Author Archives: Mike Conley

Things I’ve Learned This Week (May 4 – May 8, 2015)

How to convert an NSString to a Gecko nsAString

I actually discovered this during my most recent Joy of Coding episode – there is a static utility method to convert between native Cocoa NSStrings and Gecko nsAStrings – nsCocoaUtils::GetStringForNSString. Very handy, and works exactly as advertised.

An “Attach to Process by pid” Keyboard Shortcut for XCode

I actually have colleague Garvan Keeley to thank for this one, and technically I learned this on April 24th. It was only this week that I remembered I had learned it!

When I’m debugging Firefox on OS X, I tend to use XCode, and I usually attach to Firefox after it has started running. I have to navigate some menus in order to bring up the dialog to attach to a process by pid, and I was getting tired of doing that over and over again.

So, as usual, I tweeted my frustration:

AND LO, THE INTERNET SPOKE BACK:

It seems small, but the savings in time for something that I do so frequently quickly adds up. And it always feels good to go faster!

Things I’ve Learned This Week (April 27 – May 1, 2015)

Another short one this week.

You can pass DOM Promises back through XPIDL

XPIDL is what we use to define XPCOM interfaces in Gecko. I think we’re trying to avoid XPCOM where we can, but sometimes you have to work with pre-existing XPCOM interfaces, and, well, you’re just stuck using it unless you want to rewrite what you’re working on.

What I’m working on lately is nsIProfiler, which is the interface to “SPS”, AKA the Gecko Profiler. nsIProfiler allows me to turn profiling on and off with various features, and then retrieve those profiles to send to a file, or to Cleopatra1.

What I’ve been working on recently is Bug 1116188 – [e10s] Stop using sync messages for Gecko profiler, which will probably have me adding new methods to nsIProfiler for async retrieval of profiles.

In the past, doing async stuff through XPCOM / XPIDL has meant using (or defining a new) callback interface which can be passed as an argument to the async method.

I was just about to go down that road, when ehsan (or was it jrmuizel? One of them, anyhow) suggested that I just pass a DOM Promise back.

I find that Promises are excellent. I really like them, and if I could pass a Promise back, that’d be incredible. But I had no idea how to do it.

It turns out that if I can ensure that the async methods are called such that there is a JS context on the stack, I can generate a DOM Promise, and pass it back to the caller as an “nsISupports”. According to ehsan, XPConnect will do the necessary magic so that the caller, upon receiving the return value, doesn’t just get this opaque nsISupports thing, but an actual DOM Promise. This is because, I believe, that DOM Promise is something that is defined via WebIDL. I think. I can’t say I fully understand the mechanics of XPConnect2, but this all sounded wonderful.

I even found an example in our new Service Worker code:

From dom/workers/ServiceWorkerManager.cpp (I’ve edited the method to highlight the Promise stuff):

// If we return an error code here, the ServiceWorkerContainer will
// automatically reject the Promise.
NS_IMETHODIMP
ServiceWorkerManager::Register(nsIDOMWindow* aWindow,
                               nsIURI* aScopeURI,
                               nsIURI* aScriptURI,
                               nsISupports** aPromise)
{
  AssertIsOnMainThread();

  // XXXnsm Don't allow chrome callers for now, we don't support chrome
  // ServiceWorkers.
  MOZ_ASSERT(!nsContentUtils::IsCallerChrome());

  nsCOMPtr<nsPIDOMWindow> window = do_QueryInterface(aWindow);

  // ...

  nsCOMPtr<nsIGlobalObject> sgo = do_QueryInterface(window);
  ErrorResult result;
  nsRefPtr<Promise> promise = Promise::Create(sgo, result);
  if (result.Failed()) {
    return result.StealNSResult();
  }

  // ...

  nsRefPtr<ServiceWorkerResolveWindowPromiseOnUpdateCallback> cb =
    new ServiceWorkerResolveWindowPromiseOnUpdateCallback(window, promise);

  nsRefPtr<ServiceWorkerRegisterJob> job =
    new ServiceWorkerRegisterJob(queue, cleanedScope, spec, cb, documentPrincipal);
  queue->Append(job);

  promise.forget(aPromise);
  return NS_OK;
}

Notice that the outparam aPromise is an nsISupports**, and yet, I do believe the caller will end up handling a DOM Promise. Wicked!

  1. Cleopatra is the web application that can be used to browse a profile retrieved via nsIProfiler ↩

  2. Like being able to read the black speech of Mordor, there are few who can. ↩

The Joy of Coding (Ep. 12): Making “Save Page As” Work

After giving some updates on the last bug we were working on together, I started a new bug: Bug 1128050 – [e10s] Save page as… doesn’t always load from cache. The problem here is that if the user were to reach a page via a POST request, attempting to save that page from the Save Page item in the menu would result in silent failure1.

Luckily, the last bug we were working on was related to this – we had a lot of context about cache keys swapped in already.

The other important thing to realize is that fixing this bug is a bandage fix, or a wallpaper fix. I don’t think those are official terms, but it’s what I use. Basically, we’re fixing a thing with the minimum required effort because something else is going to fix it properly down the line. So we just need to do what we can to get the feature to limp along until such time as the proper fix lands.

My proposed solution was to serialize an nsISHEntry on the content process side, deserialize it on the parent side, and pass it off to nsIWebBrowserPersist.

So did it work? Watch the episode and find out!

I also want to briefly apologize for some construction noise during the video – I think it occurs somewhere halfway through minute 20 of the video. It doesn’t last long, I promise!

Episode Agenda

References

Bug 1128050 – [e10s] Save page as… doesn’t always load from cache – Notes

  1. Well, it’d show something in the Browser Console, but for a typical user, I think that’s still a silent failure. ↩

Things I’ve Learned This Week (April 20 – April 24, 2015)

Short one this week. I must not have learned much! 😀

If you’re using Sublime Text to hack on Firefox or Gecko, make sure it’s not indexing your objdir.

Sublime has this wicked cool feature that lets you quickly search for files within your project folders. On my MBP, the shortcut is Cmd-P. It’s probably something like Ctrl-P on Windows and Linux.

That feature is awesome, because when I need to get to a file, instead of searching the folder hierarchy, I just hit Cmd-P, jam in a few of the characters (they can even be out of order – Sublime does fuzzy matching), and then as soon as my desired file is the top entry, just hit Enter, and BLAM – opened file. It really saves time!

At least, it saves time in theory. I noticed that sometimes, I’d hit Cmd-P, and the UI to enter my search string would take ages to show up. I had no idea why.

Then I noticed that this slowness seemed to show up after I had done a build. My objdir resides beneath my srcdir (as is the defaults with a mozilla-central checkout), so I figured perhaps Sublime was trying to index all of those binaries and choking on them.

I went to Project > Edit Project, and added this to the configuration file that opened:

{
    "folders":
    [
        {
            "path": "/Users/mikeconley/Projects/mozilla-central",
      "folder_exclude_patterns": ["*.sublime-workspace", "obj-*"]
        }
    ]
}

I added the workspace thing too1, because I figure it’s unlikely I’ll ever want to open that thing.

Anyhow, after setting that, I restarted Sublime, and everything was crazy-fast. \o/

If you’re using Sublime, and your objdir is under your srcdir, maybe consider adding the same thing. Even if you’re not using Cmd-P, it’ll probably save your machine from needlessly burning cycles indexing stuff.

  1. That’s where Sublime holds my session state for my project. ↩