This is motivated by three separate but related problems:
1. Our concept of recursion depth is broken for things that run from AfterProcessNextEvent observers (e.g. Promises). We decrement the recursionDepth counter before firing observers, so a Promise callback running at the lowest event loop depth has a recursion depth of 0 (whereas a regular nsIRunnable would be 1). This is a problem because it's impossible to distinguish a Promise running after a sync XHR's onreadystatechange handler from a top-level event (since the former runs with depth 2 - 1 = 1, and the latter runs with just 1).
2. The nsIThreadObserver mechanism that is used by a lot of code to run "after" the current event is a poor fit for anything that runs script. First, the order the observers fire in is the order they were added, not anything fixed by spec. Additionally, running script can cause the event loop to spin, which is a big source of pain here (bholley has some nasty bug caused by this).
3. We run Promises from different points in the code for workers and main thread. The latter runs from XPConnect's nsIThreadObserver callbacks, while the former runs from a hardcoded call to run Promises in the worker event loop. What workers do is particularly problematic because it means we can't get the right recursion depth no matter what we do to nsThread.
The solve this, this patch does the following:
1. Consolidate some handling of microtasks and all handling of stable state from appshell and WorkerPrivate into CycleCollectedJSRuntime.
2. Make the recursionDepth counter only available to CycleCollectedJSRuntime (and its consumers) and remove it from the nsIThreadInternal and nsIThreadObserver APIs.
3. Adjust the recursionDepth counter so that microtasks run with the recursionDepth of the task they are associated with.
4. Introduce the concept of metastable state to replace appshell's RunBeforeNextEvent. Metastable state is reached after every microtask or task is completed. This provides the semantics that bent and I want for IndexedDB, where transactions autocommit at the end of a microtask and do not "spill" from one microtask into a subsequent microtask. This differs from appshell's RunBeforeNextEvent in two ways:
a) It fires between microtasks, which was the motivation for starting this.
b) It no longer ensures that we're at the same event loop depth in the native event queue. bent decided we don't care about this.
5. Reorder stable state to happen after microtasks such as Promises, per HTML. Right now we call the regular thread observers, including appshell, before the main thread observer (XPConnect), so stable state tasks happen before microtasks.
This conversion was done with the script:
find . -name '*.cpp' -o -name '*.h' -o -name '*.mm' -o -name '*.idl' | \
egrep -v 'cairo-win32-refptr.h|RefPtr.h|TestRefPtr.cpp' | \
xargs sed -i -e 's/mozilla::TemporaryRef</already_AddRefed</g' \
-e 's/TemporaryRef</already_AddRefed</g'
Manual fixups were performed in the following instances:
- We handled mfbt/RefPtr.h manually so as to not convert TemporaryRef itself
into already_AddRefed.
- The following files had explicit Move() calls added to make up for the lack
of a copy constructor on already_AddRefed:
dom/base/ImageEncoder.cpp
dom/media/MediaTaskQueue.{h,cpp}
dom/media/webaudio/PannerNode.cpp
- A redundant overload for MediaTaskQueue::Dispatch was deleted.
- A few manual fixups were required in mfbt/tests/TestRefPtr.cpp.
- Comments, using declarations, and forward declarations relating to
TemporaryRef in dom/canvas/ and gfx/layers/ were changed to refer to
already_AddRefed.
This patch was generated with the following command:
find . -name "*.h" -o -name "*.cpp" | xargs perl -pi -e 's/return ([a-zA-Z0-9]+)\.ErrorCode\(\);/return \1.StealNSResult();/'
This patch also updates the method names on PendingAnimationTracker but leaves
a number of local variables which will be fixed in a subsequent patch.
--HG--
rename : dom/animation/PendingPlayerTracker.cpp => dom/animation/PendingAnimationTracker.cpp
rename : dom/animation/PendingPlayerTracker.h => dom/animation/PendingAnimationTracker.h
Ryan VanderMeulen
Brad Lassey
Kyle Huey
Kearwood (Kip) Gilbert
Nathan Froyd
Xidorn Quan
Kartikaya Gupta
Cameron McCormack
Carsten "Tomcat" Book
Lee Salzman
Jan Varga
Andreas Pehrson
Karl Tomlinson
Mason Chang
David Anderson
Timothy Nikkel
Andrea Marchesini
Phil Ringnalda
Chris Pearce
Andrew McCreight
Matt Woodrow
Boris Zbarsky
Ehsan Akhgari
Brian Birtles
Denis Volk
Botond Ballo