[Bug 241849] lang/ghc: Enable USE_PTHREAD_FOR_ITIMER

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241849

            Bug ID: 241849
           Summary: lang/ghc: Enable USE_PTHREAD_FOR_ITIMER
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Individual Port(s)
          Assignee: haskell at FreeBSD.org
          Reporter: kevinz5000 at gmail.com
             Flags: maintainer-feedback?(haskell at FreeBSD.org)
          Assignee: haskell at FreeBSD.org

Created attachment 209025
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=209025&action=edit
Patch

The GHC RTS has an interval timer that is apparently used both in the threaded
(-threaded) and non-threaded runtime.

On some operating systems, rts/posix/Itimer.c sets USE_PTHREAD_FOR_ITIMER,
which uses usleep() at regular intervals instead of SIGVALRM.

Per the comments about Darwin (macOS) above:

 * We want to avoid using the SIGALRM signals whenever possible as these
signals
 * interrupt system calls (see #10840) and can be overridden by user code. On
 * Darwin we can use a dedicated thread and usleep.

The attached patch applies this for FreeBSD as well.

To see the difference, run 'truss' on a simple test program:

== test.hs ==
main = do
    c <- getChar
    putChar c

$ ghc test.hs
$ truss ./test.hs
... some stuff
SIGNAL 26 (SIGVTALRM) code=SI_TIMER value=0x0 timerid=3 overrun=0
sigprocmask(SIG_SETMASK,{ SIGVTALRM },0x0)       = 0 (0x0)
sigreturn(0x7fffffff9880)                        = 0 (0x0)
ioctl(0,TIOCGETA,0x7fffffffa270)                 = 0 (0x0)
poll({ 0/POLLIN },1,0)                           = 0 (0x0)
select(1,{ 0 },{ },0x0,0x0)                      ERR#4 'Interrupted system
call'
SIGNAL 26 (SIGVTALRM) code=SI_TIMER value=0x0 timerid=3 overrun=0
sigprocmask(SIG_SETMASK,{ SIGVTALRM },0x0)       = 0 (0x0)
sigreturn(0x7fffffffd6c0)                        ERR#4 'Interrupted system
call'
select(1,{ 0 },{ },0x0,0x0)                      ERR#4 'Interrupted system
call'
... more of this

After this change:
... some stuff
ioctl(0,TIOCGETA,0x7fffffffa270)                 = 0 (0x0)
poll({ 0/POLLIN },1,0)                           = 0 (0x0)
nanosleep({ 0.010000000 })                       = 0 (0x0)
nanosleep({ 0.010000000 })                       = 0 (0x0)
nanosleep({ 0.010000000 })                       = 0 (0x0)
... more of this
select(1,{ 0 },{ },0x0,0x0)                      = 1 (0x1)
... more stuff

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