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.
After this change, we have ShallowSizeOf{In,Ex}cludingThis(), which don't do
anything to measure children. (They can be combined with iteration to measure
children.)
After this change, we have ShallowSizeOf{In,Ex}cludingThis(), which don't do
anything to measure children. (They can be combined with iteration to measure
children.)
And we still have the existing single-arg SizeOf{In,Ex}cluding() functions,
which work if the entry type itself defines SizeOfExcludingThis().
This can cause leaks that are invisible to our XPCOM leak detection system.
To avoid this, classes should not addref or release in their Traverse methods.
After this change, we have PLDHashTable::ShallowSizeOf{In,Ex}cludingThis(),
which don't do anything to measure children. (They can be combined with
iteration to measure children.)
This patch also removes the PL_DHashTableSizeOf{In,Ex}cludingThis() functions.
They're not necessary because the methods can be used instead.
Finally, the patch deliberately converts some SizeOfExcludingThis() calls to
SizeOfIncludingThis(). These are all done on heap pointers so this change is
valid.
This makes it clearer that, unlike how SizeOf*() functions usually work, this
doesn't measure any children hanging off the array.
And do likewise for nsTObserverArray.
StorensRefPtrPassByPtr is currently used for storing reference-counted
types passed as T* template arguments to NS_NewRunnableMethodWith*.
We'd also like to use it to store nsRefPtr<T> template arguments. While
it could be used in its current form, it'd be better for its constructor
to support forwarding, so that something like:
NS_NewRunnableMethodWithArg<nsRefPtr<T>>(..., nsRefPtr<T>(local));
doesn't cause unnecessary reference counting.
