libpod/Container.readFromJournal(): don't skip the first entry

When reading log entries from the journal, don't skip past the first
matching entry after we've positioned the cursor at it.

Make the first blank-line entry that we logged so that the container
would always have at least one log entry for us to find (until it gets
vacuumed out, at least) a fake history entry, so that `logs` doesn't
pass it on for display.

CI already has tests that exercise journal-based logging, so
[NO TESTS NEEDED]

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

libpod/container_log_linux.go

@@ -38,6 +38,8 @@ func (c *Container) initializeJournal(ctx context.Context) error {
 	m["SYSLOG_IDENTIFIER"] = "podman"
 	m["PODMAN_ID"] = c.ID()
 	m["CONTAINER_ID_FULL"] = c.ID()
+	history := events.History
+	m["PODMAN_EVENT"] = history.String()
 	return journal.Send("", journal.PriInfo, m)
 }
 
@@ -89,10 +91,10 @@ func (c *Container) readFromJournal(ctx context.Context, options *logs.LogOption
 	var cursorError error
 	for i := 1; i <= 3; i++ {
 		cursor, cursorError = journal.GetCursor()
-		if err != nil {
+		if cursorError != nil {
+			time.Sleep(time.Duration(i*100) * time.Millisecond)
 			continue
 		}
-		time.Sleep(time.Duration(i*100) * time.Millisecond)
 		break
 	}
 	if cursorError != nil {
@@ -116,6 +118,7 @@ func (c *Container) readFromJournal(ctx context.Context, options *logs.LogOption
 
 		tailQueue := []*logs.LogLine{} // needed for options.Tail
 		doTail := options.Tail > 0
+		lastReadCursor := ""
 		for {
 			select {
 			case <-ctx.Done():
@@ -125,18 +128,25 @@ func (c *Container) readFromJournal(ctx context.Context, options *logs.LogOption
 				// Fallthrough
 			}
 
+			if lastReadCursor != "" {
+				// Advance to next entry if we read this one.
 				if _, err := journal.Next(); err != nil {
 					logrus.Errorf("Failed to move journal cursor to next entry: %v", err)
 					return
 				}
-			latestCursor, err := journal.GetCursor()
+			}
+
+			// Fetch the location of this entry, presumably either
+			// the one that follows the last one we read, or that
+			// same last one, if there is no next entry (yet).
+			cursor, err = journal.GetCursor()
 			if err != nil {
 				logrus.Errorf("Failed to get journal cursor: %v", err)
 				return
 			}
 
-			// Hit the end of the journal.
+			// Hit the end of the journal (so far?).
-			if cursor == latestCursor {
+			if cursor == lastReadCursor {
 				if doTail {
 					// Flush *once* we hit the end of the journal.
 					startIndex := int64(len(tailQueue)-1) - options.Tail
@@ -157,8 +167,9 @@ func (c *Container) readFromJournal(ctx context.Context, options *logs.LogOption
 				journal.Wait(sdjournal.IndefiniteWait)
 				continue
 			}
-			cursor = latestCursor
+			lastReadCursor = cursor
 
+			// Read the journal entry.
 			entry, err := journal.GetEntry()
 			if err != nil {
 				logrus.Errorf("Failed to get journal entry: %v", err)