nscd: Improved support for tracking startup failure in nscd service (BZ #16639)

Currently, the nscd parent process parses commandline options and
configuration, forks on startup and immediately exits with a success.
If the child process encounters some error after this, it goes
undetected and any services started up after it may have to repeatedly
check to make sure that the nscd service did actually start up and is
serving requests.

To make this process more reliable, I have added a pipe between the
parent and child process, through which the child process sends a
notification to the parent informing it of its status.  The parent
waits for this status and once it receives it, exits with the
corresponding exit code.  So if the child service sends a success
status (0), the parent exits with a success status.  Similarly for
error conditions, the child sends the non-zero status code, which the
parent passes on as the exit code.

This, along with setting the nscd service type to forking in its
systemd configuration file, allows systemd to be certain that the nscd
service is ready and is accepting connections.

1 files changed, 103 insertions, 5 deletions

diff --git a/nscd/nscd.c b/nscd/nscd.c
index 63d9d83599..56803785e1 100644
--- a/nscd/nscd.c
+++ b/nscd/nscd.c
@@ -39,6 +39,8 @@
 #include <sys/stat.h>
 #include <sys/uio.h>
 #include <sys/un.h>
+#include <sys/wait.h>
+#include <stdarg.h>
 
 #include "dbg_log.h"
 #include "nscd.h"
@@ -101,6 +103,7 @@ gid_t old_gid;
 
 static int check_pid (const char *file);
 static int write_pid (const char *file);
+static int monitor_child (int fd);
 
 /* Name and version of program.  */
 static void print_version (FILE *stream, struct argp_state *state);
@@ -142,6 +145,7 @@ static struct argp argp =
 
 /* True if only statistics are requested.  */
 static bool get_stats;
+static int parent_fd = -1;
 
 int
 main (int argc, char **argv)
@@ -196,11 +200,27 @@ main (int argc, char **argv)
       /* Behave like a daemon.  */
       if (run_mode == RUN_DAEMONIZE)
 	{
+	  int fd[2];
+
+	  if (pipe (fd) != 0)
+	    error (EXIT_FAILURE, errno,
+		   _("cannot create a pipe to talk to the child"));
+
 	  pid = fork ();
 	  if (pid == -1)
 	    error (EXIT_FAILURE, errno, _("cannot fork"));
 	  if (pid != 0)
-	    exit (0);
+	    {
+	      /* The parent only reads from the child.  */
+	      close (fd[1]);
+	      exit (monitor_child (fd[0]));
+	    }
+	  else
+	    {
+	      /* The child only writes to the parent.  */
+	      close (fd[0]);
+	      parent_fd = fd[1];
+	    }
 	}
 
       int nullfd = open (_PATH_DEVNULL, O_RDWR);
@@ -242,7 +262,8 @@ main (int argc, char **argv)
 	      char *endp;
 	      long int fdn = strtol (dirent->d_name, &endp, 10);
 
-	      if (*endp == '\0' && fdn != dfdn && fdn >= min_close_fd)
+	      if (*endp == '\0' && fdn != dfdn && fdn >= min_close_fd
+		  && fdn != parent_fd)
 		close ((int) fdn);
 	    }
 
@@ -250,13 +271,14 @@ main (int argc, char **argv)
 	}
       else
 	for (i = min_close_fd; i < getdtablesize (); i++)
-	  close (i);
+	  if (i != parent_fd)
+	    close (i);
 
       setsid ();
 
       if (chdir ("/") != 0)
-	error (EXIT_FAILURE, errno,
-	       _("cannot change current working directory to \"/\""));
+	do_exit (EXIT_FAILURE, errno,
+		 _("cannot change current working directory to \"/\""));
 
       openlog ("nscd", LOG_CONS | LOG_ODELAY, LOG_DAEMON);
 
@@ -592,3 +614,79 @@ write_pid (const char *file)
 
   return result;
 }
+
+static int
+monitor_child (int fd)
+{
+  int child_ret = 0;
+  int ret = read (fd, &child_ret, sizeof (child_ret));
+
+  /* The child terminated with an error, either via exit or some other abnormal
+     method, like a segfault.  */
+  if (ret <= 0 || child_ret != 0)
+    {
+      int err = wait (&child_ret);
+
+      if (err < 0)
+	{
+	  fprintf (stderr, _("wait failed"));
+	  return 1;
+	}
+
+      fprintf (stderr, _("child exited with status %d"),
+	       WEXITSTATUS (child_ret));
+      if (WIFSIGNALED (child_ret))
+	fprintf (stderr, _(", terminated by signal %d.\n"),
+		 WTERMSIG (child_ret));
+      else
+	fprintf (stderr, ".\n");
+    }
+
+  /* We have the child status, so exit with that code.  */
+  close (fd);
+
+  return child_ret;
+}
+
+void
+do_exit (int child_ret, int errnum, const char *format, ...)
+{
+  if (parent_fd != -1)
+    {
+      int ret = write (parent_fd, &child_ret, sizeof (child_ret));
+      assert (ret == sizeof (child_ret));
+      close (parent_fd);
+    }
+
+  if (format != NULL)
+    {
+      /* Emulate error() since we don't have a va_list variant for it.  */
+      va_list argp;
+
+      fflush (stdout);
+
+      fprintf (stderr, "%s: ", program_invocation_name);
+
+      va_start (argp, format);
+      vfprintf (stderr, format, argp);
+      va_end (argp);
+
+      fprintf (stderr, ": %s\n", strerror (errnum));
+      fflush (stderr);
+    }
+
+  /* Finally, exit.  */
+  exit (child_ret);
+}
+
+void
+notify_parent (int child_ret)
+{
+  if (parent_fd == -1)
+    return;
+
+  int ret = write (parent_fd, &child_ret, sizeof (child_ret));
+  assert (ret == sizeof (child_ret));
+  close (parent_fd);
+  parent_fd = -1;
+}