Partial fix for #2068.

This will not protect against the index file being completely removed.

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

server/filestore.go

@@ -616,6 +616,11 @@ func (mb *msgBlock) rebuildState() (*LostStreamData, error) {
 		index += rl
 	}
 
+	// For empty msg blocks make sure we recover last seq correctly based off of first.
+	if mb.msgs == 0 {
+		mb.last.seq = mb.first.seq - 1
+	}
+
 	return nil, nil
 }
 

server/filestore_test.go

@@ -2769,3 +2769,63 @@ func TestFileStoreStreamDeleteCacheBug(t *testing.T) {
 		t.Fatalf("Unexpected error looking up msg: %v", err)
 	}
 }
+
+// https://github.com/nats-io/nats-server/issues/2068
+func TestFileStoreStreamPurgeAndDirtyRestartBug(t *testing.T) {
+	storeDir := createDir(t, JetStreamStoreDir)
+	defer removeDir(t, storeDir)
+
+	fs, err := newFileStore(FileStoreConfig{StoreDir: storeDir}, StreamConfig{Name: "zzz", Storage: FileStorage})
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	defer fs.Stop()
+
+	// Load up some messages.
+	num, subj, hdr, msg := 100, "foo", []byte("name:derek"), []byte("Hello World")
+	for i := 0; i < num; i++ {
+		fs.StoreMsg(subj, hdr, msg)
+	}
+	// Now purge
+	fs.Purge()
+
+	// Snapshot state.
+	state := fs.State()
+	if state.FirstSeq != uint64(num+1) || state.LastSeq != uint64(num) {
+		t.Fatalf("Unexpected state: %+v", state)
+	}
+
+	// Now we will stop the store and corrupt the index such that on restart it will do a rebuild.
+	fs.mu.Lock()
+	lmb := fs.lmb
+	fs.mu.Unlock()
+
+	lmb.mu.RLock()
+	ifn := lmb.ifn
+	lmb.mu.RUnlock()
+
+	fs.Stop()
+
+	fd, err := os.OpenFile(ifn, os.O_RDWR, 0644)
+	if err != nil {
+		t.Fatalf("Error opening the index file: %v", err)
+	}
+	defer fd.Close()
+	fi, _ := fd.Stat()
+	if _, err = fd.WriteAt([]byte{1, 1}, fi.Size()-2); err != nil {
+		t.Fatalf("Error writing the index file: %v", err)
+	}
+	fd.Close()
+
+	// Restart
+	fs, err = newFileStore(FileStoreConfig{StoreDir: storeDir}, StreamConfig{Name: "zzz", Storage: FileStorage})
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	defer fs.Stop()
+
+	state = fs.State()
+	if state.FirstSeq != uint64(num+1) || state.LastSeq != uint64(num) {
+		t.Fatalf("Unexpected state: %+v", state)
+	}
+}