otelhttp: Ignore informational response status codes when persisting status

CHANGELOG.md

@@ -11,6 +11,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Changed
 
 - Jaeger remote sampler's probabilistic strategy now uses the same sampling algorithm as `trace.TraceIDRatioBased` in `go.opentelemetry.io/contrib/samplers/jaegerremote`. (#6892)
+- Ignore informational response status codes (`100-199`) when storing the HTTP status code in `go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp` and `go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux`. (#6913)
 
 ### Removed
 

instrumentation/github.com/gorilla/mux/otelmux/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 response status codes).
 // 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) {
@@ -77,6 +77,13 @@ func (w *RespWriterWrapper) WriteHeader(statusCode int) {
 // parent method.
 func (w *RespWriterWrapper) writeHeader(statusCode int) {
 	if !w.wroteHeader {
+		// Ignore informational response status codes.
+		// 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 {
+			w.ResponseWriter.WriteHeader(statusCode)
+			return
+		}
+
 		w.wroteHeader = true
 		w.statusCode = statusCode
 	}

instrumentation/github.com/gorilla/mux/otelmux/internal/request/resp_writer_wrapper_test.go

@@ -25,6 +25,18 @@ func TestRespWriterWriteHeader(t *testing.T) {
 	assert.Equal(t, http.StatusTeapot, rw.statusCode)
 }
 
+func TestRespWriterWriteInformationalStatusCode(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) {})
 

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 response status codes).
 // 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) {
@@ -77,6 +77,13 @@ func (w *RespWriterWrapper) WriteHeader(statusCode int) {
 // parent method.
 func (w *RespWriterWrapper) writeHeader(statusCode int) {
 	if !w.wroteHeader {
+		// Ignore informational response status codes.
+		// 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 {
+			w.ResponseWriter.WriteHeader(statusCode)
+			return
+		}
+
 		w.wroteHeader = true
 		w.statusCode = 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 TestRespWriterWriteInformationalStatusCode(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) {})
 

internal/shared/request/resp_writer_wrapper.go.tmpl

@@ -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 response status codes).
 // 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) {
@@ -77,6 +77,13 @@ func (w *RespWriterWrapper) WriteHeader(statusCode int) {
 // parent method.
 func (w *RespWriterWrapper) writeHeader(statusCode int) {
 	if !w.wroteHeader {
+		// Ignore informational response status codes.
+		// 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 {
+			w.ResponseWriter.WriteHeader(statusCode)
+			return
+		}
+
 		w.wroteHeader = true
 		w.statusCode = statusCode
 	}

internal/shared/request/resp_writer_wrapper_test.go.tmpl

@@ -25,6 +25,18 @@ func TestRespWriterWriteHeader(t *testing.T) {
 	assert.Equal(t, http.StatusTeapot, rw.statusCode)
 }
 
+func TestRespWriterWriteInformationalStatusCode(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) {})