RFR: 8301219: JavaFX crash when closing with the escape key

[Kevin Rushforth]

On Thu, 14 Dec 2023 00:58:56 GMT, Martin Fox <mfox at openjdk.org> wrote:

> While processing a key down event the Glass GTK code sends out PRESSED and TYPED KeyEvents back to back. If the stage is closed during the PRESSED event the code will end up referencing freed memory while sending out the TYPED event. This can lead to intermittent crashes.
> 
> In GlassApplication.cpp the EventCounterHelper object ensures the WindowContext isn't deleted while processing an event. Currently the helper object is being created *after* IME handling instead of before. If the IME is enabled it's possible for the WindowContext to be deleted in the middle of executing a number of keyboard-related events.
> 
> The fix is simple; instantiate the EventCounterHelper object earlier. There isn't always a WindowContext so I tweaked the EventCounterHelper to do nothing if the context is null.
> 
> To make the crash more reproducible I altered the WindowContext such that when it's deleted the freed memory is filled with 0xCC. This made the crash more reproducible and allowed me to test the fix. I did the same with GlassView since that's the only other Glass GTK class that's instantiated with `new` and discarded with `delete`.

The fix looks good. I tested it by reverting your fix and leaving in the debug memory fill on delete. The system test added by this PR crashes without the fix and passes with the fix. So do the tests associated with the fixed bugs.

I also ran a headful test run on our lab systems. All good.

As for the debug fill-on-delete code, it seems useful to have it available, but not enabled in production. I recommend to ifdef it out on a flag, either DEBUG or some unique flag defined in `DeletedMemDebug.h`, but off by default.

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PR Review: https://git.openjdk.org/jfx/pull/1307#pullrequestreview-1784710942