[Haskell-cafe] SIGVTALRM and Unbound threads

[Lane Seppala]

Howdy,

I've banged into this issue of FFI errors stemming from a C library
improperly handling system call interrupts caused by the SIGVTALRM/SIGALRMs
emitted by the Haskell runtime.
Usually the 'proper' solution is to fix the C library—easier said than done
for most C quagmires. The most popular workaround, described by Bryan
O'Sullivan[1] and several others, is to block the culprit signals from
reaching the FFI call.

I found another workaround using unbound threads, and it seems to work
generally for this problem. However, I'm not satisfied with my
understanding about *why* it works given what I know about FFI, bound vs
unbound threads, and sys calls.

As the simplest example, compiled with the threaded runtime (and neglecting
imports):

    main = void $ sleep 10

will be interrupted, whereas

    main = void $ runInUnboundThread $ sleep 10

will complete its 10 second sleep.

How does running the FFI call on an unbound thread block SIGVTALRM or
otherwise avoid the interrupt? My understanding is that these signals
*should* reach the FFI process, since they're on the same OS thread the
timers were set. Is there any danger to consider before saying that this is
a viable workaround?

Kindly,
Lane


[1]:
http://www.serpentine.com/blog/2010/09/04/dealing-with-fragile-c-libraries-e-g-mysql-from-haskell/
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