#13497: GHC does not use select()/poll() correctly on non-Linux platforms -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #8684, #12912 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nh2): Actually before that, I should probably fix the Windows version too (for the case that the FD is not a socket). https://github.com/ghc/ghc/blob/f46369b8a1bf90a3bdc30f2b566c3a7e03672518%5E/... {{{ while (1) // keep trying until we find a real key event { rc = WaitForSingleObject( hFile, msecs ); ... }}} Looking at this, it is not clear to me how this should ever have worked (how this should have provided the semantics that after the maximum given time, `hWaitForInput` returns), as this would simply loop forever if interrupted by a signal. I just wrote the above sentence and then realised that the exact same was true for any non-Linux select, so I got suspicious why it worked at all so far on my FreeBSD VM before my fix, because any EINTR should just return out of `fdReady()` just to have it restarted with the original full duration. [1 hour later] I now know what's going on. It only worked on non-Linux because of the idle GC! Looking at `truss`, it keeps calling `select()` until suddenly a `ktimer_settime` appears, and a quick `printf` reveals that it's issued by `stopTicker()` called by `stopTimer()` caused by idle GC. When the idle GC is set to a value larger than the `msecs` passed to `fdReady()`, e.g. when `msecs = 500` and `+RTS -I1`, then `fdReady` will never return and the program will be stuck. BSD and Mac OS users (and probably Windows) were just lucky so far that idle GC defaults to 0.3 seconds. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13497#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler