Sync with grandchild

Without this, it's possible that father process exit with
0 before grandchild exit with error.

Signed-off-by: Qiang Huang <[email protected]>

libcontainer/nsenter/nsexec.c

@@ -33,6 +33,7 @@ enum sync_t {
 	SYNC_USERMAP_ACK = 0x41, /* Mapping finished by the parent. */
 	SYNC_RECVPID_PLS = 0x42, /* Tell parent we're sending the PID. */
 	SYNC_RECVPID_ACK = 0x43, /* PID was correctly received by parent. */
+	SYNC_CHILD_READY = 0x44, /* The grandchild is ready to return. */
 
 	/* XXX: This doesn't help with segfaults and other such issues. */
 	SYNC_ERR = 0xFF, /* Fatal error, no turning back. The error code follows. */
@@ -497,7 +498,7 @@ void nsexec(void)
 	 *          process.
 	 */
 	case JUMP_PARENT: {
-			int len;
+			int len, ready = 0;
 			pid_t child;
 			char buf[JSON_MAX];
 
@@ -509,8 +510,14 @@ void nsexec(void)
 			if (child < 0)
 				bail("unable to fork: child_func");
 
-			/* State machine for synchronisation with the children. */
-			while (true) {
+			/*
+			 * State machine for synchronisation with the children.
+			 *
+			 * Father only return when both child and grandchild are
+			 * ready, so we can receive all possible error codes
+			 * generated by children.
+			 */
+			while (ready < 2) {
 				enum sync_t s;
 
 				/* This doesn't need to be global, we're in the parent. */
@@ -568,17 +575,22 @@ void nsexec(void)
 						}
 					}
 
-					/* Leave the loop. */
-					goto out;
+					ready++;
+					break;
 				case SYNC_RECVPID_ACK:
 					/* We should _never_ receive acks. */
 					kill(child, SIGKILL);
 					bail("failed to sync with child: unexpected SYNC_RECVPID_ACK");
 					break;
+				case SYNC_CHILD_READY:
+					ready++;
+					break;
+				default:
+					bail("unexpected sync value");
+					break;
 				}
 			}
 
-		out:
 			/* Send the init_func pid back to our parent. */
 			len = snprintf(buf, JSON_MAX, "{\"pid\": %d}\n", child);
 			if (len < 0) {
@@ -700,6 +712,7 @@ void nsexec(void)
 			 * start_child() code after forking in the parent.
 			 */
 			int consolefd = config.consolefd;
+			enum sync_t s;
 
 			/* We're in a child and thus need to tell the parent if we die. */
 			syncfd = syncpipe[0];
@@ -730,6 +743,10 @@ void nsexec(void)
 					bail("failed to dup stderr");
 			}
 
+			s = SYNC_CHILD_READY;
+			if (write(syncfd, &s, sizeof(s)) != sizeof(s))
+				bail("failed to sync with patent: write(SYNC_CHILD_READY)");
+
 			/* Close sync pipes. */
 			close(syncpipe[0]);
 			close(syncpipe[1]);