[JOBS] Fix dowait signal race

This test program by Alexey Gladkov can cause dash to enter an
infinite loop in waitcmd.

#!/bin/dash
trap "echo TRAP" USR1
stub() {
echo ">>> STUB $1" >&2
sleep $1
echo "<<< STUB $1" >&2
kill -USR1 $$
}
stub 3 &
stub 2 &
until { echo "###"; wait; } do
echo "*** $?"
done

The problem is that if we get a signal after the wait3 system
call has returned but before we get to INTON in dowait, then
we can jump back up to the top and lose the exit status.  So
if we then wait for the job that has just exited, then it'll
stay there forever.

I made the original change that caused this bug to fix pretty
much the same bug but in the opposite direction.  That is, if
we get a signal after we enter wait3 but before we hit the kernel
then it too can cause the wait to go on forever (assuming the
child doesn't exit).

In fact this is pretty much exactly the scenario that you'll
find in glibc's documentation on pause().  The solution is given
there too, in the form of sigsuspend, which is the only way to
do the check and wait atomically.

So this patch fixes Alexey's race without reintroducing the old
bug by converting the blocking wait3 to a sigsuspend.

In order to do this we need to set a signal handler for SIGCHLD,
so the code has been modified to always do that.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 4 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 1d18cbf..8f58d96 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2009-02-22  Herbert Xu <herbert@gondor.apana.org.au>
+
+	* Fix dowait signal race.
+
 2009-01-14  Herbert Xu <herbert@gondor.apana.org.au>
 
 	* Add arith_yacc.h to dash_SOURCES.