Fix lttng sessiond spawn wait race

If the child is scheduled before the parent, using "pause()" in the
parent could be executed after the signal has been handled: that's
entirely racy. Use a proper signal handler and flag to handle this
racelessly.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/src/bin/lttng/lttng.c b/src/bin/lttng/lttng.c
index 57927d4f3d301f87ceb11a0ec85ea489b4e3f4bd..aeaad1469725db898871c6a644f1b2a00119d8a7 100644 (file)
--- a/src/bin/lttng/lttng.c
+++ b/src/bin/lttng/lttng.c
@@ -40,6 +40,7 @@ int opt_verbose;
 static int opt_no_sessiond;
 static char *opt_sessiond_path;
 static pid_t sessiond_pid;
+static volatile int recv_child_signal;
 
 enum {
        OPT_SESSION_PATH,
@@ -181,12 +182,14 @@ static void sighandler(int sig)
                case SIGCHLD:
                        DBG("SIGCHLD caugth");
                        waitpid(sessiond_pid, &status, 0);
+                       recv_child_signal = 1;
                        /* Indicate that the session daemon died */
                        sessiond_pid = 0;
                        ERR("Session daemon died (exit status %d)", WEXITSTATUS(status));
                        break;
                case SIGUSR1:
                        /* Notify is done */
+                       recv_child_signal = 1;
                        DBG("SIGUSR1 caugth");
                        break;
                default:
@@ -294,6 +297,7 @@ static int spawn_sessiond(char *pathname)
        pid_t pid;
 
        MSG("Spawning a session daemon");
+       recv_child_signal = 0;
        pid = fork();
        if (pid == 0) {
                /*
@@ -307,12 +311,24 @@ static int spawn_sessiond(char *pathname)
                } else {
                        perror("execlp");
                }
-               kill(getppid(), SIGTERM);       /* unpause parent */
+               kill(getppid(), SIGTERM);       /* wake parent */
                exit(EXIT_FAILURE);
        } else if (pid > 0) {
                sessiond_pid = pid;
-               /* Wait for lttng-sessiond to start */
-               pause();
+               /*
+                * Wait for lttng-sessiond to start. We need to use a
+                * flag to check if the signal has been sent to us,
+                * because the child can be scheduled before the parent,
+                * and thus send the signal before this check. In the
+                * signal handler, we set the recv_child_signal flag, so
+                * anytime we check it after the fork is fine. Note that
+                * sleep() is interrupted before the 1 second delay as
+                * soon as the signal is received, so it will not cause
+                * visible delay for the user.
+                */
+               while (!recv_child_signal) {
+                       sleep(1);
+               }
                if (!sessiond_pid) {
                        exit(EXIT_FAILURE);
                }