journalctl: add new --sync switch for syncing the journal to disk

With this new "--sync" switch we add a synchronous way to sync
everything queued to disk, and return only after that's complete. This
command gives the guarantee that anything queued before has hit the disk
before the command returns.

While we are at it, also improve the man pages and help text for
journalctl a bit.

1 files changed, 32 insertions, 3 deletions

diff --git a/src/journal/journald-server.c b/src/journal/journald-server.c
index a6e5e4a20f..f0a3c82d98 100644
--- a/src/journal/journald-server.c
+++ b/src/journal/journald-server.c
@@ -1243,7 +1243,7 @@ static int dispatch_sigusr1(sd_event_source *es, const struct signalfd_siginfo *
 
         assert(s);
 
-        log_info("Received request to flush runtime journal from PID %"PRIu32, si->ssi_pid);
+        log_info("Received request to flush runtime journal from PID " PID_FMT, si->ssi_pid);
 
         server_flush_to_var(s);
         server_sync(s);
@@ -1259,7 +1259,7 @@ static int dispatch_sigusr2(sd_event_source *es, const struct signalfd_siginfo *
 
         assert(s);
 
-        log_info("Received request to rotate journal from PID %"PRIu32, si->ssi_pid);
+        log_info("Received request to rotate journal from PID " PID_FMT, si->ssi_pid);
         server_rotate(s);
         server_vacuum(s, true, true);
 
@@ -1277,12 +1277,27 @@ static int dispatch_sigterm(sd_event_source *es, const struct signalfd_siginfo *
         return 0;
 }
 
+static int dispatch_sigrtmin1(sd_event_source *es, const struct signalfd_siginfo *si, void *userdata) {
+        Server *s = userdata;
+
+        assert(s);
+
+        log_debug("Received request to sync from PID " PID_FMT, si->ssi_pid);
+
+        server_sync(s);
+
+        /* Let clients know when the most recent sync happened. */
+        (void) touch("/run/systemd/journal/synced");
+
+        return 0;
+}
+
 static int setup_signals(Server *s) {
         int r;
 
         assert(s);
 
-        assert(sigprocmask_many(SIG_SETMASK, NULL, SIGINT, SIGTERM, SIGUSR1, SIGUSR2, -1) >= 0);
+        assert(sigprocmask_many(SIG_SETMASK, NULL, SIGINT, SIGTERM, SIGUSR1, SIGUSR2, SIGRTMIN+1, -1) >= 0);
 
         r = sd_event_add_signal(s->event, &s->sigusr1_event_source, SIGUSR1, dispatch_sigusr1, s);
         if (r < 0)
@@ -1312,6 +1327,19 @@ static int setup_signals(Server *s) {
         if (r < 0)
                 return r;
 
+        /* SIGRTMIN+1 causes an immediate sync. We process this very
+         * late, so that everything else queued at this point is
+         * really written to disk. Clients can watch
+         * /run/systemd/journal/synced with inotify until its mtime
+         * changes to see when a sync happened. */
+        r = sd_event_add_signal(s->event, &s->sigrtmin1_event_source, SIGRTMIN+1, dispatch_sigrtmin1, s);
+        if (r < 0)
+                return r;
+
+        r = sd_event_source_set_priority(s->sigrtmin1_event_source, SD_EVENT_PRIORITY_NORMAL+15);
+        if (r < 0)
+                return r;
+
         return 0;
 }
 
@@ -1869,6 +1897,7 @@ void server_done(Server *s) {
         sd_event_source_unref(s->sigusr2_event_source);
         sd_event_source_unref(s->sigterm_event_source);
         sd_event_source_unref(s->sigint_event_source);
+        sd_event_source_unref(s->sigrtmin1_event_source);
         sd_event_source_unref(s->hostname_event_source);
         sd_event_source_unref(s->notify_event_source);
         sd_event_source_unref(s->watchdog_event_source);