Code review updates

- Fixes ENG-1903

.changeset/wicked-foxes-cry.md

+---
+'@eth-optimism/proxyd': minor
+---
+
+Add integration tests and batching

go/proxyd/integration_tests/failover_test.go

@@ -11,8 +11,10 @@ import (
 	"time"
 )
 
-const goodResponse = "{\"jsonrpc\": \"2.0\", \"result\": \"hello\", \"id\": 999}"
-const noBackendsResponse = "{\"error\":{\"code\":-32011,\"message\":\"no backends available for method\"},\"id\":999,\"jsonrpc\":\"2.0\"}"
+const (
+	goodResponse = `{"jsonrpc": "2.0", "result": "hello", "id": 999}`
+	noBackendsResponse = `{"error":{"code":-32011,"message":"no backends available for method"},"id":999,"jsonrpc":"2.0"}`
+)
 
 func TestFailover(t *testing.T) {
 	goodBackend := NewMockBackend(SingleResponseHandler(200, goodResponse))
@@ -82,7 +84,7 @@ func TestFailover(t *testing.T) {
 		})
 	}
 
-	t.Run("backend times out", func(t *testing.T) {
+	t.Run("backend times out and falls back to another", func(t *testing.T) {
 		badBackend.SetHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			time.Sleep(2 * time.Second)
 			w.Write([]byte("{}"))
@@ -93,12 +95,11 @@ func TestFailover(t *testing.T) {
 		RequireEqualJSON(t, []byte(goodResponse), res)
 		require.Equal(t, 1, len(badBackend.Requests()))
 		require.Equal(t, 1, len(goodBackend.Requests()))
-	})
-
-	t.Run("works with a batch request", func(t *testing.T) {
 		goodBackend.Reset()
 		badBackend.Reset()
+	})
 
+	t.Run("works with a batch request", func(t *testing.T) {
 		badBackend.SetHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			w.WriteHeader(500)
 		}))
@@ -155,7 +156,7 @@ func TestRetries(t *testing.T) {
 func TestOutOfServiceInterval(t *testing.T) {
 	goodBackend := NewMockBackend(SingleResponseHandler(200, goodResponse))
 	defer goodBackend.Close()
-	badBackend := NewMockBackend(SingleResponseHandler(200, goodResponse))
+	badBackend := NewMockBackend(nil)
 	defer badBackend.Close()
 
 	require.NoError(t, os.Setenv("GOOD_BACKEND_RPC_URL", goodBackend.URL()))

go/proxyd/integration_tests/validation_test.go

@@ -9,12 +9,12 @@ import (
 )
 
 const (
-	notWhitelistedResponse        = "{\"jsonrpc\":\"2.0\",\"error\":{\"code\":-32001,\"message\":\"rpc method is not whitelisted\"},\"id\":999}"
-	parseErrResponse              = "{\"jsonrpc\":\"2.0\",\"error\":{\"code\":-32700,\"message\":\"parse error\"},\"id\":null}"
-	invalidJSONRPCVersionResponse = "{\"error\":{\"code\":-32601,\"message\":\"invalid JSON-RPC version\"},\"id\":null,\"jsonrpc\":\"2.0\"}"
-	invalidIDResponse             = "{\"error\":{\"code\":-32601,\"message\":\"invalid ID\"},\"id\":null,\"jsonrpc\":\"2.0\"}"
-	invalidMethodResponse         = "{\"error\":{\"code\":-32601,\"message\":\"no method specified\"},\"id\":null,\"jsonrpc\":\"2.0\"}"
-	invalidBatchLenResponse       = "{\"error\":{\"code\":-32601,\"message\":\"must specify at least one batch call\"},\"id\":null,\"jsonrpc\":\"2.0\"}"
+	notWhitelistedResponse        = `{"jsonrpc":"2.0","error":{"code":-32001,"message":"rpc method is not whitelisted"},"id":999}`
+	parseErrResponse              = `{"jsonrpc":"2.0","error":{"code":-32700,"message":"parse error"},"id":null}`
+	invalidJSONRPCVersionResponse = `{"error":{"code":-32601,"message":"invalid JSON-RPC version"},"id":null,"jsonrpc":"2.0"}`
+	invalidIDResponse             = `{"error":{"code":-32601,"message":"invalid ID"},"id":null,"jsonrpc":"2.0"}`
+	invalidMethodResponse         = `{"error":{"code":-32601,"message":"no method specified"},"id":null,"jsonrpc":"2.0"}`
+	invalidBatchLenResponse       = `{"error":{"code":-32601,"message":"must specify at least one batch call"},"id":null,"jsonrpc":"2.0"}`
 )
 
 func TestSingleRPCValidation(t *testing.T) {

go/proxyd/server.go

@@ -338,6 +338,7 @@ func writeRPCRes(ctx context.Context, w http.ResponseWriter, res *RPCRes) {
 		statusCode = res.Error.HTTPErrorCode
 	}
 
+	w.Header().Set("content-type", "application/json")
 	w.WriteHeader(statusCode)
 	ww := &recordLenWriter{Writer: w}
 	enc := json.NewEncoder(ww)
@@ -351,6 +352,7 @@ func writeRPCRes(ctx context.Context, w http.ResponseWriter, res *RPCRes) {
 }
 
 func writeBatchRPCRes(ctx context.Context, w http.ResponseWriter, res []*RPCRes) {
+	w.Header().Set("content-type", "application/json")
 	w.WriteHeader(200)
 	ww := &recordLenWriter{Writer: w}
 	enc := json.NewEncoder(ww)