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Source file src/net/http/client.go

Documentation: net/http

  // 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 2616.
  //
  // 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 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 = resp.Request.URL.String()
  		} else {
  			urlStr = req.URL.String()
  		}
  		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))
  			}
  			ireq := reqs[0]
  			req = &Request{
  				Method:   redirectMethod,
  				Response: resp,
  				URL:      u,
  				Header:   make(Header),
  				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 string, 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 string, 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 DefaultClient.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.
  
  		// TODO(bradfitz): once issue 16142 is fixed, make
  		// this code use those URL accessors, and consider
  		// "http://foo.com" and "http://foo.com:80" as
  		// equivalent?
  
  		// TODO(bradfitz): better hostname canonicalization,
  		// at least once we figure out IDNA/Punycode (issue
  		// 13835).
  		ihost := strings.ToLower(initial.Host)
  		dhost := strings.ToLower(dest.Host)
  		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] == '.'
  }
  

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