Source file src/net/http/transfer.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.
  
  package http
  
  import (
  	"bufio"
  	"bytes"
  	"errors"
  	"fmt"
  	"io"
  	"io/ioutil"
  	"net/http/internal"
  	"net/textproto"
  	"sort"
  	"strconv"
  	"strings"
  	"sync"
  	"time"
  
  	"golang_org/x/net/lex/httplex"
  )
  
  // ErrLineTooLong is returned when reading request or response bodies
  // with malformed chunked encoding.
  var ErrLineTooLong = internal.ErrLineTooLong
  
  type errorReader struct {
  	err error
  }
  
  func (r errorReader) Read(p []byte) (n int, err error) {
  	return 0, r.err
  }
  
  type byteReader struct {
  	b    byte
  	done bool
  }
  
  func (br *byteReader) Read(p []byte) (n int, err error) {
  	if br.done {
  		return 0, io.EOF
  	}
  	if len(p) == 0 {
  		return 0, nil
  	}
  	br.done = true
  	p[0] = br.b
  	return 1, io.EOF
  }
  
  // transferBodyReader is an io.Reader that reads from tw.Body
  // and records any non-EOF error in tw.bodyReadError.
  // It is exactly 1 pointer wide to avoid allocations into interfaces.
  type transferBodyReader struct{ tw *transferWriter }
  
  func (br transferBodyReader) Read(p []byte) (n int, err error) {
  	n, err = br.tw.Body.Read(p)
  	if err != nil && err != io.EOF {
  		br.tw.bodyReadError = err
  	}
  	return
  }
  
  // transferWriter inspects the fields of a user-supplied Request or Response,
  // sanitizes them without changing the user object and provides methods for
  // writing the respective header, body and trailer in wire format.
  type transferWriter struct {
  	Method           string
  	Body             io.Reader
  	BodyCloser       io.Closer
  	ResponseToHEAD   bool
  	ContentLength    int64 // -1 means unknown, 0 means exactly none
  	Close            bool
  	TransferEncoding []string
  	Header           Header
  	Trailer          Header
  	IsResponse       bool
  	bodyReadError    error // any non-EOF error from reading Body
  
  	FlushHeaders bool            // flush headers to network before body
  	ByteReadCh   chan readResult // non-nil if probeRequestBody called
  }
  
  func newTransferWriter(r interface{}) (t *transferWriter, err error) {
  	t = &transferWriter{}
  
  	// Extract relevant fields
  	atLeastHTTP11 := false
  	switch rr := r.(type) {
  	case *Request:
  		if rr.ContentLength != 0 && rr.Body == nil {
  			return nil, fmt.Errorf("http: Request.ContentLength=%d with nil Body", rr.ContentLength)
  		}
  		t.Method = valueOrDefault(rr.Method, "GET")
  		t.Close = rr.Close
  		t.TransferEncoding = rr.TransferEncoding
  		t.Header = rr.Header
  		t.Trailer = rr.Trailer
  		t.Body = rr.Body
  		t.BodyCloser = rr.Body
  		t.ContentLength = rr.outgoingLength()
  		if t.ContentLength < 0 && len(t.TransferEncoding) == 0 && t.shouldSendChunkedRequestBody() {
  			t.TransferEncoding = []string{"chunked"}
  		}
  		atLeastHTTP11 = true // Transport requests are always 1.1 or 2.0
  	case *Response:
  		t.IsResponse = true
  		if rr.Request != nil {
  			t.Method = rr.Request.Method
  		}
  		t.Body = rr.Body
  		t.BodyCloser = rr.Body
  		t.ContentLength = rr.ContentLength
  		t.Close = rr.Close
  		t.TransferEncoding = rr.TransferEncoding
  		t.Header = rr.Header
  		t.Trailer = rr.Trailer
  		atLeastHTTP11 = rr.ProtoAtLeast(1, 1)
  		t.ResponseToHEAD = noResponseBodyExpected(t.Method)
  	}
  
  	// Sanitize Body,ContentLength,TransferEncoding
  	if t.ResponseToHEAD {
  		t.Body = nil
  		if chunked(t.TransferEncoding) {
  			t.ContentLength = -1
  		}
  	} else {
  		if !atLeastHTTP11 || t.Body == nil {
  			t.TransferEncoding = nil
  		}
  		if chunked(t.TransferEncoding) {
  			t.ContentLength = -1
  		} else if t.Body == nil { // no chunking, no body
  			t.ContentLength = 0
  		}
  	}
  
