functional/tester: fix trigger snapshot retry

Fix negative commit entry diff:

> {"level":"info","ts":1528740474.775448,"caller":"tester/case.go:299","msg":"trigger snapshot PROGRESS","retries":53,"committed-entries":-238470,"etcd-snapshot-count":10000,"start-revision":238470,"last-revision":0,"took":77.302668629,"error":"context deadline exceeded"}

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

functional/tester/case.go

@@ -275,6 +275,18 @@ func (c *caseUntilSnapshot) Inject(clus *Cluster) error {
 
 	for i := 0; i < retries; i++ {
 		lastRev, err = clus.maxRev()
+		if lastRev == 0 {
+			clus.lg.Info(
+				"trigger snapshot RETRY",
+				zap.Int("retries", i),
+				zap.Int64("etcd-snapshot-count", snapshotCount),
+				zap.Int64("start-revision", startRev),
+				zap.Error(err),
+			)
+			time.Sleep(3 * time.Second)
+			continue
+		}
+
 		// If the number of proposals committed is bigger than snapshot count,
 		// a new snapshot should have been created.
 		diff := lastRev - startRev
@@ -292,12 +304,8 @@ func (c *caseUntilSnapshot) Inject(clus *Cluster) error {
 			return nil
 		}
 
-		dur := time.Second
-		if diff < 0 || err != nil {
-			dur = 3 * time.Second
-		}
 		clus.lg.Info(
-			"trigger snapshot PROGRESS",
+			"trigger snapshot RETRY",
 			zap.Int("retries", i),
 			zap.Int64("committed-entries", diff),
 			zap.Int64("etcd-snapshot-count", snapshotCount),
@@ -306,7 +314,10 @@ func (c *caseUntilSnapshot) Inject(clus *Cluster) error {
 			zap.Duration("took", time.Since(now)),
 			zap.Error(err),
 		)
-		time.Sleep(dur)
+		time.Sleep(time.Second)
+		if err != nil {
+			time.Sleep(2 * time.Second)
+		}
 	}
 
 	return fmt.Errorf("cluster too slow: only %d commits in %d retries", lastRev-startRev, retries)