Creates a new root singleton: OWSMessagePipelineSupervisor
As of right now, this singleton has two responsibilities:
- Track all message processing pipeline stages that have registered
themselves
- Post processing suspension updates to interested stages
Four classes will now register themselves as pipeline stages:
- OWSMessageContentQueue
- YAPDBMessageDecryptQueue
- IncomingGroupsV2MessageQueue
- SSKMessageDecryptJobQueue
At initialization, OWSMessagePipelineSupervisor will take out a pipeline
suspension while it waits for a UUIDBackfillTask to complete. Other
interested objects are also able to suspend the message processing
pipeline by invoking suspendMessageProcessing(for:) on the supervisor
(though, currently this is not used by anything except tests)
This also adds some supporting improvements to UnfairLock. Now,
UnfairLock closures will return the value returned from its critical
section closure.
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