network scripts: don't block SIGCHLD before forking

This patch fixes a bug where child processes of launch_script() can
misbehave due to SIGCHLD being blocked. In the case of `sudo`, this
causes a permanent hang.

Previously a SIGCHLD handler was added to reap fork_exec()'d zombie
processes by calling waitpid(-1, ...). This required other
fork()/waitpid() callers to temporarilly block SIGCHILD to avoid
having the final wait status being intercepted by the SIGCHLD
handler:

7c3370d4

Since then, the qemu_add_child_watch() interface was added to allow
registration of such processes and reap only from that specific set
of PIDs:

4d54ec78

As a result, we can now avoid blocking SIGCHLD in launch_script(), so
drop that behavior.
Reviewed-by: NJan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

net/tap.c

@@ -346,15 +346,10 @@ static TAPState *net_tap_fd_init(VLANState *vlan,
 
 static int launch_script(const char *setup_script, const char *ifname, int fd)
 {
-    sigset_t oldmask, mask;
     int pid, status;
     char *args[3];
     char **parg;
 
-    sigemptyset(&mask);
-    sigaddset(&mask, SIGCHLD);
-    sigprocmask(SIG_BLOCK, &mask, &oldmask);
-
     /* try to launch network script */
     pid = fork();
     if (pid == 0) {
@@ -378,7 +373,6 @@ static int launch_script(const char *setup_script, const char *ifname, int fd)
         while (waitpid(pid, &status, 0) != pid) {
             /* loop */
         }
-        sigprocmask(SIG_SETMASK, &oldmask, NULL);
 
         if (WIFEXITED(status) && WEXITSTATUS(status) == 0) {
             return 0;