open, except that OpenNoNavigate doesn’t send the window anywhere – it just returns it so that the caller can send it to a URI.

Through the magic of do_GetInterface, nsDocShell::GetInterface, EnsureScriptEnvironment, and NS_NewScriptGlobalObject, I know that the nsPIDOMWindow is being implemented by nsGlobalWindow, and that’s where I should go to to find the OpenNoNavigate implementation.

So off we go!

nsGlobalWindow::OpenNoNavigate just seems to forward the call, after some argument setting, to nsGlobalWindow::OpenInternal, like this:

  return OpenInternal(aUrl, aName, aOptions,
                      false,          // aDialog
                      false,          // aContentModal
                      true,           // aCalledNoScript
                      false,          // aDoJSFixups
                      false,          // aNavigate
                      nullptr, nullptr,  // No args
                      GetPrincipal(),    // aCalleePrincipal
                      nullptr,           // aJSCallerContext
                      _retval);

Having a glance around at the rest of the nsGlobalWindow::Open[foo] methods, it looks like they all call into OpenInternal. It’s the big-mamma opening method.

This method does a few things, including making sure that we’re not being abused by web content that’s trying to spam the user with popups.

Eventually, we get to this:

      rv = pwwatch->OpenWindow2(this, url.get(), name_ptr, options_ptr,
                                /* aCalledFromScript = */ false,
                                aDialog, aNavigate, aExtraArgument,
                                getter_AddRefs(domReturn));

and return the domReturn pointer back after a few more checks to our caller. Remember that the caller is going to take this new window, and navigate it to some URI.

Ok, so, pwwatch. What is that? Well, that appears to be a private interface to nsWindowWatcher, which gives us access to the OpenWindow2 method.

After prepping some arguments, much like nsGlobalWindow::OpenNoNavigate did, we forward the call over to nsWindowWatcher::OpenWindowInternal.

And now we’re almost done – we’re almost at the point where we’re actually going to open a window!

Some key things need to happen though. First, we do this:

nsCOMPtr<nsIDocShellTreeOwner>  parentTreeOwner;  // from the parent window, if any
...
GetWindowTreeOwner(aParent, getter_AddRefs(parentTreeOwner));

So what that does is it tries to get the docshell owner of the docshell that’s attempting to open the window (and that’d be the docshell that we clicked the link in).

After a few more things, we check to see if there’s an existing window with that target name which we can re-use:

  // try to find an extant window with the given name
  nsCOMPtr<nsIDOMWindow> foundWindow = SafeGetWindowByName(name, aParent);
  GetWindowTreeItem(foundWindow, getter_AddRefs(newDocShellItem));

And if so, we set it to newDocShellItem.

After some more security stuff, we check to see if newDocShellItem exists. Because name is nullptr (since we had target=”_blank”, and nsDocShell::FindItemWithName returned nullptr), newDocShellItem is null.

Because it doesn’t exist, we know we’re opening a brand new window!

More security things seem to happen, and then we get to the part that I’m starting to focus on:

      nsCOMPtr<nsIWindowProvider> provider = do_GetInterface(parentTreeOwner);
      if (provider) {
        NS_ASSERTION(aParent, "We've _got_ to have a parent here!");

        nsCOMPtr<nsIDOMWindow> newWindow;
        rv = provider->ProvideWindow(aParent, chromeFlags, aCalledFromJS,
                                     sizeSpec.PositionSpecified(),
                                     sizeSpec.SizeSpecified(),
                                     uriToLoad, name, features, &windowIsNew,
                                     getter_AddRefs(newWindow));

We ask the parentTreeOwner to get us something that it knows about that implements nsIWindowProvider. In the Electrolysis / content process case, that’d be TabChild. In the normal, non-Electrolysis case, that’s nsContentTreeOwner.

The nsIWindowProvider is the thing that we’ll use to get a new window from! So we call ProvideWindow on it, to give us a pointer to new nsIDOMWindow window, assigned to newWindow.

Here’s TabChild::ProvideWindow:

NS_IMETHODIMP
TabChild::ProvideWindow(nsIDOMWindow* aParent, uint32_t aChromeFlags,
                        bool aCalledFromJS,
                        bool aPositionSpecified, bool aSizeSpecified,
                        nsIURI* aURI, const nsAString& aName,
                        const nsACString& aFeatures, bool* aWindowIsNew,
                        nsIDOMWindow** aReturn)
{
    *aReturn = nullptr;

    // If aParent is inside an <iframe mozbrowser> or <iframe mozapp> and this
    // isn't a request to open a modal-type window, we're going to create a new
    // <iframe mozbrowser/mozapp> and return its window here.
    nsCOMPtr<nsIDocShell> docshell = do_GetInterface(aParent);
    if (docshell && docshell->GetIsInBrowserOrApp() &&
        !(aChromeFlags & (nsIWebBrowserChrome::CHROME_MODAL |
                          nsIWebBrowserChrome::CHROME_OPENAS_DIALOG |
                          nsIWebBrowserChrome::CHROME_OPENAS_CHROME))) {

