Synchronize the cursor returned by followJournal

Make sure that the cursor value returned by followJournal() is the last
of the values returned by its goroutine's calls to drainJournal() by
waiting for it, rather than returning a value that may be superceded by
another if we're singalling the goroutine that it should exit by closing
a pipe.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Author: nalind

daemon/logger/journald/read.go

@@ -248,6 +248,9 @@ func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.Re
 	s.readers.mu.Lock()
 	s.readers.readers[logWatcher] = logWatcher
 	s.readers.mu.Unlock()
+
+	newCursor := make(chan *C.char)
+
 	go func() {
 		// Keep copying journal data out until we're notified to stop
 		// or we hit an error.
@@ -275,6 +278,7 @@ func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.Re
 		delete(s.readers.readers, logWatcher)
 		s.readers.mu.Unlock()
 		close(logWatcher.Msg)
+		newCursor <- cursor
 	}()
 	// Wait until we're told to stop.
 	select {
@@ -283,6 +287,8 @@ func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.Re
 		C.close(pfd[1])
 	}
 
+	cursor = <-newCursor
+
 	return cursor
 }