  	// Sanitize Trailer
  	if !chunked(t.TransferEncoding) {
  		t.Trailer = nil
  	}
  
  	return t, nil
  }
  
  // shouldSendChunkedRequestBody reports whether we should try to send a
  // chunked request body to the server. In particular, the case we really
  // want to prevent is sending a GET or other typically-bodyless request to a
  // server with a chunked body when the body has zero bytes, since GETs with
  // bodies (while acceptable according to specs), even zero-byte chunked
  // bodies, are approximately never seen in the wild and confuse most
  // servers. See Issue 18257, as one example.
  //
  // The only reason we'd send such a request is if the user set the Body to a
  // non-nil value (say, ioutil.NopCloser(bytes.NewReader(nil))) and didn't
  // set ContentLength, or NewRequest set it to -1 (unknown), so then we assume
  // there's bytes to send.
  //
  // This code tries to read a byte from the Request.Body in such cases to see
  // whether the body actually has content (super rare) or is actually just
  // a non-nil content-less ReadCloser (the more common case). In that more
  // common case, we act as if their Body were nil instead, and don't send
  // a body.
  func (t *transferWriter) shouldSendChunkedRequestBody() bool {
  	// Note that t.ContentLength is the corrected content length
  	// from rr.outgoingLength, so 0 actually means zero, not unknown.
  	if t.ContentLength >= 0 || t.Body == nil { // redundant checks; caller did them
  		return false
  	}
  	if requestMethodUsuallyLacksBody(t.Method) {
  		// Only probe the Request.Body for GET/HEAD/DELETE/etc
  		// requests, because it's only those types of requests
  		// that confuse servers.
  		t.probeRequestBody() // adjusts t.Body, t.ContentLength
  		return t.Body != nil
  	}
  	// For all other request types (PUT, POST, PATCH, or anything
  	// made-up we've never heard of), assume it's normal and the server
  	// can deal with a chunked request body. Maybe we'll adjust this
  	// later.
  	return true
  }
  
  // probeRequestBody reads a byte from t.Body to see whether it's empty
  // (returns io.EOF right away).
  //
  // But because we've had problems with this blocking users in the past
  // (issue 17480) when the body is a pipe (perhaps waiting on the response
  // headers before the pipe is fed data), we need to be careful and bound how
  // long we wait for it. This delay will only affect users if all the following
  // are true:
  //   * the request body blocks
  //   * the content length is not set (or set to -1)
  //   * the method doesn't usually have a body (GET, HEAD, DELETE, ...)
  //   * there is no transfer-encoding=chunked already set.
  // In other words, this delay will not normally affect anybody, and there
  // are workarounds if it does.
  func (t *transferWriter) probeRequestBody() {
  	t.ByteReadCh = make(chan readResult, 1)
  	go func(body io.Reader) {
  		var buf [1]byte
  		var rres readResult
  		rres.n, rres.err = body.Read(buf[:])
  		if rres.n == 1 {
  			rres.b = buf[0]
  		}
  		t.ByteReadCh <- rres
  	}(t.Body)
  	timer := time.NewTimer(200 * time.Millisecond)
  	select {
  	case rres := <-t.ByteReadCh:
  		timer.Stop()
  		if rres.n == 0 && rres.err == io.EOF {
  			// It was empty.
  			t.Body = nil
  			t.ContentLength = 0
  		} else if rres.n == 1 {
  			if rres.err != nil {
  				t.Body = io.MultiReader(&byteReader{b: rres.b}, errorReader{rres.err})
  			} else {
  				t.Body = io.MultiReader(&byteReader{b: rres.b}, t.Body)
  			}
  		} else if rres.err != nil {
  			t.Body = errorReader{rres.err}
  		}
  	case <-timer.C:
  		// Too slow. Don't wait. Read it later, and keep
  		// assuming that this is ContentLength == -1
  		// (unknown), which means we'll send a
  		// "Transfer-Encoding: chunked" header.
  		t.Body = io.MultiReader(finishAsyncByteRead{t}, t.Body)
  		// Request that Request.Write flush the headers to the
  		// network before writing the body, since our body may not
  		// become readable until it's seen the response headers.
  		t.FlushHeaders = true
  	}
  }
  
