net/http: deep copy Request.URL also in Request.WithContext's copy

Despite the previously known behavior of Request.WithContext
shallow copying a request, usage of the request inside server.ServeHTTP
mutates the request's URL. This CL implements deep copying of the URL.

Fixes #20068

Change-Id: I86857d7259e23ac624d196401bf12dde401c42af
Reviewed-on: https://go-review.googlesource.com/41308
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httputil/reverseproxy_test.go

@@ -698,3 +698,41 @@ func BenchmarkServeHTTP(b *testing.B) {
 		proxy.ServeHTTP(w, r)
 	}
 }
+
+func TestServeHTTPDeepCopy(t *testing.T) {
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte("Hello Gopher!"))
+	}))
+	defer backend.Close()
+	backendURL, err := url.Parse(backend.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	type result struct {
+		before, after string
+	}
+
+	resultChan := make(chan result, 1)
+	proxyHandler := NewSingleHostReverseProxy(backendURL)
+	frontend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		before := r.URL.String()
+		proxyHandler.ServeHTTP(w, r)
+		after := r.URL.String()
+		resultChan <- result{before: before, after: after}
+	}))
+	defer frontend.Close()
+
+	want := result{before: "/", after: "/"}
+
+	res, err := frontend.Client().Get(frontend.URL)
+	if err != nil {
+		t.Fatalf("Do: %v", err)
+	}
+	res.Body.Close()
+
+	got := <-resultChan
+	if got != want {
+		t.Errorf("got = %+v; want = %+v", got, want)
+	}
+}

src/net/http/request.go

@@ -329,6 +329,14 @@ func (r *Request) WithContext(ctx context.Context) *Request {
 	r2 := new(Request)
 	*r2 = *r
 	r2.ctx = ctx
+
+	// Deep copy the URL because it isn't
+	// a map and the URL is mutable by users
+	// of WithContext.
+	r2URL := new(url.URL)
+	*r2URL = *r.URL
+	r2.URL = r2URL
+
 	return r2
 }
 

src/net/http/request_test.go

@@ -7,6 +7,7 @@ package http_test
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"encoding/base64"
 	"fmt"
 	"io"
@@ -785,6 +786,21 @@ func TestMaxBytesReaderStickyError(t *testing.T) {
 	}
 }
 
+func TestWithContextDeepCopiesURL(t *testing.T) {
+	req, err := NewRequest("POST", "https://golang.org/", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	reqCopy := req.WithContext(context.Background())
+	reqCopy.URL.Scheme = "http"
+
+	firstURL, secondURL := req.URL.String(), reqCopy.URL.String()
+	if firstURL == secondURL {
+		t.Errorf("unexpected change to original request's URL")
+	}
+}
+
 // verify that NewRequest sets Request.GetBody and that it works
 func TestNewRequestGetBody(t *testing.T) {
 	tests := []struct {