qemu-ga: guest-shutdown: become synchronous

Last commit dropped qemu-ga's SIGCHLD handler, used to automatically
reap terminated children processes. This introduced a bug to
qmp_guest_shutdown(): it will generate zombies.

This problem probably doesn't matter in the success case, as the VM
will shutdown anyway, but let's do the right thing and reap the
created process. This ultimately means that guest-shutdown is now a
synchronous command.

An interesting side effect is that guest-shutdown is now able to
report an error to the client if shutting down fails.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qga/commands-posix.c

@@ -36,8 +36,9 @@
 
 void qmp_guest_shutdown(bool has_mode, const char *mode, Error **err)
 {
-    int ret;
     const char *shutdown_flag;
+    int ret, status;
+    pid_t rpid, pid;
 
     slog("guest-shutdown called, mode: %s", mode);
     if (!has_mode || strcmp(mode, "powerdown") == 0) {
@@ -52,8 +53,8 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, Error **err)
         return;
     }
 
-    ret = fork();
-    if (ret == 0) {
+    pid = fork();
+    if (pid == 0) {
         /* child, start the shutdown */
         setsid();
         fclose(stdin);
@@ -66,9 +67,19 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, Error **err)
             slog("guest-shutdown failed: %s", strerror(errno));
         }
         exit(!!ret);
-    } else if (ret < 0) {
-        error_set(err, QERR_UNDEFINED_ERROR);
+    } else if (pid < 0) {
+        goto exit_err;
     }
+
+    do {
+        rpid = waitpid(pid, &status, 0);
+    } while (rpid == -1 && errno == EINTR);
+    if (rpid == pid && WIFEXITED(status) && !WEXITSTATUS(status)) {
+        return;
+    }
+
+exit_err:
+    error_set(err, QERR_UNDEFINED_ERROR);
 }
 
 typedef struct GuestFileHandle {