daemon: Allow daemon child process to report success or failure to parent.

There are conflicting pressures in startup of a daemon process:

    * The parent process should exit with an error code if the daemon
      cannot start up successfully.

    * Some startup actions must be performed in the child process, not in
      the parent.  The most obvious of these are file locking, since
      child processes do not inherit locks, and anything that requires
      knowing the child process's PID (e.g. unixctl sockets).

Until now, this conflict has usually been handled by giving up part of the
first property, i.e. in some cases the parent process would exit
successfully and the child immediately afterward exit with a failure code.

This commit introduces a better approach, by allowing daemons to perform
startup work in the child and only then signal the parent that they have
successfully started.  If the child instead exits without signaling
success, the parent passes this exit code along to its own parent.

This commit also modifies the daemons that can usefully take advantage of
this new feature to do so.

lib/daemon.c

@@ -20,6 +20,7 @@
 #include <fcntl.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/wait.h>
 #include <unistd.h>
 #include "fatal-signal.h"
 #include "dirs.h"
@@ -39,9 +40,12 @@ static char *pidfile;
 /* Create pidfile even if one already exists and is locked? */
 static bool overwrite_pidfile;
 
-/* Should we chdir to "/". */
+/* Should we chdir to "/"? */
 static bool chdir_ = true;
 
+/* File descriptors used by daemonize_start() and daemonize_complete(). */
+static int daemonize_fds[2];
+
 /* Returns the file name that would be used for a pidfile if 'name' were
  * provided to set_pidfile().  The caller must free the returned string. */
 char *
@@ -210,48 +214,83 @@ make_pidfile(void)
  * detaches from the foreground session.  */
 void
 daemonize(void)
+{
+    daemonize_start();
+    daemonize_complete();
+}
+
+/* If daemonization is configured, then starts daemonization, by forking and
+ * returning in the child process.  The parent process hangs around until the
+ * child lets it know either that it completed startup successfully (by calling
+ * daemon_complete()) or that it failed to start up (by exiting with a nonzero
+ * exit code). */
+void
+daemonize_start(void)
 {
     if (detach) {
-        char c = 0;
-        int fds[2];
-        if (pipe(fds) < 0) {
+        pid_t pid;
+
+        if (pipe(daemonize_fds) < 0) {
             ovs_fatal(errno, "pipe failed");
         }
 
-        switch (fork()) {
-        default:
-            /* Parent process: wait for child to create pidfile, then exit. */
-            close(fds[1]);
+        pid = fork();
+        if (pid > 0) {
+            /* Running in parent process. */
+            char c;
+
+            close(daemonize_fds[1]);
             fatal_signal_fork();
-            if (read(fds[0], &c, 1) != 1) {
+            if (read(daemonize_fds[0], &c, 1) != 1) {
+                int retval;
+                int status;
+
+                do {
+                    retval = waitpid(pid, &status, 0);
+                } while (retval == -1 && errno == EINTR);
+
+                if (retval == pid
+                    && WIFEXITED(status)
+                    && WEXITSTATUS(status)) {
+                    /* Child exited with an error.  Convey the same error to
+                     * our parent process as a courtesy. */
+                    exit(WEXITSTATUS(status));
+                }
+
                 ovs_fatal(errno, "daemon child failed to signal startup");
             }
             exit(0);
-
-        case 0:
-            /* Child process. */
-            close(fds[0]);
+        } else if (!pid) {
+            /* Running in child process. */
+            close(daemonize_fds[0]);
             make_pidfile();
-            ignore(write(fds[1], &c, 1));
-            close(fds[1]);
-            setsid();
-            if (chdir_) {
-                ignore(chdir("/"));
-            }
             time_postfork();
             lockfile_postfork();
-            break;
-
-        case -1:
-            /* Error. */
+        } else {
             ovs_fatal(errno, "could not fork");
-            break;
         }
     } else {
         make_pidfile();
     }
 }
 
+/* If daemonization is configured, then this function notifies the parent
+ * process that the child process has completed startup successfully. */
+void
+daemonize_complete(void)
+{
+    if (detach) {
+        char c = 0;
+
+        ignore(write(daemonize_fds[1], &c, 1));
+        close(daemonize_fds[1]);
+        setsid();
+        if (chdir_) {
+            ignore(chdir("/"));
+        }
+    }
+}
+
 void
 daemon_usage(void)
 {