  func noResponseBodyExpected(requestMethod string) bool {
  	return requestMethod == "HEAD"
  }
  
  func (t *transferWriter) shouldSendContentLength() bool {
  	if chunked(t.TransferEncoding) {
  		return false
  	}
  	if t.ContentLength > 0 {
  		return true
  	}
  	if t.ContentLength < 0 {
  		return false
  	}
  	// Many servers expect a Content-Length for these methods
  	if t.Method == "POST" || t.Method == "PUT" {
  		return true
  	}
  	if t.ContentLength == 0 && isIdentity(t.TransferEncoding) {
  		if t.Method == "GET" || t.Method == "HEAD" {
  			return false
  		}
  		return true
  	}
  
  	return false
  }
  
  func (t *transferWriter) WriteHeader(w io.Writer) error {
  	if t.Close && !hasToken(t.Header.get("Connection"), "close") {
  		if _, err := io.WriteString(w, "Connection: close\r\n"); err != nil {
  			return err
  		}
  	}
  
  	// Write Content-Length and/or Transfer-Encoding whose values are a
  	// function of the sanitized field triple (Body, ContentLength,
  	// TransferEncoding)
  	if t.shouldSendContentLength() {
  		if _, err := io.WriteString(w, "Content-Length: "); err != nil {
  			return err
  		}
  		if _, err := io.WriteString(w, strconv.FormatInt(t.ContentLength, 10)+"\r\n"); err != nil {
  			return err
  		}
  	} else if chunked(t.TransferEncoding) {
  		if _, err := io.WriteString(w, "Transfer-Encoding: chunked\r\n"); err != nil {
  			return err
  		}
  	}
  
  	// Write Trailer header
  	if t.Trailer != nil {
  		keys := make([]string, 0, len(t.Trailer))
  		for k := range t.Trailer {
  			k = CanonicalHeaderKey(k)
  			switch k {
  			case "Transfer-Encoding", "Trailer", "Content-Length":
  				return &badStringError{"invalid Trailer key", k}
  			}
  			keys = append(keys, k)
  		}
  		if len(keys) > 0 {
  			sort.Strings(keys)
  			// TODO: could do better allocation-wise here, but trailers are rare,
  			// so being lazy for now.
  			if _, err := io.WriteString(w, "Trailer: "+strings.Join(keys, ",")+"\r\n"); err != nil {
  				return err
  			}
  		}
  	}
  
  	return nil
  }
  
  func (t *transferWriter) WriteBody(w io.Writer) error {
  	var err error
  	var ncopy int64
  
  	// Write body
  	if t.Body != nil {
  		var body = transferBodyReader{t}
  		if chunked(t.TransferEncoding) {
  			if bw, ok := w.(*bufio.Writer); ok && !t.IsResponse {
  				w = &internal.FlushAfterChunkWriter{Writer: bw}
  			}
  			cw := internal.NewChunkedWriter(w)
  			_, err = io.Copy(cw, body)
  			if err == nil {
  				err = cw.Close()
  			}
  		} else if t.ContentLength == -1 {
  			ncopy, err = io.Copy(w, body)
  		} else {
  			ncopy, err = io.Copy(w, io.LimitReader(body, t.ContentLength))
  			if err != nil {
  				return err
  			}
  			var nextra int64
  			nextra, err = io.Copy(ioutil.Discard, body)
  			ncopy += nextra
  		}
  		if err != nil {
  			return err
  		}
  	}
  	if t.BodyCloser != nil {
  		if err := t.BodyCloser.Close(); err != nil {
  			return err
  		}
  	}
  
  	if !t.ResponseToHEAD && t.ContentLength != -1 && t.ContentLength != ncopy {
  		return fmt.Errorf("http: ContentLength=%d with Body length %d",
  			t.ContentLength, ncopy)
  	}
  
  	if chunked(t.TransferEncoding) {
  		// Write Trailer header
  		if t.Trailer != nil {
  			if err := t.Trailer.Write(w); err != nil {
  				return err
  			}
  		}
  		// Last chunk, empty trailer
  		_, err = io.WriteString(w, "\r\n")
  	}
  	return err
  }
  
