[5.5] Inject hop_to_executor after self is fully initialized in unrestricted actor inits
#38490
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I ran into this while implementing async actor inits; where I was emitting an actor hop in the middle of an access region, which caused the access set to be unexpectedly empty.
Because `self` can be used unrestricted after it is fully-initialized in an async actor init, we perform a hop_to_executor(self) immediately after `self` is fully-initialized on all paths within the initializer. If we did not, then code could race with the initializer if it, e.g., spawns a task from the initializer and mutates actor state that the initializer then reads. By hopping to the executor, we prevent that task from running until _after_ the initializer completes.
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DougGregor
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Jul 20, 2021
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Rationale: The typechecker permits unfettered use of
selfthroughout an actor'sasyncinitializer. This patch brings the implementation of such initializers in line with what the typechecker permits, by actually hopping toself's initializer in order to reserve it. This prevents a possible race with the initializer's body.Risk: Low
Risk Detail: This change is focused on correcting the code generated by the compiler.
Reward: High
Reward Details: Fixes a miscompile that would lead to a concurrency bug.
Original PR: #38215
Issue: rdar://78790683
Code Reviewed By: Konrad Malawski
Testing Details: Regression test is included.