sh: Fix EINTR race condition in "wait" and "set -T" using sigsuspend().

When waiting for child processes using "wait" or if "set -T" is in effect, a
signal interrupts the wait. Make sure there is no window where the signal
handler may be invoked (setting a flag) just before going to sleep.

There is a similar race condition in the shell language, but scripts can
avoid it by exiting from the trap handler or enforcing synchronization using
a fifo.

If SIGCHLD is not trapped, a signal handler must be installed for it. Only
install this handler for the duration of the wait to avoid triggering
unexpected [EINTR] errors elsewhere.

Note that for some reason only SIGINT and SIGQUIT interrupt a "wait"
command. This remains the case.

1 files changed, 8 insertions, 0 deletions

diff --git a/bin/sh/trap.c b/bin/sh/trap.c
index 94cf129..521c511 100644
--- a/bin/sh/trap.c
+++ b/bin/sh/trap.c
@@ -368,6 +368,14 @@ ignoresig(int signo)
 }
 
 
+int
+issigchldtrapped(void)
+{
+
+	return (trap[SIGCHLD] != NULL && *trap[SIGCHLD] != '\0');
+}
+
+
 /*
  * Signal handler.
  */