fix: last-modified of empty shard directory shouldn't be Unix epoch. (#21481)

Co-authored-by: davidby-influx <72418212+davidby-influx@users.noreply.github.com>

CHANGELOG.md

@@ -14,6 +14,7 @@
 1. [21375](https://github.com/influxdata/influxdb/pull/21375): Add logging to NATS streaming server to help debug startup failures.
 1. [21477](https://github.com/influxdata/influxdb/pull/21477): Accept `--input` instead of a positional arg in `influx restore`.
 1. [21477](https://github.com/influxdata/influxdb/pull/21477): Print error instead of panicking when `influx restore` fails to find backup manifests.
+1. [21481](https://github.com/influxdata/influxdb/pull/21481): Set last-modified time of empty shard directory to the directory's mod time instead of Unix epoch.
 
 ## v2.0.6 [2021-04-29]
 

tsdb/engine/tsm1/file_store.go

@@ -576,6 +576,7 @@ func (f *FileStore) Open() error {
 	}
 
 	var lm int64
+	isEmpty := true
 	for range files {
 		res := <-readerC
 		if res.err != nil {
@@ -595,9 +596,18 @@ func (f *FileStore) Open() error {
 		if res.r.LastModified() > lm {
 			lm = res.r.LastModified()
 		}
-
+		isEmpty = false
+	}
+	if isEmpty {
+		if fi, err := os.Stat(f.dir); err == nil {
+			f.lastModified = fi.ModTime().UTC()
+		} else {
+			close(readerC)
+			return err
+		}
+	} else {
+		f.lastModified = time.Unix(0, lm).UTC()
 	}
-	f.lastModified = time.Unix(0, lm).UTC()
 	close(readerC)
 
 	sort.Sort(tsmReaders(f.files))