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// 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/textproto"
        "strconv"
        "strings"
)

// 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
}

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 = rr.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)
                if t.Body != nil && len(t.TransferEncoding) == 0 && atLeastHTTP11 {
                        if t.ContentLength == 0 {
                                // Test to see if it's actually zero or just unset.
                                var buf [1]byte
                                n, _ := io.ReadFull(t.Body, buf[:])
                                if n == 1 {
                                        // Oh, guess there is data in this Body Reader after all.
                                        // The ContentLength field just wasn't set.
                                        // Stich the Body back together again, re-attaching our
                                        // consumed byte.
                                        t.ContentLength = -1
                                        t.Body = io.MultiReader(bytes.NewBuffer(buf[:]), t.Body)
                                } else {
                                        // Body is actually empty.
                                        t.Body = nil
                                        t.BodyCloser = nil
                                }
                        }
                        if t.ContentLength < 0 {
                                t.TransferEncoding = []string{"chunked"}
                        }
                }
        case *Response:
                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 = noBodyExpected(rr.Request.Method)
        }

        // Sanitize Body,ContentLength,TransferEncoding
        if t.ResponseToHEAD {
                t.Body = nil
                t.TransferEncoding = nil
                // ContentLength is expected to hold Content-Length
                if t.ContentLength < 0 {
                        return nil, ErrMissingContentLength
                }
        } 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
}

func noBodyExpected(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.ResponseToHEAD {
                return true
        }
        // 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) {
                return true
        }

        return false
}

func (t *transferWriter) WriteHeader(w io.Writer) (err error) {
        if t.Close {
                _, err = io.WriteString(w, "Connection: close\r\n")
                if err != nil {
                        return
                }
        }

        // Write Content-Length and/or Transfer-Encoding whose values are a
        // function of the sanitized field triple (Body, ContentLength,
        // TransferEncoding)
        if t.shouldSendContentLength() {
                io.WriteString(w, "Content-Length: ")
                _, err = io.WriteString(w, strconv.FormatInt(t.ContentLength, 10)+"\r\n")
                if err != nil {
                        return
                }
        } else if chunked(t.TransferEncoding) {
                _, err = io.WriteString(w, "Transfer-Encoding: chunked\r\n")
                if err != nil {
                        return
                }
        }

        // Write Trailer header
        if t.Trailer != nil {
                // TODO: At some point, there should be a generic mechanism for
                // writing long headers, using HTTP line splitting
                io.WriteString(w, "Trailer: ")
                needComma := false
                for k := range t.Trailer {
                        k = CanonicalHeaderKey(k)
                        switch k {
                        case "Transfer-Encoding", "Trailer", "Content-Length":
                                return &badStringError{"invalid Trailer key", k}
                        }
                        if needComma {
                                io.WriteString(w, ",")
                        }
                        io.WriteString(w, k)
                        needComma = true
                }
                _, err = io.WriteString(w, "\r\n")
        }

        return
}

func (t *transferWriter) WriteBody(w io.Writer) (err error) {
        var ncopy int64

        // Write body
        if t.Body != nil {
                if chunked(t.TransferEncoding) {
                        cw := 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))
                        nextra, err := io.Copy(ioutil.Discard, t.Body)
                        if err != nil {
                                return err
                        }
                        ncopy += nextra
                }
                if err != nil {
                        return err
                }
                if err = t.BodyCloser.Close(); err != nil {
                        return err
                }
        }

        if t.ContentLength != -1 && t.ContentLength != ncopy {
                return fmt.Errorf("http: Request.ContentLength=%d with Body length %d",
                        t.ContentLength, ncopy)
        }

        // TODO(petar): Place trailer writer code here.
        if chunked(t.TransferEncoding) {
                // Last chunk, empty trailer
                _, err = io.WriteString(w, "\r\n")
        }

        return
}

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
}

// bodyAllowedForStatus returns whether a given response status code
// permits a body.  See RFC2616, 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
}

// msg is *Request or *Response.
func readTransfer(msg interface{}, r *bufio.Reader) (err error) {
        t := &transferReader{}

        // Unify input
        isResponse := false
        switch rr := msg.(type) {
        case *Response:
                t.Header = rr.Header
                t.StatusCode = rr.StatusCode
                t.RequestMethod = rr.Request.Method
                t.ProtoMajor = rr.ProtoMajor
                t.ProtoMinor = rr.ProtoMinor
                t.Close = shouldClose(t.ProtoMajor, t.ProtoMinor, t.Header)
                isResponse = true
        case *Request:
                t.Header = rr.Header
                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.RequestMethod = "GET"
        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
        t.TransferEncoding, err = fixTransferEncoding(t.RequestMethod, t.Header)
        if err != nil {
                return err
        }

        t.ContentLength, err = fixLength(isResponse, t.StatusCode, t.RequestMethod, t.Header, t.TransferEncoding)
        if err != nil {
                return err
        }