  type transferReader struct {
  	// Input
  	Header        Header
  	StatusCode    int
  	RequestMethod string
  	ProtoMajor    int
  	ProtoMinor    int
  	// Output
  	Body             io.ReadCloser
  	ContentLength    int64
  	TransferEncoding []string
  	Close            bool
  	Trailer          Header
  }
  
  func (t *transferReader) protoAtLeast(m, n int) bool {
  	return t.ProtoMajor > m || (t.ProtoMajor == m && t.ProtoMinor >= n)
  }
  
  // bodyAllowedForStatus reports whether a given response status code
  // permits a body. See RFC 2616, section 4.4.
  func bodyAllowedForStatus(status int) bool {
  	switch {
  	case status >= 100 && status <= 199:
  		return false
  	case status == 204:
  		return false
  	case status == 304:
  		return false
  	}
  	return true
  }
  
  var (
  	suppressedHeaders304    = []string{"Content-Type", "Content-Length", "Transfer-Encoding"}
  	suppressedHeadersNoBody = []string{"Content-Length", "Transfer-Encoding"}
  )
  
  func suppressedHeaders(status int) []string {
  	switch {
  	case status == 304:
  		// RFC 2616 section 10.3.5: "the response MUST NOT include other entity-headers"
  		return suppressedHeaders304
  	case !bodyAllowedForStatus(status):
  		return suppressedHeadersNoBody
  	}
  	return nil
  }
  
  // msg is *Request or *Response.
  func readTransfer(msg interface{}, r *bufio.Reader) (err error) {
  	t := &transferReader{RequestMethod: "GET"}
  
  	// Unify input
  	isResponse := false
  	switch rr := msg.(type) {
  	case *Response:
  		t.Header = rr.Header
  		t.StatusCode = rr.StatusCode
  		t.ProtoMajor = rr.ProtoMajor
  		t.ProtoMinor = rr.ProtoMinor
  		t.Close = shouldClose(t.ProtoMajor, t.ProtoMinor, t.Header, true)
  		isResponse = true
  		if rr.Request != nil {
  			t.RequestMethod = rr.Request.Method
  		}
  	case *Request:
  		t.Header = rr.Header
  		t.RequestMethod = rr.Method
  		t.ProtoMajor = rr.ProtoMajor
  		t.ProtoMinor = rr.ProtoMinor
  		// Transfer semantics for Requests are exactly like those for
  		// Responses with status code 200, responding to a GET method
  		t.StatusCode = 200
  		t.Close = rr.Close
  	default:
  		panic("unexpected type")
  	}
  
  	// Default to HTTP/1.1
  	if t.ProtoMajor == 0 && t.ProtoMinor == 0 {
  		t.ProtoMajor, t.ProtoMinor = 1, 1
  	}
  
  	// Transfer encoding, content length
  	err = t.fixTransferEncoding()
  	if err != nil {
  		return err
  	}
  
  	realLength, err := fixLength(isResponse, t.StatusCode, t.RequestMethod, t.Header, t.TransferEncoding)
  	if err != nil {
  		return err
  	}
  	if isResponse && t.RequestMethod == "HEAD" {
  		if n, err := parseContentLength(t.Header.get("Content-Length")); err != nil {
  			return err
  		} else {
  			t.ContentLength = n
  		}
  	} else {
  		t.ContentLength = realLength
  	}
  
  	// Trailer
  	t.Trailer, err = fixTrailer(t.Header, t.TransferEncoding)
  	if err != nil {
  		return err
  	}
  
  	// If there is no Content-Length or chunked Transfer-Encoding on a *Response
  	// and the status is not 1xx, 204 or 304, then the body is unbounded.
  	// See RFC 2616, section 4.4.
  	switch msg.(type) {
  	case *Response:
  		if realLength == -1 &&
  			!chunked(t.TransferEncoding) &&
  			bodyAllowedForStatus(t.StatusCode) {
  			// Unbounded body.
  			t.Close = true
  		}
  	}
  
