src/net/http/client.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// HTTP client. See RFC 7230 through 7235.
//
// This is the high-level Client interface.
// The low-level implementation is in transport.go.

package http

import (
	"crypto/tls"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"net/url"
	"sort"
	"strings"
	"sync"
	"time"
)

// A Client is an HTTP client. Its zero value (DefaultClient) is a
// usable client that uses DefaultTransport.
//
// The Client's Transport typically has internal state (cached TCP
// connections), so Clients should be reused instead of created as
// needed. Clients are safe for concurrent use by multiple goroutines.
//
// A Client is higher-level than a RoundTripper (such as Transport)
// and additionally handles HTTP details such as cookies and
// redirects.
//
// When following redirects, the Client will forward all headers set on the
// initial Request except:
//
// • when forwarding sensitive headers like "Authorization",
// "WWW-Authenticate", and "Cookie" to untrusted targets.
// These headers will be ignored when following a redirect to a domain
// that is not a subdomain match or exact match of the initial domain.
// For example, a redirect from "foo.com" to either "foo.com" or "sub.foo.com"
// will forward the sensitive headers, but a redirect to "bar.com" will not.
//
// • when forwarding the "Cookie" header with a non-nil cookie Jar.
// Since each redirect may mutate the state of the cookie jar,
// a redirect may possibly alter a cookie set in the initial request.
// When forwarding the "Cookie" header, any mutated cookies will be omitted,
// with the expectation that the Jar will insert those mutated cookies
// with the updated values (assuming the origin matches).
// If Jar is nil, the initial cookies are forwarded without change.
//
type Client struct {
	// Transport specifies the mechanism by which individual
	// HTTP requests are made.
	// If nil, DefaultTransport is used.
	Transport RoundTripper

	// CheckRedirect specifies the policy for handling redirects.
	// If CheckRedirect is not nil, the client calls it before
	// following an HTTP redirect. The arguments req and via are
	// the upcoming request and the requests made already, oldest
	// first. If CheckRedirect returns an error, the Client's Get
	// method returns both the previous Response (with its Body
	// closed) and CheckRedirect's error (wrapped in a url.Error)
	// instead of issuing the Request req.
	// As a special case, if CheckRedirect returns ErrUseLastResponse,
	// then the most recent response is returned with its body
	// unclosed, along with a nil error.
	//
	// If CheckRedirect is nil, the Client uses its default policy,
	// which is to stop after 10 consecutive requests.
	CheckRedirect func(req *Request, via []*Request) error

	// Jar specifies the cookie jar.
	//
	// The Jar is used to insert relevant cookies into every
	// outbound Request and is updated with the cookie values
	// of every inbound Response. The Jar is consulted for every
	// redirect that the Client follows.
	//
	// If Jar is nil, cookies are only sent if they are explicitly
	// set on the Request.
	Jar CookieJar

	// Timeout specifies a time limit for requests made by this
	// Client. The timeout includes connection time, any
	// redirects, and reading the response body. The timer remains
	// running after Get, Head, Post, or Do return and will
	// interrupt reading of the Response.Body.
	//
	// A Timeout of zero means no timeout.
	//
	// The Client cancels requests to the underlying Transport
	// using the Request.Cancel mechanism. Requests passed
	// to Client.Do may still set Request.Cancel; both will
	// cancel the request.
	//
	// For compatibility, the Client will also use the deprecated
	// CancelRequest method on Transport if found. New
	// RoundTripper implementations should use Request.Cancel
	// instead of implementing CancelRequest.
	Timeout time.Duration
}

// DefaultClient is the default Client and is used by Get, Head, and Post.
var DefaultClient = &Client{}

