journald/read: simplify walking backwards

In case Tail=N parameter is requested, we need to show N lines.
It does not make sense to walk backwards one by one if we can
do it at once. Now, if Since=T is also provided, make sure we
haven't jumped too far (before T), and if we did, move forward.

The primary motivation for this was to make the code simpler.

This also fixes a tiny bug in the "since" implementation.

Before this commit:
> $ docker logs -t --tail=6000 --since="2019-03-10T03:54:25.00" $ID | head
> 2019-03-10T03:54:24.999821000Z 95981

After:
> $ docker logs -t --tail=6000 --since="2019-03-10T03:54:25.00" $ID | head
> 2019-03-10T03:54:25.000013000Z 95982

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit ff3cd167ea4d089b7695a263ba2fc4caa0a0750c)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/logger/journald/read.go

@@ -361,7 +361,6 @@ func (s *journald) readLogs(logWatcher *logger.LogWatcher, config logger.ReadCon
 		untilUnixMicro = uint64(nano / 1000)
 	}
 	if config.Tail > 0 {
-		lines := config.Tail
 		// If until time provided, start from there.
 		// Otherwise start at the end of the journal.
 		if untilUnixMicro != 0 && C.sd_journal_seek_realtime_usec(j, C.uint64_t(untilUnixMicro)) < 0 {
@@ -371,29 +370,13 @@ func (s *journald) readLogs(logWatcher *logger.LogWatcher, config logger.ReadCon
 			logWatcher.Err <- fmt.Errorf("error seeking to end of journal")
 			return
 		}
-		if C.sd_journal_previous(j) < 0 {
-			logWatcher.Err <- fmt.Errorf("error backtracking to previous journal entry")
-			return
-		}
-		// Walk backward.
-		for lines > 0 {
-			// Stop if the entry time is before our cutoff.
-			// We'll need the entry time if it isn't, so go
-			// ahead and parse it now.
-			if C.sd_journal_get_realtime_usec(j, &stamp) != 0 {
-				break
-			} else {
-				// Compare the timestamp on the entry to our threshold value.
-				if sinceUnixMicro != 0 && sinceUnixMicro > uint64(stamp) {
-					break
-				}
-			}
-			lines--
-			// If we're at the start of the journal, or
-			// don't need to back up past any more entries,
-			// stop.
-			if lines == 0 || C.sd_journal_previous(j) <= 0 {
-				break
+		// (Try to) skip backwards by the requested number of lines...
+		if C.sd_journal_previous_skip(j, C.uint64_t(config.Tail)) > 0 {
+			// ...but not before "since"
+			if sinceUnixMicro != 0 &&
+				C.sd_journal_get_realtime_usec(j, &stamp) == 0 &&
+				uint64(stamp) < sinceUnixMicro {
+				C.sd_journal_seek_realtime_usec(j, C.uint64_t(sinceUnixMicro))
 			}
 		}
 	} else {