  	// Prepare body reader. ContentLength < 0 means chunked encoding
  	// or close connection when finished, since multipart is not supported yet
  	switch {
  	case chunked(t.TransferEncoding):
  		if noResponseBodyExpected(t.RequestMethod) || !bodyAllowedForStatus(t.StatusCode) {
  			t.Body = NoBody
  		} else {
  			t.Body = &body{src: internal.NewChunkedReader(r), hdr: msg, r: r, closing: t.Close}
  		}
  	case realLength == 0:
  		t.Body = NoBody
  	case realLength > 0:
  		t.Body = &body{src: io.LimitReader(r, realLength), closing: t.Close}
  	default:
  		// realLength < 0, i.e. "Content-Length" not mentioned in header
  		if t.Close {
  			// Close semantics (i.e. HTTP/1.0)
  			t.Body = &body{src: r, closing: t.Close}
  		} else {
  			// Persistent connection (i.e. HTTP/1.1)
  			t.Body = NoBody
  		}
  	}
  
  	// Unify output
  	switch rr := msg.(type) {
  	case *Request:
  		rr.Body = t.Body
  		rr.ContentLength = t.ContentLength
  		rr.TransferEncoding = t.TransferEncoding
  		rr.Close = t.Close
  		rr.Trailer = t.Trailer
  	case *Response:
  		rr.Body = t.Body
  		rr.ContentLength = t.ContentLength
  		rr.TransferEncoding = t.TransferEncoding
  		rr.Close = t.Close
  		rr.Trailer = t.Trailer
  	}
  
  	return nil
  }
  
  // Checks whether chunked is part of the encodings stack
  func chunked(te []string) bool { return len(te) > 0 && te[0] == "chunked" }
  
  // Checks whether the encoding is explicitly "identity".
  func isIdentity(te []string) bool { return len(te) == 1 && te[0] == "identity" }
  
  // fixTransferEncoding sanitizes t.TransferEncoding, if needed.
  func (t *transferReader) fixTransferEncoding() error {
  	raw, present := t.Header["Transfer-Encoding"]
  	if !present {
  		return nil
  	}
  	delete(t.Header, "Transfer-Encoding")
  
  	// Issue 12785; ignore Transfer-Encoding on HTTP/1.0 requests.
  	if !t.protoAtLeast(1, 1) {
  		return nil
  	}
  
  	encodings := strings.Split(raw[0], ",")
  	te := make([]string, 0, len(encodings))
  	// TODO: Even though we only support "identity" and "chunked"
  	// encodings, the loop below is designed with foresight. One
  	// invariant that must be maintained is that, if present,
  	// chunked encoding must always come first.
  	for _, encoding := range encodings {
  		encoding = strings.ToLower(strings.TrimSpace(encoding))
  		// "identity" encoding is not recorded
  		if encoding == "identity" {
  			break
  		}
  		if encoding != "chunked" {
  			return &badStringError{"unsupported transfer encoding", encoding}
  		}
  		te = te[0 : len(te)+1]
  		te[len(te)-1] = encoding
  	}
  	if len(te) > 1 {
  		return &badStringError{"too many transfer encodings", strings.Join(te, ",")}
  	}
  	if len(te) > 0 {
  		// RFC 7230 3.3.2 says "A sender MUST NOT send a
  		// Content-Length header field in any message that
  		// contains a Transfer-Encoding header field."
  		//
  		// but also:
  		// "If a message is received with both a
  		// Transfer-Encoding and a Content-Length header
  		// field, the Transfer-Encoding overrides the
  		// Content-Length. Such a message might indicate an
  		// attempt to perform request smuggling (Section 9.5)
  		// or response splitting (Section 9.4) and ought to be
  		// handled as an error. A sender MUST remove the
  		// received Content-Length field prior to forwarding
  		// such a message downstream."
  		//
  		// Reportedly, these appear in the wild.
  		delete(t.Header, "Content-Length")
  		t.TransferEncoding = te
  		return nil
  	}
  
  	return nil
  }
  
  // Determine the expected body length, using RFC 2616 Section 4.4. This
  // function is not a method, because ultimately it should be shared by
  // ReadResponse and ReadRequest.
  func fixLength(isResponse bool, status int, requestMethod string, header Header, te []string) (int64, error) {
  	isRequest := !isResponse
  	contentLens := header["Content-Length"]
  