// RoundTripper is an interface representing the ability to execute a
// single HTTP transaction, obtaining the Response for a given Request.
//
// A RoundTripper must be safe for concurrent use by multiple
// goroutines.
type RoundTripper interface {
	// RoundTrip executes a single HTTP transaction, returning
	// a Response for the provided Request.
	//
	// RoundTrip should not attempt to interpret the response. In
	// particular, RoundTrip must return err == nil if it obtained
	// a response, regardless of the response's HTTP status code.
	// A non-nil err should be reserved for failure to obtain a
	// response. Similarly, RoundTrip should not attempt to
	// handle higher-level protocol details such as redirects,
	// authentication, or cookies.
	//
	// RoundTrip should not modify the request, except for
	// consuming and closing the Request's Body. RoundTrip may
	// read fields of the request in a separate goroutine. Callers
	// should not mutate the request until the Response's Body has
	// been closed.
	//
	// RoundTrip must always close the body, including on errors,
	// but depending on the implementation may do so in a separate
	// goroutine even after RoundTrip returns. This means that
	// callers wanting to reuse the body for subsequent requests
	// must arrange to wait for the Close call before doing so.
	//
	// The Request's URL and Header fields must be initialized.
	RoundTrip(*Request) (*Response, error)
}

// refererForURL returns a referer without any authentication info or
// an empty string if lastReq scheme is https and newReq scheme is http.
func refererForURL(lastReq, newReq *url.URL) string {
	// https://tools.ietf.org/html/rfc7231#section-5.5.2
	//   "Clients SHOULD NOT include a Referer header field in a
	//    (non-secure) HTTP request if the referring page was
	//    transferred with a secure protocol."
	if lastReq.Scheme == "https" && newReq.Scheme == "http" {
		return ""
	}
	referer := lastReq.String()
	if lastReq.User != nil {
		// This is not very efficient, but is the best we can
		// do without:
		// - introducing a new method on URL
		// - creating a race condition
		// - copying the URL struct manually, which would cause
		//   maintenance problems down the line
		auth := lastReq.User.String() + "@"
		referer = strings.Replace(referer, auth, "", 1)
	}
	return referer
}

// didTimeout is non-nil only if err != nil.
func (c *Client) send(req *Request, deadline time.Time) (resp *Response, didTimeout func() bool, err error) {
	if c.Jar != nil {
		for _, cookie := range c.Jar.Cookies(req.URL) {
			req.AddCookie(cookie)
		}
	}
	resp, didTimeout, err = send(req, c.transport(), deadline)
	if err != nil {
		return nil, didTimeout, err
	}
	if c.Jar != nil {
		if rc := resp.Cookies(); len(rc) > 0 {
			c.Jar.SetCookies(req.URL, rc)
		}
	}
	return resp, nil, nil
}

func (c *Client) deadline() time.Time {
	if c.Timeout > 0 {
		return time.Now().Add(c.Timeout)
	}
	return time.Time{}
}

func (c *Client) transport() RoundTripper {
	if c.Transport != nil {
		return c.Transport
	}
	return DefaultTransport
}

// send issues an HTTP request.
// Caller should close resp.Body when done reading from it.
func send(ireq *Request, rt RoundTripper, deadline time.Time) (resp *Response, didTimeout func() bool, err error) {
	req := ireq // req is either the original request, or a modified fork

	if rt == nil {
		req.closeBody()
		return nil, alwaysFalse, errors.New("http: no Client.Transport or DefaultTransport")
	}

	if req.URL == nil {
		req.closeBody()
		return nil, alwaysFalse, errors.New("http: nil Request.URL")
	}

	if req.RequestURI != "" {
		req.closeBody()
		return nil, alwaysFalse, errors.New("http: Request.RequestURI can't be set in client requests.")
	}

	// forkReq forks req into a shallow clone of ireq the first
	// time it's called.
	forkReq := func() {
		if ireq == req {
			req = new(Request)
			*req = *ireq // shallow clone
		}
	}

