journald logs: drain 1 more time at container exit

In the journald log driver, attempt to drain the journal 1 more time
after being told to stop following the log. Due to a possible race
condition, sometimes data is written to the journal at almost the same
time the log watch is closed, and depending on the order of operations,
sometimes you miss the last journal entry.

Signed-off-by: Andy Goldstein <agoldste@redhat.com>

daemon/logger/journald/read.go

@@ -245,20 +245,28 @@ func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.Re
 	s.readers.mu.Lock()
 	s.readers.readers[logWatcher] = logWatcher
 	s.readers.mu.Unlock()
+
 	go func() {
-		// Keep copying journal data out until we're notified to stop
-		// or we hit an error.
-		status := C.wait_for_data_cancelable(j, pfd[0])
-		for status == 1 {
+		for {
+			// Keep copying journal data out until we're notified to stop
+			// or we hit an error.
+			status := C.wait_for_data_cancelable(j, pfd[0])
+			if status < 0 {
+				cerrstr := C.strerror(C.int(-status))
+				errstr := C.GoString(cerrstr)
+				fmtstr := "error %q while attempting to follow journal for container %q"
+				logrus.Errorf(fmtstr, errstr, s.vars["CONTAINER_ID_FULL"])
+				break
+			}
+
 			cursor = s.drainJournal(logWatcher, config, j, cursor)
-			status = C.wait_for_data_cancelable(j, pfd[0])
-		}
-		if status < 0 {
-			cerrstr := C.strerror(C.int(-status))
-			errstr := C.GoString(cerrstr)
-			fmtstr := "error %q while attempting to follow journal for container %q"
-			logrus.Errorf(fmtstr, errstr, s.vars["CONTAINER_ID_FULL"])
+
+			if status != 1 {
+				// We were notified to stop
+				break
+			}
 		}
+
 		// Clean up.
 		C.close(pfd[0])
 		s.readers.mu.Lock()
@@ -267,6 +275,7 @@ func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.Re
 		C.sd_journal_close(j)
 		close(logWatcher.Msg)
 	}()
+
 	// Wait until we're told to stop.
 	select {
 	case <-logWatcher.WatchClose():