otelhttp.NewHandler body wrapper alters default behaviour of Expect: 100-continue header handling #6908

Description

otelhttp.NewHandler adds extra body wrapper on top of r.Body: https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/instrumentation/net/http/otelhttp/handler.go#L138-L144. This changes default behavior of handling Expect: 100-continue header by Go HTTP server.

Here: https://github.com/golang/go/blob/go1.24.1/src/net/http/server.go#L1418-L1420 since r.Body is not the expectContinueReader (it is our custom wrapper) this means that we won't trigger close after reply so instead of that we will go here: https://github.com/golang/go/blob/go1.24.1/src/net/http/server.go#L1461-L1462 and we will wait for the body without sending 100 Continue response back (which means that client won't send us data until the specified timeout and thus whole operations takes seconds instead of milliseconds).

Environment

• OS: MacOS
• Architecture: arch64
• Go Version: 1.24
• otelhttp version: v0.56.0

Steps To Reproduce

1. This is toy example on the problem itself:

   package main
   
   import (
   	"bytes"
   	"crypto/rand"
   	"fmt"
   	"io"
   	"net/http"
   	"time"
   )
   
   type customBodyWrapper struct {
   	rc io.ReadCloser
   }
   
   func (cbw customBodyWrapper) Read(p []byte) (n int, err error) {
   	return cbw.rc.Read(p)
   }
   
   func (cbw customBodyWrapper) Close() error {
   	return cbw.rc.Close()
   }
   
   func startRedirectServer() {
   	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
   		r.Body = customBodyWrapper{rc: r.Body} // <- THIS IS THE PROBLEM. REMOVING THIS FIXES IT.
   		http.Redirect(w, r, "http://localhost:8082/target", http.StatusTemporaryRedirect)
   	})
   
   	fmt.Println("Redirect server listening on :8081")
   	go func() {
   		http.ListenAndServe(":8081", nil)
   	}()
   }
   
   func startTargetServer() {
   	http.HandleFunc("/target", func(w http.ResponseWriter, r *http.Request) {
   		body, _ := io.ReadAll(r.Body)
   		w.Write(body)
   	})
   
   	fmt.Println("Target server listening on :8082")
   	go func() {
   		http.ListenAndServe(":8082", nil)
   	}()
   }
   
   func sendClientRequest() {
   	client := &http.Client{
   		Transport: &http.Transport{
   			ExpectContinueTimeout: 5 * time.Second,
   		},
   	}
   
   	body := make([]byte, 5*1024*1024)
   	rand.Read(body)
   	
   	req, _ := http.NewRequest("PUT", "http://localhost:8081/", bytes.NewReader(body))
   	req.Header.Set("Expect", "100-continue")
   	req.ContentLength = int64(len(body))
   
   	resp, _ := client.Do(req)
   	defer resp.Body.Close()
   	responseBody, _ := io.ReadAll(resp.Body)
   	fmt.Printf("Code: %d, Response: %d\n", resp.StatusCode, len(responseBody))
   }
   
   func main() {
   	startRedirectServer()
   	startTargetServer()
   	time.Sleep(time.Second)
   	fmt.Println("")
   
   	sendClientRequest()
   }

2. Run this file.
3. See how long it takes with and without custom wrapper.

Expected behavior

No extra delay when handling Expect: 100-continue headers during redirect (without reading the r.Body).

Further notes

In my opinion Go handling of this header is not ideal. Expecting specific type might not be a good idea... Not sure if we should instead file a ticket in Go to improve this...

... but in the meantime one of the thing that otelhttp library could do is something like:

	bw := request.NewBodyWrapper(r.Body, readRecordFunc)
	if r.Body != nil && r.Body != http.NoBody {
+		prevBody := r.Body
		r.Body = bw
+		defer func() { r.Body = prevBody }
	}

But not sure if this actually works properly and if it doesn't breaking something else instead :D