	// Most the callers of send (Get, Post, et al) don't need
	// Headers, leaving it uninitialized. We guarantee to the
	// Transport that this has been initialized, though.
	if req.Header == nil {
		forkReq()
		req.Header = make(Header)
	}

	if u := req.URL.User; u != nil && req.Header.Get("Authorization") == "" {
		username := u.Username()
		password, _ := u.Password()
		forkReq()
		req.Header = cloneHeader(ireq.Header)
		req.Header.Set("Authorization", "Basic "+basicAuth(username, password))
	}

	if !deadline.IsZero() {
		forkReq()
	}
	stopTimer, didTimeout := setRequestCancel(req, rt, deadline)

	resp, err = rt.RoundTrip(req)
	if err != nil {
		stopTimer()
		if resp != nil {
			log.Printf("RoundTripper returned a response & error; ignoring response")
		}
		if tlsErr, ok := err.(tls.RecordHeaderError); ok {
			// If we get a bad TLS record header, check to see if the
			// response looks like HTTP and give a more helpful error.
			// See golang.org/issue/11111.
			if string(tlsErr.RecordHeader[:]) == "HTTP/" {
				err = errors.New("http: server gave HTTP response to HTTPS client")
			}
		}
		return nil, didTimeout, err
	}
	if !deadline.IsZero() {
		resp.Body = &cancelTimerBody{
			stop:          stopTimer,
			rc:            resp.Body,
			reqDidTimeout: didTimeout,
		}
	}
	return resp, nil, nil
}

// setRequestCancel sets the Cancel field of req, if deadline is
// non-zero. The RoundTripper's type is used to determine whether the legacy
// CancelRequest behavior should be used.
//
// As background, there are three ways to cancel a request:
// First was Transport.CancelRequest. (deprecated)
// Second was Request.Cancel (this mechanism).
// Third was Request.Context.
func setRequestCancel(req *Request, rt RoundTripper, deadline time.Time) (stopTimer func(), didTimeout func() bool) {
	if deadline.IsZero() {
		return nop, alwaysFalse
	}

	initialReqCancel := req.Cancel // the user's original Request.Cancel, if any

	cancel := make(chan struct{})
	req.Cancel = cancel

	doCancel := func() {
		// The newer way (the second way in the func comment):
		close(cancel)

		// The legacy compatibility way, used only
		// for RoundTripper implementations written
		// before Go 1.5 or Go 1.6.
		type canceler interface {
			CancelRequest(*Request)
		}
		switch v := rt.(type) {
		case *Transport, *http2Transport:
			// Do nothing. The net/http package's transports
			// support the new Request.Cancel channel
		case canceler:
			v.CancelRequest(req)
		}
	}

	stopTimerCh := make(chan struct{})
	var once sync.Once
	stopTimer = func() { once.Do(func() { close(stopTimerCh) }) }

	timer := time.NewTimer(time.Until(deadline))
	var timedOut atomicBool

	go func() {
		select {
		case <-initialReqCancel:
			doCancel()
			timer.Stop()
		case <-timer.C:
			timedOut.setTrue()
			doCancel()
		case <-stopTimerCh:
			timer.Stop()
		}
	}()

	return stopTimer, timedOut.isSet
}

// See 2 (end of page 4) http://www.ietf.org/rfc/rfc2617.txt
// "To receive authorization, the client sends the userid and password,
// separated by a single colon (":") character, within a base64
// encoded string in the credentials."
// It is not meant to be urlencoded.
func basicAuth(username, password string) string {
	auth := username + ":" + password
	return base64.StdEncoding.EncodeToString([]byte(auth))
}

// Get issues a GET to the specified URL. If the response is one of
// the following redirect codes, Get follows the redirect, up to a
// maximum of 10 redirects:
//
//    301 (Moved Permanently)
//    302 (Found)
//    303 (See Other)
//    307 (Temporary Redirect)
//    308 (Permanent Redirect)
//
// An error is returned if there were too many redirects or if there
// was an HTTP protocol error. A non-2xx response doesn't cause an
// error.
//
// When err is nil, resp always contains a non-nil resp.Body.
// Caller should close resp.Body when done reading from it.
//
// Get is a wrapper around DefaultClient.Get.
//
// To make a request with custom headers, use NewRequest and
// DefaultClient.Do.
func Get(url string) (resp *Response, err error) {
	return DefaultClient.Get(url)
}

