otelhttp: Ignore informational header when persisting status

Signed-off-by: Janusz Marcinkiewicz <[email protected]>

Author: VirrageS

instrumentation/net/http/otelhttp/internal/request/resp_writer_wrapper.go

@@ -61,7 +61,7 @@ func (w *RespWriterWrapper) Write(p []byte) (int, error) {
 
 // WriteHeader persists initial statusCode for span attribution.
 // All calls to WriteHeader will be propagated to the underlying ResponseWriter
-// and will persist the statusCode from the first call.
+// and will persist the statusCode from the first call (except for informational headers).
 // Blocking consecutive calls to WriteHeader alters expected behavior and will
 // remove warning logs from net/http where developers will notice incorrect handler implementations.
 func (w *RespWriterWrapper) WriteHeader(statusCode int) {
@@ -77,9 +77,17 @@ func (w *RespWriterWrapper) WriteHeader(statusCode int) {
 // parent method.
 func (w *RespWriterWrapper) writeHeader(statusCode int) {
 	if !w.wroteHeader {
+		// Ignore informational headers.
+		// Based on https://github.com/golang/go/blob/go1.24.1/src/net/http/server.go#L1216
+		if statusCode >= 100 && statusCode <= 199 && statusCode != http.StatusSwitchingProtocols {
+			goto propagate
+		}
+		
 		w.wroteHeader = true
 		w.statusCode = statusCode
 	}
+
+propagate:
 	w.ResponseWriter.WriteHeader(statusCode)
 }
 

instrumentation/net/http/otelhttp/internal/request/resp_writer_wrapper_test.go

@@ -25,6 +25,18 @@ func TestRespWriterWriteHeader(t *testing.T) {
 	assert.Equal(t, http.StatusTeapot, rw.statusCode)
 }
 
+func TestRespWriterWriteInformationalHeader(t *testing.T) {
+	rw := NewRespWriterWrapper(&httptest.ResponseRecorder{}, func(int64) {})
+
+	rw.WriteHeader(http.StatusContinue)
+	assert.Equal(t, http.StatusOK, rw.statusCode)
+	assert.False(t, rw.wroteHeader)
+
+	rw.WriteHeader(http.StatusGone)
+	assert.Equal(t, http.StatusGone, rw.statusCode)
+	assert.True(t, rw.wroteHeader)
+}
+
 func TestRespWriterFlush(t *testing.T) {
 	rw := NewRespWriterWrapper(&httptest.ResponseRecorder{}, func(int64) {})