905698 - Displaying any tab-modal prompt causes metroFX to hang at 100% cpu

Status: RESOLVED FIXED

(Firefox for Metro Graveyard :: Browser, defect)

Description

[Tim Abraldes [:TimAbraldes] [:tabraldes]]

This became an issue in the last couple days. Not yet sure what the cause is but this needs to be fixed ASAP

Comment 1

[Tim Abraldes [:TimAbraldes] [:tabraldes]]

OK I just tested with older versions of metroFx, including versions prior to tab-modal prompts landing, and reproduced this issue with all of them.

Either something has gone wonky with my hardware, or I'm seeing bug 840855 with increased frequency.

I did upgrade to Windows 8.1 recently; it's possible that that upgrade is related to this issue.

Comment 2

[Tim Abraldes [:TimAbraldes] [:tabraldes]]

Further investigation shows that, while the cpu is spinning, we're spamming a Windows OriginateError. If we inspect the arguments to the RoOriginateError call, we see:

combase!RoOriginateErrorW(HRESULT error = 0x8000ffff,
                          unsigned int cchMax = 0x34,
                          wchar_t * message = 0x088b0916 "Nested calls to ProcessEvents method is not allowed.")

This is bad news: We've implemented our native event processing in terms of calls to ProcessEvents [1]. Any call to MetroAppShell::ProcessNextNativeEvent is going to encounter this exception, and any consumer that tries to spin an event loop by calling MetroAppShell::ProcessNextNativeEvent in a loop is going to spin forever without actually processing any events. This is exactly what is happening with tab-modal prompts in metro; the prompt service is attempting to spin an event loop at [2], and instead of actually processing events it is just throwing OriginateErrors.

[1] https://mxr.mozilla.org/mozilla-central/source/widget/windows/winrt/MetroAppShell.cpp?rev=74f983165c02#111
[2] https://mxr.mozilla.org/mozilla-central/source/toolkit/components/prompts/src/nsPrompter.js?rev=72bc1aebb5d0#434

Comment 3

[Tim Abraldes [:TimAbraldes] [:tabraldes]]

This bug should be fixed by fixing bug 907410. If there's any cleanup to do after bug 907410 lands I'll pick this bug back up.