Fix missing boundary when MultipartMixed transport aggregates initial and deferred response

graphql/handler/testserver/testserver.go

@@ -44,7 +44,7 @@ func New() *TestServer {
 			switch opCtx.Operation.Operation {
 			case ast.Query:
 				ran := false
-				// If the query contains @defer, we will mimick a deferred response.
+				// If the query contains @defer, we will mimic a deferred response.
 				if strings.Contains(opCtx.RawQuery, "@defer") {
 					initialResponse := true
 					return func(context context.Context) *graphql.Response {

graphql/handler/transport/http_multipart_mixed.go

@@ -256,6 +256,13 @@ func (a *multipartResponseAggregator) flush(w http.ResponseWriter) {
 
 		writeJson(w, a.initialResponse)
 		hasNext = a.initialResponse.HasNext != nil && *a.initialResponse.HasNext
+
+		// Handle when initial is aggregated with deferred responses.
+		if len(a.deferResponses) > 0 {
+			fmt.Fprintf(w, "\r\n")
+			writeBoundary(w, a.boundary, false)
+		}
+
 		// Reset the initial response so we don't send it again
 		a.initialResponse = nil
 	}

graphql/handler/transport/http_multipart_mixed_test.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -102,7 +103,7 @@ func TestMultipartMixed(t *testing.T) {
 		return bs
 	}
 
-	t.Run("initial and incremental patches", func(t *testing.T) {
+	t.Run("initial and incremental patches un-aggregated", func(t *testing.T) {
 		handler, srv := initializeWithServer()
 		defer srv.Close()
 
@@ -167,4 +168,68 @@ func TestMultipartMixed(t *testing.T) {
 
 		wg.Wait()
 	})
+
+	t.Run("initial and incremental patches aggregated", func(t *testing.T) {
+		handler := testserver.New()
+		handler.AddTransport(transport.MultipartMixed{
+			Boundary:        "graphql",
+			DeliveryTimeout: time.Hour,
+		})
+
+		srv := httptest.NewServer(handler)
+		defer srv.Close()
+
+		var err error
+		var res *http.Response
+
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			client := &http.Client{}
+			req := createHTTPRequest(
+				srv.URL,
+				`{"query":"query { ... @defer { name } }"}`,
+			)
+			res, err = client.Do(req) //nolint:bodyclose // false positive
+		}()
+
+		handler.SendNextSubscriptionMessage()
+		handler.SendNextSubscriptionMessage()
+		handler.SendCompleteSubscriptionMessage()
+		wg.Wait()
+
+		require.NoError(t, err, "Request threw error -> %s", err)
+		defer func() {
+			require.NoError(t, res.Body.Close())
+		}()
+
+		assert.Equal(t, 200, res.StatusCode, "Request return wrong status -> %d", res.Status)
+		assert.Equal(t, "keep-alive", res.Header.Get("Connection"))
+		assert.Contains(t, res.Header.Get("Content-Type"), "multipart/mixed")
+		assert.Contains(t, res.Header.Get("Content-Type"), `boundary="graphql"`)
+
+		br := bufio.NewReader(res.Body)
+		assert.Equal(t, "--graphql\r\n", readLine(br))
+		assert.Equal(t, "Content-Type: application/json\r\n", readLine(br))
+		assert.Equal(t, "\r\n", readLine(br))
+		assert.Equal(t,
+			"{\"data\":{\"name\":null},\"hasNext\":true}\r\n",
+			readLine(br),
+		)
+
+		assert.Equal(t, "--graphql\r\n", readLine(br))
+		assert.Equal(t, "Content-Type: application/json\r\n", readLine(br))
+		assert.Equal(t, "\r\n", readLine(br))
+		assert.Equal(
+			t,
+			"{\"incremental\":[{\"data\":{\"name\":\"test\"},\"hasNext\":false}],\"hasNext\":false}\r\n",
+			readLine(br),
+		)
+
+		assert.Equal(t, "--graphql--\r\n", readLine(br))
+
+		_, err = br.ReadByte()
+		assert.Equal(t, err, io.EOF)
+	})
 }