Call sd_journal_get_fd() earlier, only if needed

1. The journald client library initializes inotify watch(es)
during the first call to sd_journal_get_fd(), and it make sense
to open it earlier in order to not lose any journal file rotation
events.

2. It only makes sense to call this if we're going to use it
later on -- so add a check for config.Follow.

3. Remove the redundant call to sd_journal_get_fd().

NOTE that any subsequent calls to sd_journal_get_fd() return
the same file descriptor, so there's no real need to save it
for later use in wait_for_data_cancelable().

Based on earlier patch by Nalin Dahyabhai <nalin@redhat.com>.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 981c016)
Signed-off-by: Robert Günzler <robertg@balena.io>

daemon/logger/journald/read.go

@@ -330,6 +330,15 @@ func (s *journald) readLogs(logWatcher *logger.LogWatcher, config logger.ReadCon
 		close(logWatcher.Msg)
 		return
 	}
+	if config.Follow {
+		// Initialize library inotify watches early
+		rc = C.sd_journal_get_fd(j)
+		if rc < 0 {
+			logWatcher.Err <- errors.New("error getting journald fd: " + CErr(rc))
+			close(logWatcher.Msg)
+			return
+		}
+	}
 	// If we end up following the log, we can set the journal context
 	// pointer and the channel pointer to nil so that we won't close them
 	// here, potentially while the goroutine that uses them is still
@@ -402,21 +411,16 @@ func (s *journald) readLogs(logWatcher *logger.LogWatcher, config logger.ReadCon
 	}
 	cursor, _ = s.drainJournal(logWatcher, j, nil, untilUnixMicro)
 	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.
-		if fd := C.sd_journal_get_fd(j); fd < C.int(0) {
-			logWatcher.Err <- fmt.Errorf("error opening journald follow descriptor: %q", C.GoString(C.strerror(-fd)))
+		// Create a pipe that we can poll at the same time as
+		// the journald descriptor.
+		ret := C.pipe(&pipes[0])
+		if ret < 0 {
+			logWatcher.Err <- fmt.Errorf("error creating journald notification pipe")
 		} else {
-			// Create a pipe that we can poll at the same time as
-			// the journald descriptor.
-			if C.pipe(&pipes[0]) == C.int(-1) {
-				logWatcher.Err <- fmt.Errorf("error opening journald close notification pipe")
-			} else {
-				cursor = s.followJournal(logWatcher, j, pipes, cursor, untilUnixMicro)
-				// Let followJournal handle freeing the journal context
-				// object and closing the channel.
-				following = true
-			}
+			cursor = s.followJournal(logWatcher, j, pipes, cursor, untilUnixMicro)
+			// Let followJournal handle freeing the journal context
+			// object and closing the channel.
+			following = true
 		}
 	}