// Get issues a GET to the specified URL. If the response is one of the
// following redirect codes, Get follows the redirect after calling the
// Client's CheckRedirect function:
//
//    301 (Moved Permanently)
//    302 (Found)
//    303 (See Other)
//    307 (Temporary Redirect)
//    308 (Permanent Redirect)
//
// An error is returned if the Client's CheckRedirect function fails
// or if there was an HTTP protocol error. A non-2xx response doesn't
// cause an error.
//
// When err is nil, resp always contains a non-nil resp.Body.
// Caller should close resp.Body when done reading from it.
//
// To make a request with custom headers, use NewRequest and Client.Do.
func (c *Client) Get(url string) (resp *Response, err error) {
	req, err := NewRequest("GET", url, nil)
	if err != nil {
		return nil, err
	}
	return c.Do(req)
}

func alwaysFalse() bool { return false }

// ErrUseLastResponse can be returned by Client.CheckRedirect hooks to
// control how redirects are processed. If returned, the next request
// is not sent and the most recent response is returned with its body
// unclosed.
var ErrUseLastResponse = errors.New("net/http: use last response")

// checkRedirect calls either the user's configured CheckRedirect
// function, or the default.
func (c *Client) checkRedirect(req *Request, via []*Request) error {
	fn := c.CheckRedirect
	if fn == nil {
		fn = defaultCheckRedirect
	}
	return fn(req, via)
}

// redirectBehavior describes what should happen when the
// client encounters a 3xx status code from the server
func redirectBehavior(reqMethod string, resp *Response, ireq *Request) (redirectMethod string, shouldRedirect, includeBody bool) {
	switch resp.StatusCode {
	case 301, 302, 303:
		redirectMethod = reqMethod
		shouldRedirect = true
		includeBody = false

		// RFC 2616 allowed automatic redirection only with GET and
		// HEAD requests. RFC 7231 lifts this restriction, but we still
		// restrict other methods to GET to maintain compatibility.
		// See Issue 18570.
		if reqMethod != "GET" && reqMethod != "HEAD" {
			redirectMethod = "GET"
		}
	case 307, 308:
		redirectMethod = reqMethod
		shouldRedirect = true
		includeBody = true

		// Treat 307 and 308 specially, since they're new in
		// Go 1.8, and they also require re-sending the request body.
		if resp.Header.Get("Location") == "" {
			// 308s have been observed in the wild being served
			// without Location headers. Since Go 1.7 and earlier
			// didn't follow these codes, just stop here instead
			// of returning an error.
			// See Issue 17773.
			shouldRedirect = false
			break
		}
		if ireq.GetBody == nil && ireq.outgoingLength() != 0 {
			// We had a request body, and 307/308 require
			// re-sending it, but GetBody is not defined. So just
			// return this response to the user instead of an
			// error, like we did in Go 1.7 and earlier.
			shouldRedirect = false
		}
	}
	return redirectMethod, shouldRedirect, includeBody
}

// Do sends an HTTP request and returns an HTTP response, following
// policy (such as redirects, cookies, auth) as configured on the
// client.
//
// An error is returned if caused by client policy (such as
// CheckRedirect), or failure to speak HTTP (such as a network
// connectivity problem). A non-2xx status code doesn't cause an
// error.
//
// If the returned error is nil, the Response will contain a non-nil
// Body which the user is expected to close. If the Body is not
// closed, the Client's underlying RoundTripper (typically Transport)
// may not be able to re-use a persistent TCP connection to the server
// for a subsequent "keep-alive" request.
//
// The request Body, if non-nil, will be closed by the underlying
// Transport, even on errors.
//
// On error, any Response can be ignored. A non-nil Response with a
// non-nil error only occurs when CheckRedirect fails, and even then
// the returned Response.Body is already closed.
//
// Generally Get, Post, or PostForm will be used instead of Do.
//
// If the server replies with a redirect, the Client first uses the
// CheckRedirect function to determine whether the redirect should be
// followed. If permitted, a 301, 302, or 303 redirect causes
// subsequent requests to use HTTP method GET
// (or HEAD if the original request was HEAD), with no body.
// A 307 or 308 redirect preserves the original HTTP method and body,
// provided that the Request.GetBody function is defined.
// The NewRequest function automatically sets GetBody for common
// standard library body types.
func (c *Client) Do(req *Request) (*Response, error) {
	if req.URL == nil {
		req.closeBody()
		return nil, errors.New("http: nil Request.URL")
	}

	var (
		deadline      = c.deadline()
		reqs          []*Request
		resp          *Response
		copyHeaders   = c.makeHeadersCopier(req)
		reqBodyClosed = false // have we closed the current req.Body?

