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fix(gateway): buffer response headers to prevent superfluous WriteHeader #35

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Merged
olamide226 merged 1 commit into from
Mar 16, 2026
Merged

fix(gateway): buffer response headers to prevent superfluous WriteHeader #35

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1 change: 1 addition & 0 deletions .gitignore
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@ _cgo_gotypes.go
_cgo_export.*
_obj/
_test/
.gocache/

# ─── Node / TypeScript (sdk, cli) ─────────────────────────────────────────────
**/node_modules/
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31 changes: 26 additions & 5 deletions gateway/internal/inject/inject.go
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Expand Up @@ -65,16 +65,18 @@ func (m *Middleware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
rec := &responseRecorder{
ResponseWriter: w,
body: &bytes.Buffer{},
header: make(http.Header),
statusCode: http.StatusOK,
}

// Serve the request to the upstream handler.
m.next.ServeHTTP(rec, r)

// Check if the response is HTML.
contentType := rec.Header().Get("Content-Type")
contentType := rec.header.Get("Content-Type")
if !isHTML(contentType) {
// Not HTML — write the response as-is.
copyHeaders(w.Header(), rec.header)
w.WriteHeader(rec.statusCode)
if _, err := w.Write(rec.body.Bytes()); err != nil {
m.logger.Debug("rep.inject.write_error", "path", r.URL.Path, "error", err)
Expand All @@ -84,7 +86,7 @@ func (m *Middleware) ServeHTTP(w http.ResponseWriter, r *http.Request) {

// Decompress the body if the upstream ignored our Accept-Encoding removal.
body := rec.body.Bytes()
encoding := rec.Header().Get("Content-Encoding")
encoding := rec.header.Get("Content-Encoding")
if encoding != "" {
decompressed, err := decompressBody(body, encoding)
if err != nil {
Expand All @@ -93,6 +95,7 @@ func (m *Middleware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
"path", r.URL.Path,
"reason", "unsupported Content-Encoding: "+encoding,
)
copyHeaders(w.Header(), rec.header)
w.WriteHeader(rec.statusCode)
if _, err := w.Write(body); err != nil {
m.logger.Debug("rep.inject.write_error", "path", r.URL.Path, "error", err)
Expand All @@ -111,6 +114,8 @@ func (m *Middleware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
// Inject the REP script tag into the HTML.
injected := injectIntoHTML(body, tag)

copyHeaders(w.Header(), rec.header)

// Update Content-Length to reflect the injected content.
w.Header().Set("Content-Length", strconv.Itoa(len(injected)))

Expand Down Expand Up @@ -260,11 +265,16 @@ func isHTML(contentType string) bool {
// responseRecorder captures the upstream response for inspection.
type responseRecorder struct {
http.ResponseWriter
header http.Header
body *bytes.Buffer
statusCode int
wroteHeader bool
}

func (r *responseRecorder) Header() http.Header {
return r.header
}

func (r *responseRecorder) WriteHeader(code int) {
r.statusCode = code
r.wroteHeader = true
Expand All @@ -277,12 +287,23 @@ func (r *responseRecorder) Write(b []byte) (int, error) {

// Flush implements http.Flusher for streaming support.
func (r *responseRecorder) Flush() {
if f, ok := r.ResponseWriter.(http.Flusher); ok {
f.Flush()
}
// Intentionally do nothing. The middleware buffers the full upstream response
// before deciding whether to inject, so flushing here would prematurely commit
// headers/body to the client.
}

// ReadFrom implements io.ReaderFrom for efficient copies.
func (r *responseRecorder) ReadFrom(src io.Reader) (int64, error) {
return r.body.ReadFrom(src)
}

func copyHeaders(dst, src http.Header) {
for k := range dst {
dst.Del(k)
}
for k, values := range src {
for _, value := range values {
dst.Add(k, value)
}
}
}
51 changes: 51 additions & 0 deletions gateway/internal/inject/inject_test.go
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Expand Up @@ -191,6 +191,57 @@ func TestMiddleware_ContentLengthUpdated(t *testing.T) {
_ = expectedLen // The header value is set by the middleware.
}

func TestMiddleware_UpstreamFlushDoesNotCommitEarly(t *testing.T) {
upstream := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "text/html; charset=utf-8")
w.WriteHeader(http.StatusCreated)
if f, ok := w.(http.Flusher); ok {
f.Flush()
}
_, _ = w.Write([]byte(`<html><head></head><body>flushed</body></html>`))
})

m := New(upstream, testScriptTag, slog.Default())

req := httptest.NewRequest(http.MethodGet, "/", nil)
rec := httptest.NewRecorder()

m.ServeHTTP(rec, req)

if rec.Code != http.StatusCreated {
t.Fatalf("expected status %d, got %d", http.StatusCreated, rec.Code)
}
body := rec.Body.String()
if !strings.Contains(body, testScriptTag) {
t.Fatal("expected injected script tag in flushed HTML response")
}
if !strings.Contains(body, "flushed") {
t.Fatal("expected original body content to be preserved after flush")
}
}

func TestMiddleware_BuffersHeadersUntilWriteback(t *testing.T) {
upstream := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "text/html")
w.Header().Set("X-REP-Test", "buffered")
_, _ = w.Write([]byte(`<html><head></head><body></body></html>`))
})

m := New(upstream, testScriptTag, slog.Default())

req := httptest.NewRequest(http.MethodGet, "/", nil)
rec := httptest.NewRecorder()

m.ServeHTTP(rec, req)

if got := rec.Header().Get("X-REP-Test"); got != "buffered" {
t.Fatalf("expected buffered header to be copied back, got %q", got)
}
if got := rec.Header().Get("Content-Type"); got != "text/html" {
t.Fatalf("expected content type to survive buffering, got %q", got)
}
}

func TestIsHTML(t *testing.T) {
tests := []struct {
ct string
Expand Down
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