Add LOOP_VINFO_MAX_VECT_FACTOR

[official-gcc.git] / libgo / go / net / http / transfer.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.

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
}

// 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
        Trailer          Header
        IsResponse       bool

        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.Trailer = rr.Trailer
                atLeastHTTP11 = rr.protoAtLeastOutgoing(1, 1)
                t.Body = rr.Body
                t.BodyCloser = rr.Body
                t.ContentLength = rr.outgoingLength()
                if t.ContentLength < 0 && len(t.TransferEncoding) == 0 && atLeastHTTP11 && t.shouldSendChunkedRequestBody() {
                        t.TransferEncoding = []string{"chunked"}
                }
        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.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 {
                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 {
                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, t.Body)
                        if err == nil {
                                err = cw.Close()
                        }
                } else if t.ContentLength == -1 {
                        ncopy, err = io.Copy(w, t.Body)
                } else {
                        ncopy, err = io.Copy(w, io.LimitReader(t.Body, t.ContentLength))
                        if err != nil {
                                return err
                        }
                        var nextra int64
                        nextra, err = io.Copy(ioutil.Discard, t.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) {
                        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
        } else {
                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
}