The Joy of Coding (Ep. 10): The Mystery of the Cache Key

In this episode, I kept my camera off, since I was having some audio-sync issues1.

I was also under some time-pressure, because I had a meeting scheduled for 2:30 ET2, giving me exactly 1.5 hours to do what I needed to do.

And what did I need to do?

I needed to figure out why an nsISHEntry, when passed to nsIWebPageDescriptor’s loadPage, was not enough to get the document out from the HTTP cache in some cases. 1.5 hours to figure it out – the pressure was on!

I don’t recall writing a single line of code. Instead, I spent most of my time inside XCode, walking through various scenarios in the debugger, trying to figure out what was going on. And I eventually figured it out! Read this footnote for the TL;DR:3

Episode Agenda


Bug 1025146 – [e10s] Never load the source off of the network when viewing sourceNotes

  1. I should have those resolved for Episode 11! 

  2. And when the stream finished, I found out the meeting had been postponed to next week, meaning that next week will also be a short episode. 🙁 

  3. Basically, the nsIChannel used to retrieve data over the network is implemented by HttpChannelChild in the content process. HttpChannelChild is really just a proxy to a proper nsIChannel on the parent-side. On the child side, HttpChannelChild does not implement nsICachingChannel, which means we cannot get a cache key from it when creating a session history entry. With no cache key, comes no ability to retrieve the document from the network cache via nsIWebDescriptor’s loadPage.