  	// Hardening against HTTP request smuggling
  	if len(contentLens) > 1 {
  		// Per RFC 7230 Section 3.3.2, prevent multiple
  		// Content-Length headers if they differ in value.
  		// If there are dups of the value, remove the dups.
  		// See Issue 16490.
  		first := strings.TrimSpace(contentLens[0])
  		for _, ct := range contentLens[1:] {
  			if first != strings.TrimSpace(ct) {
  				return 0, fmt.Errorf("http: message cannot contain multiple Content-Length headers; got %q", contentLens)
  			}
  		}
  
  		// deduplicate Content-Length
  		header.Del("Content-Length")
  		header.Add("Content-Length", first)
  
  		contentLens = header["Content-Length"]
  	}
  
  	// Logic based on response type or status
  	if noResponseBodyExpected(requestMethod) {
  		// For HTTP requests, as part of hardening against request
  		// smuggling (RFC 7230), don't allow a Content-Length header for
  		// methods which don't permit bodies. As an exception, allow
  		// exactly one Content-Length header if its value is "0".
  		if isRequest && len(contentLens) > 0 && !(len(contentLens) == 1 && contentLens[0] == "0") {
  			return 0, fmt.Errorf("http: method cannot contain a Content-Length; got %q", contentLens)
  		}
  		return 0, nil
  	}
  	if status/100 == 1 {
  		return 0, nil
  	}
  	switch status {
  	case 204, 304:
  		return 0, nil
  	}
  
  	// Logic based on Transfer-Encoding
  	if chunked(te) {
  		return -1, nil
  	}
  
  	// Logic based on Content-Length
  	var cl string
  	if len(contentLens) == 1 {
  		cl = strings.TrimSpace(contentLens[0])
  	}
  	if cl != "" {
  		n, err := parseContentLength(cl)
  		if err != nil {
  			return -1, err
  		}
  		return n, nil
  	}
  	header.Del("Content-Length")
  
  	if isRequest {
  		// RFC 2616 neither explicitly permits nor forbids an
  		// entity-body on a GET request so we permit one if
  		// declared, but we default to 0 here (not -1 below)
  		// if there's no mention of a body.
  		// Likewise, all other request methods are assumed to have
  		// no body if neither Transfer-Encoding chunked nor a
  		// Content-Length are set.
  		return 0, nil
  	}
  
  	// Body-EOF logic based on other methods (like closing, or chunked coding)
  	return -1, nil
  }
  
  // Determine whether to hang up after sending a request and body, or
  // receiving a response and body
  // 'header' is the request headers
  func shouldClose(major, minor int, header Header, removeCloseHeader bool) bool {
  	if major < 1 {
  		return true
  	}
  
  	conv := header["Connection"]
  	hasClose := httplex.HeaderValuesContainsToken(conv, "close")
  	if major == 1 && minor == 0 {
  		return hasClose || !httplex.HeaderValuesContainsToken(conv, "keep-alive")
  	}
  
  	if hasClose && removeCloseHeader {
  		header.Del("Connection")
  	}
  
  	return hasClose
  }
  
  // Parse the trailer header
  func fixTrailer(header Header, te []string) (Header, error) {
  	vv, ok := header["Trailer"]
  	if !ok {
  		return nil, nil
  	}
  	header.Del("Trailer")
  
  	trailer := make(Header)
  	var err error
  	for _, v := range vv {
  		foreachHeaderElement(v, func(key string) {
  			key = CanonicalHeaderKey(key)
  			switch key {
  			case "Transfer-Encoding", "Trailer", "Content-Length":
  				if err == nil {
  					err = &badStringError{"bad trailer key", key}
  					return
  				}
  			}
  			trailer[key] = nil
  		})
  	}
  	if err != nil {
  		return nil, err
  	}
  	if len(trailer) == 0 {
  		return nil, nil
  	}
  	if !chunked(te) {
  		// Trailer and no chunking
  		return nil, ErrUnexpectedTrailer
  	}
  	return trailer, nil
  }
  
