fix: Anti-Entropy loops endlessly with empty shard (#21275) (#21290)

The anti-entropy service will loop trying to copy an empty shard to a
data node missing that shard.  This fix is one of two changes that
correctly create an empty shard on a new node. This fix will set the
LastModified date of an empty shard directory to the modification time
of that directory, instead of to the Unix epoch.

Fixes: https://github.com/influxdata/influxdb/issues/21273
(cherry picked from commit 7f300dc248)

CHANGELOG.md

@@ -1,6 +1,8 @@
 v1.8.6 [unreleased]
 -------------------
 
+-	[#21290](https://github.com/influxdata/influxdb/pull/21290): fix: Anti-Entropy loops endlessly with empty shard
+
 v1.8.5 [2021-04-19]
 -------------------
 

tsdb/engine/tsm1/file_store.go

@@ -566,6 +566,7 @@ func (f *FileStore) Open() error {
 	}
 
 	var lm int64
+	isEmpty := true
 	for range files {
 		res := <-readerC
 		if res.err != nil {
@@ -585,9 +586,18 @@ 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))