      // Note that BrowserFrameProvideWindow may return NS_ERROR_ABORT if the
      // open window call was canceled.  It's important that we pass this error
      // code back to our caller.
      return BrowserFrameProvideWindow(aParent, aURI, aName, aFeatures,
                                       aWindowIsNew, aReturn);
    }

    // Otherwise, create a new top-level window.
    PBrowserChild* newChild;
    if (!CallCreateWindow(&newChild)) {
        return NS_ERROR_NOT_AVAILABLE;
    }

    *aWindowIsNew = true;
    nsCOMPtr<nsIDOMWindow> win =
        do_GetInterface(static_cast<TabChild*>(newChild)->WebNavigation());
    win.forget(aReturn);
    return NS_OK;
}

The docshell->GetIsInBrowserOrApp() is basically asking “are we b2g?”, to which the answer is “no”, so we skip that block, and go right for CallCreateWindow.

CallCreateWindow is using the IPC library to communicate with TabParent in the UI process, which has a corresponding function called AnswerCreateWindow. Here it is:

bool
TabParent::AnswerCreateWindow(PBrowserParent** retval)
{
    if (!mBrowserDOMWindow) {
        return false;
    }

    // Only non-app, non-browser processes may call CreateWindow.
    if (IsBrowserOrApp()) {
        return false;
    }

    // Get a new rendering area from the browserDOMWin.  We don't want
    // to be starting any loads here, so get it with a null URI.
    nsCOMPtr<nsIFrameLoaderOwner> frameLoaderOwner;
    mBrowserDOMWindow->OpenURIInFrame(nullptr, nullptr,
                                      nsIBrowserDOMWindow::OPEN_NEWTAB,
                                      nsIBrowserDOMWindow::OPEN_NEW,
                                      getter_AddRefs(frameLoaderOwner));
    if (!frameLoaderOwner) {
        return false;
    }

    nsRefPtr<nsFrameLoader> frameLoader = frameLoaderOwner->GetFrameLoader();
    if (!frameLoader) {
        return false;
    }

    *retval = frameLoader->GetRemoteBrowser();
    return true;
}

So after some checks, we call mBrowserDOMWindow’s OpenURIInFrame, with (among other things), nsIBrowserDOMWindow::OPEN_NEWTAB. So that’s why we’ve got a new tab opening instead of a new window.

mBrowserDOMWindow is a reference to this thing implemented in browser.js:

function nsBrowserAccess() { }

nsBrowserAccess.prototype = {
  QueryInterface: XPCOMUtils.generateQI([Ci.nsIBrowserDOMWindow, Ci.nsISupports]),

  _openURIInNewTab: function(aURI, aOpener, aIsExternal) {
    let win, needToFocusWin;

    // try the current window.  if we're in a popup, fall back on the most recent browser window
    if (window.toolbar.visible)
      win = window;
    else {
      let isPrivate = PrivateBrowsingUtils.isWindowPrivate(aOpener || window);
      win = RecentWindow.getMostRecentBrowserWindow({private: isPrivate});
      needToFocusWin = true;
    }

    if (!win) {
      // we couldn't find a suitable window, a new one needs to be opened.
      return null;
    }

    if (aIsExternal && (!aURI || aURI.spec == "about:blank")) {
      win.BrowserOpenTab(); // this also focuses the location bar
      win.focus();
      return win.gBrowser.selectedBrowser;
    }

    let loadInBackground = gPrefService.getBoolPref("browser.tabs.loadDivertedInBackground");
    let referrer = aOpener ? makeURI(aOpener.location.href) : null;

    let tab = win.gBrowser.loadOneTab(aURI ? aURI.spec : "about:blank", {
                                      referrerURI: referrer,
                                      fromExternal: aIsExternal,
                                      inBackground: loadInBackground});
    let browser = win.gBrowser.getBrowserForTab(tab);

    if (needToFocusWin || (!loadInBackground && aIsExternal))
      win.focus();

    return browser;
  },

  openURI: function (aURI, aOpener, aWhere, aContext) {
    ... (removed for brevity)
  },

  openURIInFrame: function browser_openURIInFrame(aURI, aOpener, aWhere, aContext) {
    if (aWhere != Ci.nsIBrowserDOMWindow.OPEN_NEWTAB) {
      dump("Error: openURIInFrame can only open in new tabs");
      return null;
    }

    var isExternal = (aContext == Ci.nsIBrowserDOMWindow.OPEN_EXTERNAL);
    let browser = this._openURIInNewTab(aURI, aOpener, isExternal);
    if (browser)
      return browser.QueryInterface(Ci.nsIFrameLoaderOwner);

    return null;
  },

  isTabContentWindow: function (aWindow) {
    return gBrowser.browsers.some(function (browser) browser.contentWindow == aWindow);
  },

  get contentWindow() {
    return gBrowser.contentWindow;
  }
}

So nsBrowserAccess’s openURIInFrame only supports opening things in new tabs, and then it just calls _openURIInNewTab on itself, which does the job of returning the tab’s remote browser after the tab is opened.

I might follow this up with a post about how nsContentTreeOwner opens a window in the non-Electrolysis case, and how we might abstract some of that out for re-use here. We’ll see.

And that’s about it. Hopefully this is useful to future spelunkers.

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