		// Redirect behavior:
		redirectMethod string
		includeBody    bool
	)
	uerr := func(err error) error {
		// the body may have been closed already by c.send()
		if !reqBodyClosed {
			req.closeBody()
		}
		method := valueOrDefault(reqs[0].Method, "GET")
		var urlStr string
		if resp != nil && resp.Request != nil {
			urlStr = stripPassword(resp.Request.URL)
		} else {
			urlStr = stripPassword(req.URL)
		}
		return &url.Error{
			Op:  method[:1] + strings.ToLower(method[1:]),
			URL: urlStr,
			Err: err,
		}
	}
	for {
		// For all but the first request, create the next
		// request hop and replace req.
		if len(reqs) > 0 {
			loc := resp.Header.Get("Location")
			if loc == "" {
				resp.closeBody()
				return nil, uerr(fmt.Errorf("%d response missing Location header", resp.StatusCode))
			}
			u, err := req.URL.Parse(loc)
			if err != nil {
				resp.closeBody()
				return nil, uerr(fmt.Errorf("failed to parse Location header %q: %v", loc, err))
			}
			host := ""
			if req.Host != "" && req.Host != req.URL.Host {
				// If the caller specified a custom Host header and the
				// redirect location is relative, preserve the Host header
				// through the redirect. See issue #22233.
				if u, _ := url.Parse(loc); u != nil && !u.IsAbs() {
					host = req.Host
				}
			}
			ireq := reqs[0]
			req = &Request{
				Method:   redirectMethod,
				Response: resp,
				URL:      u,
				Header:   make(Header),
				Host:     host,
				Cancel:   ireq.Cancel,
				ctx:      ireq.ctx,
			}
			if includeBody && ireq.GetBody != nil {
				req.Body, err = ireq.GetBody()
				if err != nil {
					resp.closeBody()
					return nil, uerr(err)
				}
				req.ContentLength = ireq.ContentLength
			}

			// Copy original headers before setting the Referer,
			// in case the user set Referer on their first request.
			// If they really want to override, they can do it in
			// their CheckRedirect func.
			copyHeaders(req)

			// Add the Referer header from the most recent
			// request URL to the new one, if it's not https->http:
			if ref := refererForURL(reqs[len(reqs)-1].URL, req.URL); ref != "" {
				req.Header.Set("Referer", ref)
			}
			err = c.checkRedirect(req, reqs)

			// Sentinel error to let users select the
			// previous response, without closing its
			// body. See Issue 10069.
			if err == ErrUseLastResponse {
				return resp, nil
			}

			// Close the previous response's body. But
			// read at least some of the body so if it's
			// small the underlying TCP connection will be
			// re-used. No need to check for errors: if it
			// fails, the Transport won't reuse it anyway.
			const maxBodySlurpSize = 2 << 10
			if resp.ContentLength == -1 || resp.ContentLength <= maxBodySlurpSize {
				io.CopyN(ioutil.Discard, resp.Body, maxBodySlurpSize)
			}
			resp.Body.Close()

			if err != nil {
				// Special case for Go 1 compatibility: return both the response
				// and an error if the CheckRedirect function failed.
				// See https://golang.org/issue/3795
				// The resp.Body has already been closed.
				ue := uerr(err)
				ue.(*url.Error).URL = loc
				return resp, ue
			}
		}

		reqs = append(reqs, req)
		var err error
		var didTimeout func() bool
		if resp, didTimeout, err = c.send(req, deadline); err != nil {
			// c.send() always closes req.Body
			reqBodyClosed = true
			if !deadline.IsZero() && didTimeout() {
				err = &httpError{
					err:     err.Error() + " (Client.Timeout exceeded while awaiting headers)",
					timeout: true,
				}
			}
			return nil, uerr(err)
		}

