[release-branch.go1.14] net/http: deep copy Request.TransferEncoding

The existing implementation in Request.Clone() assigns the wrong
pointer to r2.TransferEncoding.

Updates #41907.
Fixes #41913.

Change-Id: I7f220a41b1b46a55d1a1005e47c6dd69478cb025
Reviewed-on: https://go-review.googlesource.com/c/go/+/261377
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Carlos Amedee <carlos@golang.org>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>

src/net/http/request.go

@@ -387,7 +387,7 @@ func (r *Request) Clone(ctx context.Context) *Request {
 	if s := r.TransferEncoding; s != nil {
 		s2 := make([]string, len(s))
 		copy(s2, s)
-		r2.TransferEncoding = s
+		r2.TransferEncoding = s2
 	}
 	r2.Form = cloneURLValues(r.Form)
 	r2.PostForm = cloneURLValues(r.PostForm)

src/net/http/request_test.go

@@ -828,6 +828,27 @@ func TestWithContextDeepCopiesURL(t *testing.T) {
 	}
 }
 
+// Ensure that Request.Clone creates a deep copy of TransferEncoding.
+// See issue 41907.
+func TestRequestCloneTransferEncoding(t *testing.T) {
+	body := strings.NewReader("body")
+	req, _ := NewRequest("POST", "https://example.org/", body)
+	req.TransferEncoding = []string{
+		"encoding1",
+	}
+
+	clonedReq := req.Clone(context.Background())
+	// modify original after deep copy
+	req.TransferEncoding[0] = "encoding2"
+
+	if req.TransferEncoding[0] != "encoding2" {
+		t.Error("expected req.TransferEncoding to be changed")
+	}
+	if clonedReq.TransferEncoding[0] != "encoding1" {
+		t.Error("expected clonedReq.TransferEncoding to be unchanged")
+	}
+}
+
 func TestNoPanicOnRoundTripWithBasicAuth_h1(t *testing.T) {
 	testNoPanicWithBasicAuth(t, h1Mode)
 }