Peculiar RTS crash on OS X

This Harbormaster build failed with an odd RTS crash from the stage0 compiler (which is 8.2.1),

ghc: internal error: RELEASE_LOCK: I do not own this lock: rts/posix/itimer/Pthread.c 187
    (GHC version 8.2.1 for x86_64_apple_darwin)
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

It's not immediately clear to me from the implementation how this could happen, but I thought I should at least note the issue.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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