		var shouldRedirect bool
		redirectMethod, shouldRedirect, includeBody = redirectBehavior(req.Method, resp, reqs[0])
		if !shouldRedirect {
			return resp, nil
		}

		req.closeBody()
	}
}

// makeHeadersCopier makes a function that copies headers from the
// initial Request, ireq. For every redirect, this function must be called
// so that it can copy headers into the upcoming Request.
func (c *Client) makeHeadersCopier(ireq *Request) func(*Request) {
	// The headers to copy are from the very initial request.
	// We use a closured callback to keep a reference to these original headers.
	var (
		ireqhdr  = ireq.Header.clone()
		icookies map[string][]*Cookie
	)
	if c.Jar != nil && ireq.Header.Get("Cookie") != "" {
		icookies = make(map[string][]*Cookie)
		for _, c := range ireq.Cookies() {
			icookies[c.Name] = append(icookies[c.Name], c)
		}
	}

	preq := ireq // The previous request
	return func(req *Request) {
		// If Jar is present and there was some initial cookies provided
		// via the request header, then we may need to alter the initial
		// cookies as we follow redirects since each redirect may end up
		// modifying a pre-existing cookie.
		//
		// Since cookies already set in the request header do not contain
		// information about the original domain and path, the logic below
		// assumes any new set cookies override the original cookie
		// regardless of domain or path.
		//
		// See https://golang.org/issue/17494
		if c.Jar != nil && icookies != nil {
			var changed bool
			resp := req.Response // The response that caused the upcoming redirect
			for _, c := range resp.Cookies() {
				if _, ok := icookies[c.Name]; ok {
					delete(icookies, c.Name)
					changed = true
				}
			}
			if changed {
				ireqhdr.Del("Cookie")
				var ss []string
				for _, cs := range icookies {
					for _, c := range cs {
						ss = append(ss, c.Name+"="+c.Value)
					}
				}
				sort.Strings(ss) // Ensure deterministic headers
				ireqhdr.Set("Cookie", strings.Join(ss, "; "))
			}
		}

		// Copy the initial request's Header values
		// (at least the safe ones).
		for k, vv := range ireqhdr {
			if shouldCopyHeaderOnRedirect(k, preq.URL, req.URL) {
				req.Header[k] = vv
			}
		}

		preq = req // Update previous Request with the current request
	}
}

func defaultCheckRedirect(req *Request, via []*Request) error {
	if len(via) >= 10 {
		return errors.New("stopped after 10 redirects")
	}
	return nil
}

// Post issues a POST to the specified URL.
//
// Caller should close resp.Body when done reading from it.
//
// If the provided body is an io.Closer, it is closed after the
// request.
//
// Post is a wrapper around DefaultClient.Post.
//
// To set custom headers, use NewRequest and DefaultClient.Do.
//
// See the Client.Do method documentation for details on how redirects
// are handled.
func Post(url, contentType string, body io.Reader) (resp *Response, err error) {
	return DefaultClient.Post(url, contentType, body)
}

// Post issues a POST to the specified URL.
//
// Caller should close resp.Body when done reading from it.
//
// If the provided body is an io.Closer, it is closed after the
// request.
//
// To set custom headers, use NewRequest and Client.Do.
//
// See the Client.Do method documentation for details on how redirects
// are handled.
func (c *Client) Post(url, contentType string, body io.Reader) (resp *Response, err error) {
	req, err := NewRequest("POST", url, body)
	if err != nil {
		return nil, err
	}
	req.Header.Set("Content-Type", contentType)
	return c.Do(req)
}

// PostForm issues a POST to the specified URL, with data's keys and
// values URL-encoded as the request body.
//
// The Content-Type header is set to application/x-www-form-urlencoded.
// To set other headers, use NewRequest and DefaultClient.Do.
//
// When err is nil, resp always contains a non-nil resp.Body.
// Caller should close resp.Body when done reading from it.
//
// PostForm is a wrapper around DefaultClient.PostForm.
//
// See the Client.Do method documentation for details on how redirects
// are handled.
func PostForm(url string, data url.Values) (resp *Response, err error) {
	return DefaultClient.PostForm(url, data)
}

