Interrupt interruptible foreign calls on HS exit

[Andreas Voellmy]

Hi GHCers,

I've been looking into issue #9284, which boils down to getting certain
foreign calls issued by HS threads to finish (i.e. return) in the exit
sequence of forkProcess.

There are several options for solving the particular problem in #9284; one
option is to issue the particular foreign calls causing that issue as
"interruptible" and then have the exit sequence interrupt interruptible
foreign calls.

The exit sequence, starting from hs_exit(), goes through hs_exit_(),
exitScheduler(), scheduleDoGC(), deleteAllThreads(), and then
deleteThread(), where deleteThread is this:

static void
deleteThread (Capability *cap STG_UNUSED, StgTSO *tso)
{
    // NOTE: must only be called on a TSO that we have exclusive
    // access to, because we will call throwToSingleThreaded() below.
    // The TSO must be on the run queue of the Capability we own, or
    // we must own all Capabilities.
  if (tso->why_blocked != BlockedOnCCall &&
tso->why_blocked != BlockedOnCCall_Interruptible) {
        throwToSingleThreaded(tso->cap,tso,NULL);
    }
}

So it looks like interruptible foreign calls are not interrupted in the
exit sequence.

Is there a good reason why we have this behavior? Could we change it to
interrupt TSO's with why_blocked == BlockedOnCCall_Interruptible in the
exit sequence?

Thanks,
Andi

P.S. It looks like this was introduced in commit
83d563cb9ede0ba792836e529b1e2929db926355.
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