daemon: restart child process if it died before signaling its readiness

The child process (the one being monitored) could die before it was able
to call fork_notify_startup() function.  If such situation arises, then
parent process (the one monitoring child process) would also terminate
with a fatal log message:

...|EMER|fork child died before signaling startup (killed (...))

This patch changes that behavior by always restarting child process
if it was able to start up at least once in the past.  However, if
child was not able to start up even once, then the monitor process
would still terminate, because that would most likely indicate a
persistent programming or system error.

To reproduce use following script:

while : ; do kill -SIGSEGV `cat /var/run/openvswitch/ovs-vswitchd.pid`; done

Signed-Off-By: Ansis Atteka <[email protected]>
Acked-by: Ben Pfaff <[email protected]>
VMware-BZ: 1273550

lib/daemon-unix.c

@@ -214,19 +214,23 @@ fork_and_clean_up(void)
 /* Forks, then:
  *
  *   - In the parent, waits for the child to signal that it has completed its
- *     startup sequence.  Then stores -1 in '*fdp' and returns the child's pid.
+ *     startup sequence.  Then stores -1 in '*fdp' and returns the child's
+ *     pid in '*child_pid' argument.
  *
- *   - In the child, stores a fd in '*fdp' and returns 0.  The caller should
- *     pass the fd to fork_notify_startup() after it finishes its startup
- *     sequence.
+ *   - In the child, stores a fd in '*fdp' and returns 0 through '*child_pid'
+ *     argument.  The caller should pass the fd to fork_notify_startup() after
+ *     it finishes its startup sequence.
  *
- * If something goes wrong with the fork, logs a critical error and aborts the
- * process. */
-static pid_t
-fork_and_wait_for_startup(int *fdp)
+ * Returns 0 on success.  If something goes wrong and child process was not
+ * able to signal its readiness by calling fork_notify_startup(), then this
+ * function returns -1. However, even in case of failure it still sets child
+ * process id in '*child_pid'. */
+static int
+fork_and_wait_for_startup(int *fdp, pid_t *child_pid)
 {
     int fds[2];
     pid_t pid;
+    int ret = 0;
 
     xpipe(fds);
 
@@ -252,8 +256,9 @@ fork_and_wait_for_startup(int *fdp)
                     exit(WEXITSTATUS(status));
                 } else {
                     char *status_msg = process_status_msg(status);
-                    VLOG_FATAL("fork child died before signaling startup (%s)",
-                               status_msg);
+                    VLOG_ERR("fork child died before signaling startup (%s)",
+                             status_msg);
+                    ret = -1;
                 }
             } else if (retval < 0) {
                 VLOG_FATAL("waitpid failed (%s)", ovs_strerror(errno));
@@ -268,8 +273,8 @@ fork_and_wait_for_startup(int *fdp)
         close(fds[0]);
         *fdp = fds[1];
     }
-
-    return pid;
+    *child_pid = pid;
+    return ret;
 }
 
 static void
@@ -317,6 +322,7 @@ monitor_daemon(pid_t daemon_pid)
     time_t last_restart;
     char *status_msg;
     int crashes;
+    bool child_ready = true;
 
     set_subprogram_name("monitor");
     status_msg = xstrdup("healthy");
@@ -329,13 +335,16 @@ monitor_daemon(pid_t daemon_pid)
         proctitle_set("monitoring pid %lu (%s)",
                       (unsigned long int) daemon_pid, status_msg);
 
-        do {
-            retval = waitpid(daemon_pid, &status, 0);
-        } while (retval == -1 && errno == EINTR);
+        if (child_ready) {
+            do {
+                retval = waitpid(daemon_pid, &status, 0);
+            } while (retval == -1 && errno == EINTR);
+            if (retval == -1) {
+                VLOG_FATAL("waitpid failed (%s)", ovs_strerror(errno));
+            }
+        }
 
-        if (retval == -1) {
-            VLOG_FATAL("waitpid failed (%s)", ovs_strerror(errno));
-        } else if (retval == daemon_pid) {
+        if (!child_ready || retval == daemon_pid) {
             char *s = process_status_msg(status);
             if (should_restart(status)) {
                 free(status_msg);
@@ -372,8 +381,11 @@ monitor_daemon(pid_t daemon_pid)
                 last_restart = time(NULL);
 
                 VLOG_ERR("%s, restarting", status_msg);
-                daemon_pid = fork_and_wait_for_startup(&daemonize_fd);
-                if (!daemon_pid) {
+                child_ready = !fork_and_wait_for_startup(&daemonize_fd,
+                                                         &daemon_pid);
+                if (child_ready && !daemon_pid) {
+                    /* Child process needs to break out of monitoring
+                     * loop. */
                     break;
                 }
             } else {
@@ -403,7 +415,12 @@ daemonize_start(void)
     daemonize_fd = -1;
 
     if (detach) {
-        if (fork_and_wait_for_startup(&daemonize_fd) > 0) {
+        pid_t pid;
+
+        if (fork_and_wait_for_startup(&daemonize_fd, &pid)) {
+            VLOG_FATAL("could not detach from foreground session");
+        }
+        if (pid > 0) {
             /* Running in parent process. */
             exit(0);
         }
@@ -416,7 +433,9 @@ daemonize_start(void)
         int saved_daemonize_fd = daemonize_fd;
         pid_t daemon_pid;
 
-        daemon_pid = fork_and_wait_for_startup(&daemonize_fd);
+        if (fork_and_wait_for_startup(&daemonize_fd, &daemon_pid)) {
+            VLOG_FATAL("could not initiate process monitoring");
+        }
         if (daemon_pid > 0) {
             /* Running in monitor process. */
             fork_notify_startup(saved_daemonize_fd);