Reparent: add ability to watch shard data

go/vt/topo/shard.go

@@ -182,7 +182,7 @@ func (ts *Server) GetShard(ctx context.Context, keyspace, shard string) (*ShardI
 	span.Annotate("shard", shard)
 	defer span.Finish()
 
-	shardPath := path.Join(KeyspacesPath, keyspace, ShardsPath, shard, ShardFile)
+	shardPath := shardFilePath(keyspace, shard)
 	data, version, err := ts.globalCell.Get(ctx, shardPath)
 	if err != nil {
 		return nil, err
@@ -212,7 +212,7 @@ func (ts *Server) updateShard(ctx context.Context, si *ShardInfo) error {
 	if err != nil {
 		return err
 	}
-	shardPath := path.Join(KeyspacesPath, si.keyspace, ShardsPath, si.shardName, ShardFile)
+	shardPath := shardFilePath(si.keyspace, si.shardName)
 	newVersion, err := ts.globalCell.Update(ctx, shardPath, data, si.version)
 	if err != nil {
 		return err
@@ -302,7 +302,7 @@ func (ts *Server) CreateShard(ctx context.Context, keyspace, shard string) (err
 	if err != nil {
 		return err
 	}
-	shardPath := path.Join(KeyspacesPath, keyspace, ShardsPath, shard, ShardFile)
+	shardPath := shardFilePath(keyspace, shard)
 	if _, err := ts.globalCell.Create(ctx, shardPath, data); err != nil {
 		// Return error as is, we need to propagate
 		// ErrNodeExists for instance.
@@ -349,7 +349,7 @@ func (ts *Server) GetOrCreateShard(ctx context.Context, keyspace, shard string)
 // DeleteShard wraps the underlying conn.Delete
 // and dispatches the event.
 func (ts *Server) DeleteShard(ctx context.Context, keyspace, shard string) error {
-	shardPath := path.Join(KeyspacesPath, keyspace, ShardsPath, shard, ShardFile)
+	shardPath := shardFilePath(keyspace, shard)
 	if err := ts.globalCell.Delete(ctx, shardPath, nil); err != nil {
 		return err
 	}
@@ -578,3 +578,66 @@ func (ts *Server) GetTabletMapForShardByCell(ctx context.Context, keyspace, shar
 	}
 	return result, gerr
 }
+
+func shardFilePath(keyspace, shard string) string {
+	return path.Join(KeyspacesPath, keyspace, ShardsPath, shard, ShardFile)
+}
+
+// WatchShardData wraps the data we receive on the watch channel
+// The WatchShard API guarantees exactly one of Value or Err will be set.
+type WatchShardData struct {
+	Value *topodatapb.Shard
+	Err   error
+}
+
+// WatchShard will set a watch on the Shard object.
+// It has the same contract as conn.Watch, but it also unpacks the
+// contents into a Shard object
+func (ts *Server) WatchShard(ctx context.Context, keyspace, shard string) (*WatchShardData, <-chan *WatchShardData, CancelFunc) {
+	shardPath := shardFilePath(keyspace, shard)
+	current, wdChannel, cancel := ts.globalCell.Watch(ctx, shardPath)
+	if current.Err != nil {
+		return &WatchShardData{Err: current.Err}, nil, nil
+	}
+	value := &topodatapb.Shard{}
+	if err := proto.Unmarshal(current.Contents, value); err != nil {
+		// Cancel the watch, drain channel.
+		cancel()
+		for range wdChannel {
+		}
+		return &WatchShardData{Err: vterrors.Wrapf(err, "error unpacking initial Shard object")}, nil, nil
+	}
+
+	changes := make(chan *WatchShardData, 10)
+	// The background routine reads any event from the watch channel,
+	// translates it, and sends it to the caller.
+	// If cancel() is called, the underlying Watch() code will
+	// send an ErrInterrupted and then close the channel. We'll
+	// just propagate that back to our caller.
+	go func() {
+		defer close(changes)
+
+		for wd := range wdChannel {
+			if wd.Err != nil {
+				// Last error value, we're done.
+				// wdChannel will be closed right after
+				// this, no need to do anything.
+				changes <- &WatchShardData{Err: wd.Err}
+				return
+			}
+
+			value := &topodatapb.Shard{}
+			if err := proto.Unmarshal(wd.Contents, value); err != nil {
+				cancel()
+				for range wdChannel {
+				}
+				changes <- &WatchShardData{Err: vterrors.Wrapf(err, "error unpacking Shard object")}
+				return
+			}
+
+			changes <- &WatchShardData{Value: value}
+		}
+	}()
+
+	return &WatchShardData{Value: value}, changes, cancel
+}