  // body turns a Reader into a ReadCloser.
  // Close ensures that the body has been fully read
  // and then reads the trailer if necessary.
  type body struct {
  	src          io.Reader
  	hdr          interface{}   // non-nil (Response or Request) value means read trailer
  	r            *bufio.Reader // underlying wire-format reader for the trailer
  	closing      bool          // is the connection to be closed after reading body?
  	doEarlyClose bool          // whether Close should stop early
  
  	mu         sync.Mutex // guards following, and calls to Read and Close
  	sawEOF     bool
  	closed     bool
  	earlyClose bool   // Close called and we didn't read to the end of src
  	onHitEOF   func() // if non-nil, func to call when EOF is Read
  }
  
  // ErrBodyReadAfterClose is returned when reading a Request or Response
  // Body after the body has been closed. This typically happens when the body is
  // read after an HTTP Handler calls WriteHeader or Write on its
  // ResponseWriter.
  var ErrBodyReadAfterClose = errors.New("http: invalid Read on closed Body")
  
  func (b *body) Read(p []byte) (n int, err error) {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  	if b.closed {
  		return 0, ErrBodyReadAfterClose
  	}
  	return b.readLocked(p)
  }
  
  // Must hold b.mu.
  func (b *body) readLocked(p []byte) (n int, err error) {
  	if b.sawEOF {
  		return 0, io.EOF
  	}
  	n, err = b.src.Read(p)
  
  	if err == io.EOF {
  		b.sawEOF = true
  		// Chunked case. Read the trailer.
  		if b.hdr != nil {
  			if e := b.readTrailer(); e != nil {
  				err = e
  				// Something went wrong in the trailer, we must not allow any
  				// further reads of any kind to succeed from body, nor any
  				// subsequent requests on the server connection. See
  				// golang.org/issue/12027
  				b.sawEOF = false
  				b.closed = true
  			}
  			b.hdr = nil
  		} else {
  			// If the server declared the Content-Length, our body is a LimitedReader
  			// and we need to check whether this EOF arrived early.
  			if lr, ok := b.src.(*io.LimitedReader); ok && lr.N > 0 {
  				err = io.ErrUnexpectedEOF
  			}
  		}
  	}
  
  	// If we can return an EOF here along with the read data, do
  	// so. This is optional per the io.Reader contract, but doing
  	// so helps the HTTP transport code recycle its connection
  	// earlier (since it will see this EOF itself), even if the
  	// client doesn't do future reads or Close.
  	if err == nil && n > 0 {
  		if lr, ok := b.src.(*io.LimitedReader); ok && lr.N == 0 {
  			err = io.EOF
  			b.sawEOF = true
  		}
  	}
  
  	if b.sawEOF && b.onHitEOF != nil {
  		b.onHitEOF()
  	}
  
  	return n, err
  }
  
  var (
  	singleCRLF = []byte("\r\n")
  	doubleCRLF = []byte("\r\n\r\n")
  )
  
  func seeUpcomingDoubleCRLF(r *bufio.Reader) bool {
  	for peekSize := 4; ; peekSize++ {
  		// This loop stops when Peek returns an error,
  		// which it does when r's buffer has been filled.
  		buf, err := r.Peek(peekSize)
  		if bytes.HasSuffix(buf, doubleCRLF) {
  			return true
  		}
  		if err != nil {
  			break
  		}
  	}
  	return false
  }
  
  var errTrailerEOF = errors.New("http: unexpected EOF reading trailer")
  
  func (b *body) readTrailer() error {
  	// The common case, since nobody uses trailers.
  	buf, err := b.r.Peek(2)
  	if bytes.Equal(buf, singleCRLF) {
  		b.r.Discard(2)
  		return nil
  	}
  	if len(buf) < 2 {
  		return errTrailerEOF
  	}
  	if err != nil {
  		return err
  	}
  
