Making actor non reentrant

[vns]

Huh, had to dig the thread a bit, but:

Cooperative pool deadlock when calling into an opaque subsystem

No need; I filed the feedback when I read the post a few days ago. This use pattern with semaphores and GCD is a long established anti-pattern. The Clang static analyzer has been detecting and diagnosing this pattern since Xcode 10, because this pattern has always caused performance problems. Swift concurrency aside, I think occurrences of this should always be considered bugs. The suggested workaround in this discussion thread to invoke the API from an overcommit queue is the right way to …

(link deserves it's own extraction, I think: Xcode 10 Release Notes: Static Analyzer)