http: make HEAD client request follow redirects

HEAD requests should in my opinion have the ability to follow redirects
like the implementation of GET requests does. My use case is polling
several thousand severs to check if they respond with 200 status codes.
Using GET requests is neither efficient in running time of the task nor
for bandwidth consumption.

This suggested patch changes the return signature of http.Head() to match
that of http.Get(), providing the final URL in a redirect chain.

`curl -IL http://google.com` follows redirects with HEAD requests just fine.

Fixes #1806.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4517058

src/pkg/http/client.go

@@ -144,6 +144,10 @@ func Get(url string) (r *Response, err os.Error) {
 //
 // Caller should close r.Body when done reading from it.
 func (c *Client) Get(url string) (r *Response, err os.Error) {
+	return c.sendFollowingRedirects("GET", url)
+}
+
+func (c *Client) sendFollowingRedirects(method, url string) (r *Response, err os.Error) {
 	// TODO: if/when we add cookie support, the redirected request shouldn't
 	// necessarily supply the same cookies as the original.
 	var base *URL
@@ -155,7 +159,7 @@ func (c *Client) Get(url string) (r *Response, err os.Error) {
 
 	for redirect := 0; ; redirect++ {
 		var req Request
-		req.Method = "GET"
+		req.Method = method
 		req.Header = make(Header)
 		if base == nil {
 			req.URL, err = ParseURL(url)
@@ -195,7 +199,7 @@ func (c *Client) Get(url string) (r *Response, err os.Error) {
 		return
 	}
 
-	err = &URLError{"Get", url, err}
+	err = &URLError{method[0:1] + strings.ToLower(method[1:]), url, err}
 	return
 }
 
@@ -283,19 +287,28 @@ func urlencode(data map[string]string) (b *bytes.Buffer) {
 	return bytes.NewBuffer([]byte(EncodeQuery(m)))
 }
 
-// Head issues a HEAD to the specified URL.
+// Head issues a HEAD to the specified URL.  If the response is one of the
+// following redirect codes, Head follows the redirect after calling the
+// Client's CheckRedirect function.
+//
+//    301 (Moved Permanently)
+//    302 (Found)
+//    303 (See Other)
+//    307 (Temporary Redirect)
 //
 // Head is a wrapper around DefaultClient.Head
 func Head(url string) (r *Response, err os.Error) {
 	return DefaultClient.Head(url)
 }
 
-// Head issues a HEAD to the specified URL.
+// Head issues a HEAD to the specified URL.  If the response is one of the
+// following redirect codes, Head follows the redirect after calling the
+// Client's CheckRedirect function.
+//
+//    301 (Moved Permanently)
+//    302 (Found)
+//    303 (See Other)
+//    307 (Temporary Redirect)
 func (c *Client) Head(url string) (r *Response, err os.Error) {
-	var req Request
-	req.Method = "HEAD"
-	if req.URL, err = ParseURL(url); err != nil {
-		return
-	}
-	return send(&req, c.Transport)
+	return c.sendFollowingRedirects("HEAD", url)
 }

src/pkg/http/client_test.go

@@ -101,6 +101,12 @@ func TestRedirects(t *testing.T) {
 		t.Errorf("with default client, expected error %q, got %q", e, g)
 	}
 
+	// HEAD request should also have the ability to follow redirects.
+	_, err = c.Head(ts.URL)
+	if e, g := "Head /?n=10: stopped after 10 redirects", fmt.Sprintf("%v", err); e != g {
+		t.Errorf("with default client, expected error %q, got %q", e, g)
+	}
+
 	var checkErr os.Error
 	var lastVia []*Request
 	c = &Client{CheckRedirect: func(_ *Request, via []*Request) os.Error {