  	// Make sure there's a header terminator coming up, to prevent
  	// a DoS with an unbounded size Trailer. It's not easy to
  	// slip in a LimitReader here, as textproto.NewReader requires
  	// a concrete *bufio.Reader. Also, we can't get all the way
  	// back up to our conn's LimitedReader that *might* be backing
  	// this bufio.Reader. Instead, a hack: we iteratively Peek up
  	// to the bufio.Reader's max size, looking for a double CRLF.
  	// This limits the trailer to the underlying buffer size, typically 4kB.
  	if !seeUpcomingDoubleCRLF(b.r) {
  		return errors.New("http: suspiciously long trailer after chunked body")
  	}
  
  	hdr, err := textproto.NewReader(b.r).ReadMIMEHeader()
  	if err != nil {
  		if err == io.EOF {
  			return errTrailerEOF
  		}
  		return err
  	}
  	switch rr := b.hdr.(type) {
  	case *Request:
  		mergeSetHeader(&rr.Trailer, Header(hdr))
  	case *Response:
  		mergeSetHeader(&rr.Trailer, Header(hdr))
  	}
  	return nil
  }
  
  func mergeSetHeader(dst *Header, src Header) {
  	if *dst == nil {
  		*dst = src
  		return
  	}
  	for k, vv := range src {
  		(*dst)[k] = vv
  	}
  }
  
  // unreadDataSizeLocked returns the number of bytes of unread input.
  // It returns -1 if unknown.
  // b.mu must be held.
  func (b *body) unreadDataSizeLocked() int64 {
  	if lr, ok := b.src.(*io.LimitedReader); ok {
  		return lr.N
  	}
  	return -1
  }
  
  func (b *body) Close() error {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  	if b.closed {
  		return nil
  	}
  	var err error
  	switch {
  	case b.sawEOF:
  		// Already saw EOF, so no need going to look for it.
  	case b.hdr == nil && b.closing:
  		// no trailer and closing the connection next.
  		// no point in reading to EOF.
  	case b.doEarlyClose:
  		// Read up to maxPostHandlerReadBytes bytes of the body, looking for
  		// for EOF (and trailers), so we can re-use this connection.
  		if lr, ok := b.src.(*io.LimitedReader); ok && lr.N > maxPostHandlerReadBytes {
  			// There was a declared Content-Length, and we have more bytes remaining
  			// than our maxPostHandlerReadBytes tolerance. So, give up.
  			b.earlyClose = true
  		} else {
  			var n int64
  			// Consume the body, or, which will also lead to us reading
  			// the trailer headers after the body, if present.
  			n, err = io.CopyN(ioutil.Discard, bodyLocked{b}, maxPostHandlerReadBytes)
  			if err == io.EOF {
  				err = nil
  			}
  			if n == maxPostHandlerReadBytes {
  				b.earlyClose = true
  			}
  		}
  	default:
  		// Fully consume the body, which will also lead to us reading
  		// the trailer headers after the body, if present.
  		_, err = io.Copy(ioutil.Discard, bodyLocked{b})
  	}
  	b.closed = true
  	return err
  }
  
  func (b *body) didEarlyClose() bool {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  	return b.earlyClose
  }
  
  // bodyRemains reports whether future Read calls might
  // yield data.
  func (b *body) bodyRemains() bool {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  	return !b.sawEOF
  }
  
  func (b *body) registerOnHitEOF(fn func()) {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  	b.onHitEOF = fn
  }
  
  // bodyLocked is a io.Reader reading from a *body when its mutex is
  // already held.
  type bodyLocked struct {
  	b *body
  }
  
  func (bl bodyLocked) Read(p []byte) (n int, err error) {
  	if bl.b.closed {
  		return 0, ErrBodyReadAfterClose
  	}
  	return bl.b.readLocked(p)
  }
  
  // parseContentLength trims whitespace from s and returns -1 if no value
  // is set, or the value if it's >= 0.
  func parseContentLength(cl string) (int64, error) {
  	cl = strings.TrimSpace(cl)
  	if cl == "" {
  		return -1, nil
  	}
  	n, err := strconv.ParseInt(cl, 10, 64)
  	if err != nil || n < 0 {
  		return 0, &badStringError{"bad Content-Length", cl}
  	}
  	return n, nil
  
  }
  
  // finishAsyncByteRead finishes reading the 1-byte sniff
  // from the ContentLength==0, Body!=nil case.
  type finishAsyncByteRead struct {
  	tw *transferWriter
  }
  
  func (fr finishAsyncByteRead) Read(p []byte) (n int, err error) {
  	if len(p) == 0 {
  		return
  	}
  	rres := <-fr.tw.ByteReadCh
  	n, err = rres.n, rres.err
  	if n == 1 {
  		p[0] = rres.b
  	}
  	return
  }
  

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