1692375 - Clean up our Objective C exception handling story a little

Status: RESOLVED FIXED

(Core :: Widget: Cocoa, task, P2)

Description

[Markus Stange [:mstange]]

Currently we try to make sure that Objective C exceptions don't propagate into C/C++ code because it would cause all kinds of trouble.
We do this with NS_OBJC_BEGIN/END_TRY_ABORT_BLOCK* macros that are placed in strategic spots. Those macros catch any exception, log its description, and keep going.

There are various problems with this:

• The macros have ABORT in their name but don't actually abort.
• Ignoring exceptions is a rather unsafe default. Much of our code is not written with the expectation of being abandoned in the middle, and this can result in broken invariants and generally bad state.
• We have no insight into which exceptions fire where and how often. The exception names end up in crash reports, but only if something else later on causes a crash.
• The stack at which the exception was thrown is lost.
• The stack at which the exception was caught is also lost.
• It's unclear whether we're catching exceptions in all the right spots.

I propose the following plan:

1. Rename the macros to reflect what they do. [done, bug 1692391]
2. Append exception.callStackReturnAddresses to the crash report notes so that we know where the exception was thrown. We'll need to symbolicate these addresses manually, which is annoying but doable. [done, bug 1692394]
3. Introduce a new macro which actually aborts. [done, bug 1692401]
4. Gradually convert all usages of the old macros to the new aborting macro. This will introduce new crashes that tell us which places need fixing, and it'll get us to a generally safer place. Also, for any expected exceptions, add custom handling. [tracked in bug 1693389]
5. Remove the non-aborting macros.
6. Do another pass over the codebase to make sure that we catch exceptions in the right places. [tracked in bug 1693392]

Comment 1

[Markus Stange [:mstange]]

I'm declaring this bug fixed, and bug 1693389 and bug 1693392 as follow-up work.