nsqd: save both old and new metadata filenames

and when loading, if both exist, ensure they match

this makes rolling-back possible without losing messages

apps/nsqd/nsqd.go

@@ -218,8 +218,11 @@ func (p *program) Start() error {
 	options.Resolve(opts, flagSet, cfg)
 	nsqd := nsqd.New(opts)
 
-	nsqd.LoadMetadata()
-	err := nsqd.PersistMetadata()
+	err := nsqd.LoadMetadata()
+	if err != nil {
+		log.Fatalf("ERROR: %s", err.Error())
+	}
+	err = nsqd.PersistMetadata()
 	if err != nil {
 		log.Fatalf("ERROR: failed to persist metadata - %s", err.Error())
 	}

nsqd/nsqd.go

@@ -1,6 +1,7 @@
 package nsqd
 
 import (
+	"bytes"
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/json"
@@ -267,32 +268,45 @@ type meta struct {
 	} `json:"topics"`
 }
 
-func (n *NSQD) LoadMetadata() {
+func (n *NSQD) LoadMetadata() error {
 	atomic.StoreInt32(&n.isLoading, 1)
 	defer atomic.StoreInt32(&n.isLoading, 0)
 
 	fn := path.Join(n.getOpts().DataPath, "nsqd.dat")
-	oldFn := fmt.Sprintf(path.Join(n.getOpts().DataPath, "nsqd.%d.dat"), n.getOpts().ID)
+	// old metadata filename with ID, maintained in parallel to enable roll-back
+	fnID := fmt.Sprintf(path.Join(n.getOpts().DataPath, "nsqd.%d.dat"), n.getOpts().ID)
 
 	data, err := ioutil.ReadFile(fn)
 	if err != nil {
 		if !os.IsNotExist(err) {
-			n.logf("ERROR: failed to read channel metadata from %s - %s", fn, err)
+			return fmt.Errorf("failed to read channel metadata from %s - %s", fn, err)
 		}
-		data, err = ioutil.ReadFile(oldFn)
-		if err != nil {
-			if !os.IsNotExist(err) {
-				n.logf("ERROR: failed to read channel metadata from %s - %s", fn, err)
-			}
-			return
+	}
+
+	dataID, errID := ioutil.ReadFile(fnID)
+	if errID != nil {
+		if !os.IsNotExist(errID) {
+			return fmt.Errorf("failed to read channel metadata from %s - %s", fnID, errID)
 		}
 	}
 
+	if err != nil && errID != nil {
+		return nil
+	}
+	if err == nil && errID == nil {
+		if bytes.Compare(data, dataID) != 0 {
+			return fmt.Errorf("metadata in %s and %s do not match (delete one)", fn, fnID)
+		}
+	}
+	if err != nil {
+		fn = fnID
+		data = dataID
+	}
+
 	var m meta
 	err = json.Unmarshal(data, &m)
 	if err != nil {
-		n.logf("ERROR: failed to parse metadata - %s", err)
-		return
+		return fmt.Errorf("failed to parse metadata in %s - %s", fn, err)
 	}
 
 	for _, t := range m.Topics {
@@ -316,13 +330,15 @@ func (n *NSQD) LoadMetadata() {
 			}
 		}
 	}
+	return nil
 }
 
 func (n *NSQD) PersistMetadata() error {
 	// persist metadata about what topics/channels we have
 	// so that upon restart we can get back to the same state
 	fileName := path.Join(n.getOpts().DataPath, "nsqd.dat")
-	oldFileName := fmt.Sprintf(path.Join(n.getOpts().DataPath, "nsqd.%d.dat"), n.getOpts().ID)
+	// old metadata filename with ID, maintained in parallel to enable roll-back
+	fileNameID := fmt.Sprintf(path.Join(n.getOpts().DataPath, "nsqd.%d.dat"), n.getOpts().ID)
 	n.logf("NSQ: persisting topic/channel metadata to %s", fileName)
 
 	js := make(map[string]interface{})
@@ -365,7 +381,19 @@ func (n *NSQD) PersistMetadata() error {
 	if err != nil {
 		return err
 	}
+	_, err = f.Write(data)
+	if err != nil {
+		f.Close()
+		return err
+	}
+	f.Sync()
+	f.Close()
 
+	tmpFileNameID := fmt.Sprintf("%s.%d.tmp", fileNameID, rand.Int())
+	f, err = os.OpenFile(tmpFileNameID, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
+	if err != nil {
+		return err
+	}
 	_, err = f.Write(data)
 	if err != nil {
 		f.Close()
@@ -378,11 +406,11 @@ func (n *NSQD) PersistMetadata() error {
 	if err != nil {
 		return err
 	}
-
-	err = os.Remove(oldFileName)
-	if err != nil && !os.IsNotExist(err) {
-		n.logf("NSQ: WARNING: failed to delete old metadata file %s: %s", oldFileName, err)
+	err = atomicRename(tmpFileNameID, fileNameID)
+	if err != nil {
+		return err
 	}
+	// technically should fsync DataPath here
 
 	return nil
 }