Deadlock in non-threaded RTS with hPutBuf

Using the non-threaded RTS, the attached program works fine in 6.8.2, but deadlocks in 6.8.3 and later GHCs (including 6.10.3 and 6.11.20090618).

The program forks (runInteractiveProcess) two copies of cat, then forks off two separate threads (forkIO) to write a large buffer to stdin of each copy, and also lazily reads the outputs of each copy of cat, forcing the results with an equality test.

From strace, it looks like 6.8.2 is correctly selecting on multiple FDs at once (though just 3, rather than all 4, which I don't understand), whereas 6.8.3 doesn't and hence ends up in a deadlock when the pipe it wants to use blocks while some other pipe is actually ready.

Trac metadata

Trac field	Value
Version	6.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Architecture