fix: stop forwarding empty response when no response was received

intercept/responses/base.go

@@ -331,6 +331,11 @@ func (r *responseCopier) readAll() ([]byte, error) {
 
 // forwardResp writes whole response as received to ResponseWriter
 func (r *responseCopier) forwardResp(w http.ResponseWriter) error {
+	// no response was received, nothing to forward
+	if !r.responseReceived.Load() {
+		return nil
+	}
+
 	w.Header().Set("Content-Type", r.responseHeaders.Get("Content-Type"))
 	w.WriteHeader(r.responseStatus)
 

intercept/responses/base_test.go

@@ -1,6 +1,7 @@
 package responses
 
 import (
+	"net/http"
 	"testing"
 	"time"
 
@@ -434,3 +435,44 @@ func TestRecordTokenUsage(t *testing.T) {
 		})
 	}
 }
+
+type mockResponseWriter struct {
+	headerCalled      bool
+	writeCalled       bool
+	writeHeaderCalled bool
+}
+
+func (mrw *mockResponseWriter) Header() http.Header {
+	mrw.headerCalled = true
+	return http.Header{}
+}
+
+func (mrw *mockResponseWriter) Write([]byte) (int, error) {
+	mrw.writeCalled = true
+	return 0, nil
+}
+
+func (mrw *mockResponseWriter) WriteHeader(statusCode int) {
+	mrw.writeHeaderCalled = true
+}
+
+func TestResponseCopierDoesntSendIfNoResponseReceived(t *testing.T) {
+	mrw := mockResponseWriter{}
+
+	respCopy := responseCopier{}
+	body := "test_body"
+	respCopy.buff.Write([]byte(body))
+
+	respCopy.forwardResp(&mrw)
+	require.False(t, mrw.headerCalled)
+	require.False(t, mrw.writeCalled)
+	require.False(t, mrw.writeHeaderCalled)
+
+	// after response is received data is forwarded
+	respCopy.responseReceived.Store(true)
+
+	respCopy.forwardResp(&mrw)
+	require.True(t, mrw.headerCalled)
+	require.True(t, mrw.writeCalled)
+	require.True(t, mrw.writeHeaderCalled)
+}

intercept/responses/blocking.go

@@ -3,6 +3,7 @@ package responses
 import (
 	"context"
 	"errors"
+	"fmt"
 	"net/http"
 	"time"
 
@@ -102,6 +103,7 @@ func (i *BlockingResponsesInterceptor) ProcessRequest(w http.ResponseWriter, r *
 	if upstreamErr != nil && !respCopy.responseReceived.Load() {
 		// no response received from upstream, return custom error
 		i.sendCustomErr(ctx, w, http.StatusInternalServerError, upstreamErr)
+		return fmt.Errorf("failed to connect to upstream: %w", upstreamErr)
 	}
 
 	err = respCopy.forwardResp(w)