Clean up journald logger

We clean up the journald logger with these four changes.

1. Make field array static
2. Make function name more appropriate
3. Initialize the file descriptors only once
4. Avoid copying the journald cursor

Point 4 is the most significant change: instead of treating the journald
cursor like a Go string we use it as a raw C.char pointer. That way we
avoid the copying by the C.CString and C.GoString functions.

Signed-off-by: Silvan Jegen <s.jegen@gmail.com>

daemon/logger/journald/read.go

@@ -56,7 +56,7 @@ package journald
 //}
 //static int is_attribute_field(const char *msg, size_t length)
 //{
-//	const struct known_field {
+//	static const struct known_field {
 //		const char *name;
 //		size_t length;
 //	} fields[] = {
@@ -101,21 +101,23 @@ package journald
 //	}
 //	return rc;
 //}
-//static int wait_for_data_or_close(sd_journal *j, int pipefd)
+//static int wait_for_data_cancelable(sd_journal *j, int pipefd)
 //{
 //	struct pollfd fds[2];
 //	uint64_t when = 0;
 //	int timeout, jevents, i;
 //	struct timespec ts;
 //	uint64_t now;
+//
+//	memset(&fds, 0, sizeof(fds));
+//	fds[0].fd = pipefd;
+//	fds[0].events = POLLHUP;
+//	fds[1].fd = sd_journal_get_fd(j);
+//	if (fds[1].fd < 0) {
+//		return fds[1].fd;
+//	}
+//
 //	do {
-//		memset(&fds, 0, sizeof(fds));
-//		fds[0].fd = pipefd;
-//		fds[0].events = POLLHUP;
-//		fds[1].fd = sd_journal_get_fd(j);
-//		if (fds[1].fd < 0) {
-//			return fds[1].fd;
-//		}
 //		jevents = sd_journal_get_events(j);
 //		if (jevents < 0) {
 //			return jevents;
@@ -167,7 +169,7 @@ func (s *journald) Close() error {
 	return nil
 }
 
-func (s *journald) drainJournal(logWatcher *logger.LogWatcher, config logger.ReadConfig, j *C.sd_journal, oldCursor string) string {
+func (s *journald) drainJournal(logWatcher *logger.LogWatcher, config logger.ReadConfig, j *C.sd_journal, oldCursor *C.char) *C.char {
 	var msg, data, cursor *C.char
 	var length C.size_t
 	var stamp C.uint64_t
@@ -177,10 +179,8 @@ func (s *journald) drainJournal(logWatcher *logger.LogWatcher, config logger.Rea
 drain:
 	for {
 		// Try not to send a given entry twice.
-		if oldCursor != "" {
-			ccursor := C.CString(oldCursor)
-			defer C.free(unsafe.Pointer(ccursor))
-			for C.sd_journal_test_cursor(j, ccursor) > 0 {
+		if oldCursor != nil {
+			for C.sd_journal_test_cursor(j, oldCursor) > 0 {
 				if C.sd_journal_next(j) <= 0 {
 					break drain
 				}
@@ -234,25 +234,24 @@ drain:
 			break
 		}
 	}
-	retCursor := ""
-	if C.sd_journal_get_cursor(j, &cursor) == 0 {
-		retCursor = C.GoString(cursor)
-		C.free(unsafe.Pointer(cursor))
-	}
-	return retCursor
+
+	// free(NULL) is safe
+	C.free(unsafe.Pointer(oldCursor))
+	C.sd_journal_get_cursor(j, &cursor)
+	return cursor
 }
 
-func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.ReadConfig, j *C.sd_journal, pfd [2]C.int, cursor string) {
+func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.ReadConfig, j *C.sd_journal, pfd [2]C.int, cursor *C.char) *C.char {
 	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_or_close(j, pfd[0])
+		status := C.wait_for_data_cancelable(j, pfd[0])
 		for status == 1 {
 			cursor = s.drainJournal(logWatcher, config, j, cursor)
-			status = C.wait_for_data_or_close(j, pfd[0])
+			status = C.wait_for_data_cancelable(j, pfd[0])
 		}
 		if status < 0 {
 			cerrstr := C.strerror(C.int(-status))
@@ -274,15 +273,16 @@ func (s *journald) followJournal(logWatcher *logger.LogWatcher, config logger.Re
 		// Notify the other goroutine that its work is done.
 		C.close(pfd[1])
 	}
+
+	return cursor
 }
 
 func (s *journald) readLogs(logWatcher *logger.LogWatcher, config logger.ReadConfig) {
 	var j *C.sd_journal
-	var cmatch *C.char
+	var cmatch, cursor *C.char
 	var stamp C.uint64_t
 	var sinceUnixMicro uint64
 	var pipes [2]C.int
-	cursor := ""
 
 	// Get a handle to the journal.
 	rc := C.sd_journal_open(&j, C.int(0))
@@ -370,7 +370,7 @@ func (s *journald) readLogs(logWatcher *logger.LogWatcher, config logger.ReadCon
 			return
 		}
 	}
-	cursor = s.drainJournal(logWatcher, config, j, "")
+	cursor = s.drainJournal(logWatcher, config, j, nil)
 	if config.Follow {
 		// Allocate a descriptor for following the journal, if we'll
 		// need one.  Do it here so that we can report if it fails.
@@ -382,13 +382,15 @@ func (s *journald) readLogs(logWatcher *logger.LogWatcher, config logger.ReadCon
 			if C.pipe(&pipes[0]) == C.int(-1) {
 				logWatcher.Err <- fmt.Errorf("error opening journald close notification pipe")
 			} else {
-				s.followJournal(logWatcher, config, j, pipes, cursor)
+				cursor = s.followJournal(logWatcher, config, j, pipes, cursor)
 				// Let followJournal handle freeing the journal context
 				// object and closing the channel.
 				following = true
 			}
 		}
 	}
+
+	C.free(unsafe.Pointer(cursor))
 	return
 }