[Concurrency runtime] Don't read from the actor after transitioning state
Once we have transitioned the actor into a new state, we report the
state change as a trace event so it can be noted by tools (e.g.,
Instruments). However, the act of transitioning to a new state can mean
that there is an opportunity for another thread to deallocate the
actor. This means that the tracing call cannot depend on dereferencing
the actor pointer.
A refactoring a few months ago to move the bit that indicates when a
distributed actor is remote from inside the atomic actor state out to a
separate field (because it's constant for a given actor instance),
which introduced a dereference of the actor instance in forming the
tracing call. This introduced a narrow window in which a race
condition could occur: the actor transitions to an idle state, and is
then deallocate before the trace event for the actor transition occurs,
leading to a use-after-free.
Fetch this bit of information earlier in the process, before any state
changes and when we know the actor is still allocated, and pass it
through to the tracing code.
Fixes rdar://108497870.
(cherry picked from commit b14e47fbe6af2fbc51e6cda7d0cf088555d63276)
swift
a57f3161 - [Concurrency runtime] Don't read from the actor after transitioning state
