SIGPIPE in the GHC runtime

18 Aug 2010

      The GHC runtime ignores SIGPIPE by setting the signal
to SIG_IGN. This means that any subprocesses (created via
System.Process or otherwise) will also have their
SIGPIPE handler set to SIG_IGN; I think this might be
a bug. The Python runtime does the same thing,
there's a good explanation of the drawbacks in:
http://bugs.python.org/issue1652

IMHO the simplest fix is the patch below: simply
avoid SIG_IGN, instead install a handler which does nothing.
This way, an exec() restores the handler to SIG_DFL. I've
included a testcase too.

Do people think this is the way to go? I think the alternative
is the approach Python took: patch the various bits of code
that fork&exec and have them restore the signal handler
manually.

---

diff -rN old-ghc/rts/posix/Signals.c new-ghc/rts/posix/Signals.c
472a473,477
...
static void
ignoreSigPipe(int sig)
{
}
528,529c533,535
<     // ignore SIGPIPE; see #1619
<     action.sa_handler = SIG_IGN;
---
...
// ignore SIGPIPE; see #1619. Don't use SIG_IGN since that'd
    // be inherited by any children that get fork&exec'd.
    action.sa_handler = &ignoreSigPipe;
531c537
<     action.sa_flags = 0;
---
...
action.sa_flags = SA_RESTART;
---
Testcase:

diff -rN old-testsuite/tests/ghc-regress/rts/all.T new-testsuite/tests/ghc-regress/rts/all.T
86a87,88
...
test('exec_signals', [cmd_prefix('$MAKE exec_signals-prep && ./exec_signals_prepare')], compile_and_run, [''])
diff -rN old-testsuite/tests/ghc-regress/rts/exec_signals_child.c new-testsuite/tests/ghc-regress/rts/exec_signals_child.c
0a1,47
...
#include 
#include 
#include 
// Prints the state of the signal handlers to stdout
int main()
{
    int open = 0, i;
    sigset_t blockedsigs;
printf("ChildInfo { masked = [");
sigprocmask(SIG_BLOCK, NULL, &blockedsigs);
    for(i = 0; i < NSIG; ++i)
    {
        int ret = sigismember(&blockedsigs, i);
        if(ret >= 0)
        {
            if(!open)
                open=1;
            else
                printf(",");
            printf("(%d,%s)", i, ret == 1 ? "True" : "False");
        }
    }
    printf("], handlers = [");
open = 0;
    for(i = 0; i < NSIG; ++i)
    {
        struct sigaction old;
        if(sigaction(i, NULL, &old) >= 0)
        {
            if(!open)
                open=1;
            else
                printf(",");
printf("(%d,%s)", i,
                    old.sa_handler == SIG_IGN ? "Ignored" :
                    (old.sa_handler == SIG_DFL ? "Default" : "Handled"));
        }
    }
    printf("]}");
return 0;
}
diff -rN old-testsuite/tests/ghc-regress/rts/exec_signals.hs new-testsuite/tests/ghc-regress/rts/exec_signals.hs
0a1,20
import System.Process
import System.Posix.Signals
import Control.Monad(when)
data SigState = Ignored | Default | Handled
    deriving (Eq, Read, Show)
data ChildInfo = ChildInfo {
        masked :: [(Int,Bool)],
        handlers :: [(Int, SigState)] }
    deriving (Read, Show)
main = do out <- readProcess "./exec_signals_child" [] ""
          let ci = read out :: ChildInfo
              blockedSigs = [x | (x, True) <- masked ci]
              ignoredSigs = [x | (x, Ignored) <- handlers ci]
          when (not $ null blockedSigs) $
            putStrLn ("signals " ++ show blockedSigs ++ " are blocked")
          when (not $ null ignoredSigs) $
            putStrLn ("signals " ++ show ignoredSigs ++ " are ignored")
diff -rN old-testsuite/tests/ghc-regress/rts/exec_signals_prepare.c new-testsuite/tests/ghc-regress/rts/exec_signals_prepare.c
0a1,29
#include 
#include 
#include 
#include 
// Invokes a process, making sure that the state of the signal
// handlers has all been set back to the unix default.
int main(int argc, char **argv)
{
    int i;
    sigset_t blockedsigs;
    struct sigaction action;
// unblock all signals
    sigemptyset(&blockedsigs);
    sigprocmask(SIG_BLOCK, NULL, NULL);
// reset all signals to SIG_DFL
    memset(&action, 0, sizeof(action));
    action.sa_handler = SIG_DFL;
    action.sa_flags = 0;
    sigemptyset(&action.sa_mask);
    for(i = 0; i < NSIG; ++i)
        sigaction(i, &action, NULL);
execv(argv[1], argv+1);
    fprintf(stderr, "failed to execv %s\n", argv[1]);
    return 0;
}
diff -rN old-testsuite/tests/ghc-regress/rts/Makefile new-testsuite/tests/ghc-regress/rts/Makefile
29a30,32
exec_signals-prep::
  $(CC) -o exec_signals_child exec_signals_child.c
  $(CC) -o exec_signals_prepare exec_signals_prepare.c

SIGPIPE in the GHC runtime

Brian Bloniarz