// PostForm issues a POST to the specified URL,
// with data's keys and values URL-encoded as the request body.
//
// The Content-Type header is set to application/x-www-form-urlencoded.
// To set other headers, use NewRequest and Client.Do.
//
// When err is nil, resp always contains a non-nil resp.Body.
// Caller should close resp.Body when done reading from it.
//
// See the Client.Do method documentation for details on how redirects
// are handled.
func (c *Client) PostForm(url string, data url.Values) (resp *Response, err error) {
	return c.Post(url, "application/x-www-form-urlencoded", strings.NewReader(data.Encode()))
}

// Head issues a HEAD to the specified URL. If the response is one of
// the following redirect codes, Head follows the redirect, up to a
// maximum of 10 redirects:
//
//    301 (Moved Permanently)
//    302 (Found)
//    303 (See Other)
//    307 (Temporary Redirect)
//    308 (Permanent Redirect)
//
// Head is a wrapper around DefaultClient.Head
func Head(url string) (resp *Response, err error) {
	return DefaultClient.Head(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)
//    308 (Permanent Redirect)
func (c *Client) Head(url string) (resp *Response, err error) {
	req, err := NewRequest("HEAD", url, nil)
	if err != nil {
		return nil, err
	}
	return c.Do(req)
}

// cancelTimerBody is an io.ReadCloser that wraps rc with two features:
// 1) on Read error or close, the stop func is called.
// 2) On Read failure, if reqDidTimeout is true, the error is wrapped and
//    marked as net.Error that hit its timeout.
type cancelTimerBody struct {
	stop          func() // stops the time.Timer waiting to cancel the request
	rc            io.ReadCloser
	reqDidTimeout func() bool
}

func (b *cancelTimerBody) Read(p []byte) (n int, err error) {
	n, err = b.rc.Read(p)
	if err == nil {
		return n, nil
	}
	b.stop()
	if err == io.EOF {
		return n, err
	}
	if b.reqDidTimeout() {
		err = &httpError{
			err:     err.Error() + " (Client.Timeout exceeded while reading body)",
			timeout: true,
		}
	}
	return n, err
}

func (b *cancelTimerBody) Close() error {
	err := b.rc.Close()
	b.stop()
	return err
}

func shouldCopyHeaderOnRedirect(headerKey string, initial, dest *url.URL) bool {
	switch CanonicalHeaderKey(headerKey) {
	case "Authorization", "Www-Authenticate", "Cookie", "Cookie2":
		// Permit sending auth/cookie headers from "foo.com"
		// to "sub.foo.com".

		// Note that we don't send all cookies to subdomains
		// automatically. This function is only used for
		// Cookies set explicitly on the initial outgoing
		// client request. Cookies automatically added via the
		// CookieJar mechanism continue to follow each
		// cookie's scope as set by Set-Cookie. But for
		// outgoing requests with the Cookie header set
		// directly, we don't know their scope, so we assume
		// it's for *.domain.com.

		ihost := canonicalAddr(initial)
		dhost := canonicalAddr(dest)
		return isDomainOrSubdomain(dhost, ihost)
	}
	// All other headers are copied:
	return true
}

// isDomainOrSubdomain reports whether sub is a subdomain (or exact
// match) of the parent domain.
//
// Both domains must already be in canonical form.
func isDomainOrSubdomain(sub, parent string) bool {
	if sub == parent {
		return true
	}
	// If sub is "foo.example.com" and parent is "example.com",
	// that means sub must end in "."+parent.
	// Do it without allocating.
	if !strings.HasSuffix(sub, parent) {
		return false
	}
	return sub[len(sub)-len(parent)-1] == '.'
}

func stripPassword(u *url.URL) string {
	pass, passSet := u.User.Password()
	if passSet {
		return strings.Replace(u.String(), pass+"@", "***@", 1)
	}

	return u.String()
}