Under the right circumstances Compact could leave a danling last msg block leading to error opening msg block file [""]: open : no such file or directory

When the compaction would not uplevel to a normal purge, but after completion all msg blocks were empty the mb.lmb was not cleared or reset properly.

Signed-off-by: Derek Collison <derek@nats.io>

server/filestore.go

@@ -4911,6 +4911,11 @@ SKIP:
 		}
 		// Update blks slice.
 		fs.blks = copyMsgBlocks(fs.blks[deleted:])
+		if lb := len(fs.blks); lb == 0 {
+			fs.lmb = nil
+		} else {
+			fs.lmb = fs.blks[lb-1]
+		}
 	}
 
 	// Update top level accounting.

server/filestore_test.go

@@ -4833,3 +4833,28 @@ func TestFileStoreBadFirstAndFailedExpireAfterRestart(t *testing.T) {
 	require_True(t, state.LastSeq == 9)
 	require_True(t, state.LastTime == ts)
 }
+
+func TestFileStoreCompactAllWithDanglingLMB(t *testing.T) {
+	storeDir := t.TempDir()
+
+	fs, err := newFileStore(
+		FileStoreConfig{StoreDir: storeDir},
+		StreamConfig{Name: "zzz", Subjects: []string{"foo"}, Storage: FileStorage},
+	)
+	require_NoError(t, err)
+	defer fs.Stop()
+
+	subj, msg := "foo", []byte("ZZ")
+	for i := 0; i < 100; i++ {
+		_, _, err := fs.StoreMsg(subj, nil, msg)
+		require_NoError(t, err)
+	}
+
+	fs.RemoveMsg(100)
+	purged, err := fs.Compact(100)
+	require_NoError(t, err)
+	require_True(t, purged == 99)
+
+	_, _, err = fs.StoreMsg(subj, nil, msg)
+	require_NoError(t, err)
+}