Lately at Collabora I have been working on helping Mozilla with the GTK+ 3 port of Firefox.

The problem

The issue we had to solve is that GTK+ 2 and GTK+ 3 cannot be loaded in the same address space. Moving Firefox from GTK+ 2 to GTK+ 3 isn’t a problem, as only GTK+ 3 gets loaded in its address space, and everything is fine. The problem comes when you load a plugin that links to GTK+ 2, e.g. Flash. Then, GTK+ 2 and GTK+ 3 get both loaded, GTK+ detects that, and aborts to avoid bigger problems. This was tracked as bug #624422.

More specifically, Firefox links to libxul.so, which in turn links to GTK+. These days, the plugins are loaded in a separate process, plugin-container, which communicates with the Firefox process through IPC. If plugin-container didn’t link to GTK+, there would be absolutely no problem, as the browser (Firefox) process could link to GTK+ 3 and plugin-container could load any plugin, including GTK+ 2 ones. However, although plugin-container doesn’t directly use GTK+, it links to libxul.so for IPC, which brings GTK+ into its address space.

The solution

In order to solve this, we evaluated various options. The first one was to split libxul.so in two parts, one with the IPC code and lower level stuff, which wouldn’t link to GTK+, and another side with the rest of the code, including all the widget and toolkit integration, which would obviously link to GTK+. However this turned not to be possible as the libxul code was too intricate.

In the end, we decided to add a thin layer between libxul and GTK+, which we called libmozgtk.so. This small layer links to GTK+ 3, and provides stubs for GTK+ 2 specific symbols. Additionally, there is a libmozgtk2.so with SONAME “libmozgtk.so”, which links to GTK+ 2 and provides stubs for GTK+ 3 symbols. We made libxul link against libmozgtk.so, and so when Firefox runs, libxul.so, libmozgtk.so, and GTK+ 3 are loaded, and Firefox uses GTK+ 3. However when plugin-container is executed, we add LD_PRELOAD=libmozgtk2.so in the environment. Since libmozgtk2.so has a libmozgtk.so SONAME, the libxul.so dependency is satisfied, and the plugin-container process ends with GTK+ 2. Since plugin-container doesn’t make use of the GTK+ code in libxul, this is safe, and we end up with a GTK+ 3 Firefox that can load GTK+ 2 plugins. The end result is that you can watch Youtube videos again!

While this solution is somewhat hacky, it means we didn’t need to mess with libxul, splitting it in two just for the Linux/GTK+ port’s sake. And when the GTK+ 2 plugins become irrelevant, or NPAPI support is removed (as it recently happened in Chrome), we should be able to easily revert this and use GTK+ 3 everywhere.

Wayland

On an unrelated note, we have looked a bit at porting Firefox to Wayland. Wayland is designed to be a replacement for X11, and is becoming very popular in the digital TV and set top box space. Those obviously need HTML engines and web browsers, and with WebKit and Chrome already having Wayland ports, we think Firefox shouldn’t fall behind.

For this, the GTK+ 3 port was a prerequisite, but that isn’t enough. There are many X11 uses on the Firefox codebase, most of which are guarded by #ifdef MOZ_X11, though not all of them are. We got Firefox to start on Weston (the Wayland reference compositor) with a bunch of hacks, one of which broke keyboard input (but avoided a segfault). As you can see from the screenshot, things aren’t perfect, but it’s at least a good start!