Since we know that corrupt startup caches can and do cause startup crashes, we
should ideally purge them any time we detect that the last startup resulted in
a crash. The alternative is waiting until the users restart enough times to
trigger safe mode, and hope that they start in safe mode and then immediately
restart in normal mode. This is not a great user experience.
Unfortunately, the normal startup crash detection mechanism relies on
the preference service, which starts up far too late for us to make this
decision effectively.
To work around that, this patch adds a new startup crash detection mechanism
that creates an incomplete startup canary file which we remove at the end of
the startup crash detection window, or on any sort of graceful exit.
MozReview-Commit-ID: ALaLsyF4meg
359716f115