daemon: Better log when fork child dies early from signals.

On one machine, "/etc/init.d/openvswitch-switch start" failed to start
with:

   ovs-vswitchd: fork child failed to signal startup (Success)
   Starting ovs-vswitchd ... failed!

"strace" revealed that the fork child was actually segfaulting, but the
message output didn't indicate that in any way.  This commit fixes the
log message (but not the segfault itself).

Reported-by: Michael Hu <mhu@nicira.com>
Bug #8457.

Author: blp

lib/daemon.c

@@ -250,16 +250,21 @@ fork_and_wait_for_startup(int *fdp)
                 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));
+            if (retval == pid) {
+                if (WIFEXITED(status) && WEXITSTATUS(status)) {
+                    /* Child exited with an error.  Convey the same error
+                     * to our parent process as a courtesy. */
+                    exit(WEXITSTATUS(status));
+                } else {
+                    char *status_msg = process_status_msg(status);
+                    VLOG_FATAL("fork child died before signaling startup (%s)",
+                               status_msg);
+                }
+            } else if (retval < 0) {
+                VLOG_FATAL("waitpid failed (%s)", strerror(errno));
+            } else {
+                NOT_REACHED();
             }
-
-            VLOG_FATAL("fork child failed to signal startup (%s)",
-                       strerror(errno));
         }
         close(fds[0]);
         *fdp = -1;

python/ovs/daemon.py

@@ -245,13 +245,19 @@ def _fork_and_wait_for_startup():
                 break
         if len(s) != 1:
             retval, status = _waitpid(pid, 0)
-            if (retval == pid and
-                os.WIFEXITED(status) and os.WEXITSTATUS(status)):
-                # Child exited with an error.  Convey the same error to
-                # our parent process as a courtesy.
-                sys.exit(os.WEXITSTATUS(status))
+            if retval == pid:
+                if os.WIFEXITED(status) and os.WEXITSTATUS(status):
+                    # Child exited with an error.  Convey the same error to
+                    # our parent process as a courtesy.
+                    sys.exit(os.WEXITSTATUS(status))
+                else:
+                    sys.stderr.write("fork child failed to signal "
+                                     "startup (%s)\n"
+                                     % ovs.process.status_msg(status))
             else:
-                sys.stderr.write("fork child failed to signal startup\n")
+                assert retval < 0
+                sys.stderr.write("waitpid failed (%s)\n"
+                                 % os.strerror(-retval))
                 sys.exit(1)
 
         os.close(rfd)