From 6946ddd22a373fb33c1f78392242294af5bc4fc4 Mon Sep 17 00:00:00 2001
From: dmathieu <42@dmathieu.com>
Date: Wed, 24 Jul 2024 09:30:15 +0200
Subject: [PATCH] acquire the lock only once when writing

---
 .../otelhttp/internal/request/resp_writer_wrapper.go | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/instrumentation/net/http/otelhttp/internal/request/resp_writer_wrapper.go b/instrumentation/net/http/otelhttp/internal/request/resp_writer_wrapper.go
index 5ddc20954dd..ca5c5e8c778 100644
--- a/instrumentation/net/http/otelhttp/internal/request/resp_writer_wrapper.go
+++ b/instrumentation/net/http/otelhttp/internal/request/resp_writer_wrapper.go
@@ -41,11 +41,11 @@ func NewRespWriterWrapper(w http.ResponseWriter, onWrite func(int64)) *RespWrite
 // Write writes the bytes array into the [ResponseWriter], and tracks the
 // number of bytes written and last error.
 func (w *RespWriterWrapper) Write(p []byte) (int, error) {
-	w.WriteHeader(http.StatusOK)
-
 	w.mu.RLock()
 	defer w.mu.RUnlock()
 
+	w.writeHeader(http.StatusOK)
+
 	n, err := w.ResponseWriter.Write(p)
 	n1 := int64(n)
 	w.OnWrite(n1)
@@ -63,6 +63,14 @@ func (w *RespWriterWrapper) WriteHeader(statusCode int) {
 	w.mu.Lock()
 	defer w.mu.Unlock()
 
+	w.writeHeader(statusCode)
+}
+
+// writeHeader persists the status code for span attribution, and propagates
+// the call to the underlying ResponseWriter.
+// It does not acquire a lock, and therefore assumes that is being handled by a
+// parent method.
+func (w *RespWriterWrapper) writeHeader(statusCode int) {
 	if !w.wroteHeader {
 		w.wroteHeader = true
 		w.statusCode = statusCode