net/http: make Client propagate Request.Cancel over redirected requests

On HTTP redirect, the HTTP client creates a new request and don't copy
over the Cancel channel. This prevents any redirected request from being
cancelled.

Fixes #14053

Change-Id: I467cdd4aadcae8351b6e9733fc582b7985b8b9d3
Reviewed-on: https://go-review.googlesource.com/18810Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/client.go

@@ -445,6 +445,7 @@ func (c *Client) doFollowingRedirects(ireq *Request, shouldRedirect func(int) bo
 	for redirect := 0; ; redirect++ {
 		if redirect != 0 {
 			nreq := new(Request)
+			nreq.Cancel = ireq.Cancel
 			nreq.Method = ireq.Method
 			if ireq.Method == "POST" || ireq.Method == "PUT" {
 				nreq.Method = "GET"

src/net/http/client_test.go

@@ -240,7 +240,9 @@ func TestClientRedirects(t *testing.T) {
 
 	var checkErr error
 	var lastVia []*Request
-	c = &Client{CheckRedirect: func(_ *Request, via []*Request) error {
+	var lastReq *Request
+	c = &Client{CheckRedirect: func(req *Request, via []*Request) error {
+		lastReq = req
 		lastVia = via
 		return checkErr
 	}}
@@ -260,6 +262,20 @@ func TestClientRedirects(t *testing.T) {
 		t.Errorf("expected lastVia to have contained %d elements; got %d", e, g)
 	}
 
+	// Test that Request.Cancel is propagated between requests (Issue 14053)
+	creq, _ := NewRequest("HEAD", ts.URL, nil)
+	cancel := make(chan struct{})
+	creq.Cancel = cancel
+	if _, err := c.Do(creq); err != nil {
+		t.Fatal(err)
+	}
+	if lastReq == nil {
+		t.Fatal("didn't see redirect")
+	}
+	if lastReq.Cancel != cancel {
+		t.Errorf("expected lastReq to have the cancel channel set on the inital req")
+	}
+
 	checkErr = errors.New("no redirects allowed")
 	res, err = c.Get(ts.URL)
 	if urlError, ok := err.(*url.Error); !ok || urlError.Err != checkErr {