proxyd: Unwrap single RPC batches (#3165)

* proxyd: Unwrap single RPC batches

* Update backend.go

.changeset/little-squids-invent.md

+---
+'@eth-optimism/proxyd': patch
+---
+
+Unwrap single RPC batches

proxyd/backend.go

@@ -349,7 +349,17 @@ func (b *Backend) setOffline() {
 }
 
 func (b *Backend) doForward(ctx context.Context, rpcReqs []*RPCReq, isBatch bool) ([]*RPCRes, error) {
-	body := mustMarshalJSON(rpcReqs)
+	isSingleElementBatch := len(rpcReqs) == 1
+
+	// Single element batches are unwrapped before being sent
+	// since Alchemy handles single requests better than batches.
+
+	var body []byte
+	if isSingleElementBatch {
+		body = mustMarshalJSON(rpcReqs[0])
+	} else {
+		body = mustMarshalJSON(rpcReqs)
+	}
 
 	httpReq, err := http.NewRequestWithContext(ctx, "POST", b.rpcURL, bytes.NewReader(body))
 	if err != nil {
@@ -402,12 +412,22 @@ func (b *Backend) doForward(ctx context.Context, rpcReqs []*RPCReq, isBatch bool
 	}
 
 	var res []*RPCRes
-	if err := json.Unmarshal(resB, &res); err != nil {
-		// Infura may return a single JSON-RPC response if, for example, the batch contains a request for an unsupported method
-		if responseIsNotBatched(resB) {
-			return nil, ErrBackendUnexpectedJSONRPC
+	if isSingleElementBatch {
+		var singleRes RPCRes
+		if err := json.Unmarshal(resB, &singleRes); err != nil {
+			return nil, ErrBackendBadResponse
+		}
+		res = []*RPCRes{
+			&singleRes,
+		}
+	} else {
+		if err := json.Unmarshal(resB, &res); err != nil {
+			// Infura may return a single JSON-RPC response if, for example, the batch contains a request for an unsupported method
+			if responseIsNotBatched(resB) {
+				return nil, ErrBackendUnexpectedJSONRPC
+			}
+			return nil, ErrBackendBadResponse
 		}
-		return nil, ErrBackendBadResponse
 	}
 
 	if len(rpcReqs) != len(res) {

proxyd/integration_tests/mock_backend_test.go

@@ -35,8 +35,12 @@ func SingleResponseHandler(code int, response string) http.HandlerFunc {
 }
 
 func BatchedResponseHandler(code int, responses ...string) http.HandlerFunc {
-	// all proxyd upstream requests are batched
 	return func(w http.ResponseWriter, r *http.Request) {
+		if len(responses) == 1 {
+			SingleResponseHandler(code, responses[0])(w, r)
+			return
+		}
+
 		var body string
 		body += "["
 		for i, response := range responses {

proxyd/integration_tests/ws_test.go

@@ -44,11 +44,14 @@ func TestConcurrentWSPanic(t *testing.T) {
 
 	<-readyCh
 
+	var wg sync.WaitGroup
+	wg.Add(2)
 	// spam messages
 	go func() {
 		for {
 			select {
 			case <-quitC:
+				wg.Done()
 				return
 			default:
 				_ = backendToProxyConn.WriteMessage(websocket.TextMessage, []byte("garbage"))
@@ -61,6 +64,7 @@ func TestConcurrentWSPanic(t *testing.T) {
 		for {
 			select {
 			case <-quitC:
+				wg.Done()
 				return
 			default:
 				_ = client.WriteMessage(websocket.TextMessage, []byte("{\"id\": 1, \"method\": \"eth_foo\", \"params\": [\"newHeads\"]}"))
@@ -72,6 +76,7 @@ func TestConcurrentWSPanic(t *testing.T) {
 	// concurrent write to websocket connection
 	time.Sleep(time.Second)
 	close(quitC)
+	wg.Wait()
 }
 
 type backendHandler struct {