The SpecialPowers set*Pref/get*Pref APIs currently use synchronous messaging
to set and get preference values from the parent process. Aside from directly
affecting callers of those APIs, it also affects callers of `pushPrefEnv`,
which is meant to be asynchronous, but is in practice usually synchronous due
to the synchronous messaging it uses.
This patch updates the getPref APIs to use the in-process preference service
(which most callers are expecting anyway), and also updates the callers of
the setPref and pushPrefEnv APIs to await the result if they're relying on it
taking effect immediately.
Unfortunately, there are some corner cases in tests that appear to only work
because of the quirks of the current sync messaging approach. The synchronous
setPref APIs, for instance, trigger preference changes in the parent
instantly, but don't update the values in the child until we've returned to
the event loop and had a chance to process the notifications from the parent.
The differnce in timing leads some tests to fail in strange ways, which this
patch works around by just adding timeouts.
There should be follow-ups for test owners to fix the flakiness.
Differential Revision: https://phabricator.services.mozilla.com/D35054
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extra : rebase_source : 941298157e7c82f420cf50ce057154ce9b85301c
extra : source : 189dc8a359815e059a4a217f788d183260bb2bfe