        // 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 RFC2616, section 4.4.
        switch msg.(type) {
        case *Response:
                if t.ContentLength == -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):
                t.Body = &body{Reader: newChunkedReader(r), hdr: msg, r: r, closing: t.Close}
        case t.ContentLength >= 0:
                // TODO: limit the Content-Length. This is an easy DoS vector.
                t.Body = &body{Reader: io.LimitReader(r, t.ContentLength), closing: t.Close}
        default:
                // t.ContentLength < 0, i.e. "Content-Length" not mentioned in header
                if t.Close {
                        // Close semantics (i.e. HTTP/1.0)
                        t.Body = &body{Reader: r, closing: t.Close}
                } else {
                        // Persistent connection (i.e. HTTP/1.1)
                        t.Body = &body{Reader: io.LimitReader(r, 0), closing: t.Close}
                }
        }

        // 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" }

// Sanitize transfer encoding
func fixTransferEncoding(requestMethod string, header Header) ([]string, error) {
        raw, present := header["Transfer-Encoding"]
        if !present {
                return nil, nil
        }

        delete(header, "Transfer-Encoding")

        // Head responses have no bodies, so the transfer encoding
        // should be ignored.
        if requestMethod == "HEAD" {
                return nil, 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 recored
                if encoding == "identity" {
                        break
                }
                if encoding != "chunked" {
                        return nil, &badStringError{"unsupported transfer encoding", encoding}
                }
                te = te[0 : len(te)+1]
                te[len(te)-1] = encoding
        }
        if len(te) > 1 {
                return nil, &badStringError{"too many transfer encodings", strings.Join(te, ",")}
        }
        if len(te) > 0 {
                // Chunked encoding trumps Content-Length. See RFC 2616
                // Section 4.4. Currently len(te) > 0 implies chunked
                // encoding.
                delete(header, "Content-Length")
                return te, nil
        }

        return nil, 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) {

        // Logic based on response type or status
        if noBodyExpected(requestMethod) {
                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
        cl := strings.TrimSpace(header.Get("Content-Length"))
        if cl != "" {
                n, err := strconv.ParseInt(cl, 10, 64)
                if err != nil || n < 0 {
                        return -1, &badStringError{"bad Content-Length", cl}
                }
                return n, nil
        } else {
                header.Del("Content-Length")
        }

        if !isResponse && requestMethod == "GET" {
                // RFC 2616 doesn't explicitly permit nor forbid 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.
                return 0, nil
        }

        // Logic based on media type. The purpose of the following code is just
        // to detect whether the unsupported "multipart/byteranges" is being
        // used. A proper Content-Type parser is needed in the future.
        if strings.Contains(strings.ToLower(header.Get("Content-Type")), "multipart/byteranges") {
                return -1, ErrNotSupported
        }

        // 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) bool {
        if major < 1 {
                return true
        } else if major == 1 && minor == 0 {
                if !strings.Contains(strings.ToLower(header.Get("Connection")), "keep-alive") {
                        return true
                }
                return false
        } else {
                // TODO: Should split on commas, toss surrounding white space,
                // and check each field.
                if strings.ToLower(header.Get("Connection")) == "close" {
                        header.Del("Connection")
                        return true
                }
        }
        return false
}

// Parse the trailer header
func fixTrailer(header Header, te []string) (Header, error) {
        raw := header.Get("Trailer")
        if raw == "" {
                return nil, nil
        }

        header.Del("Trailer")
        trailer := make(Header)
        keys := strings.Split(raw, ",")
        for _, key := range keys {
                key = CanonicalHeaderKey(strings.TrimSpace(key))
                switch key {
                case "Transfer-Encoding", "Trailer", "Content-Length":
                        return nil, &badStringError{"bad trailer key", key}
                }
                trailer.Del(key)
        }
        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 {
        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?
        closed  bool

        res *response // response writer for server requests, else nil
}

// ErrBodyReadAfterClose is returned when reading a Request 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 request Body")

func (b *body) Read(p []byte) (n int, err error) {
        if b.closed {
                return 0, ErrBodyReadAfterClose
        }
        n, err = b.Reader.Read(p)

        // Read the final trailer once we hit EOF.
        if err == io.EOF && b.hdr != nil {
                if e := b.readTrailer(); e != nil {
                        err = e
                }
                b.hdr = nil
        }
        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
}

func (b *body) readTrailer() error {
        // The common case, since nobody uses trailers.
        buf, _ := b.r.Peek(2)
        if bytes.Equal(buf, singleCRLF) {
                b.r.ReadByte()
                b.r.ReadByte()
                return nil
        }

        // 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 {
                return err
        }
        switch rr := b.hdr.(type) {
        case *Request:
                rr.Trailer = Header(hdr)
        case *Response:
                rr.Trailer = Header(hdr)
        }
        return nil
}

func (b *body) Close() error {
        if b.closed {
                return nil
        }
        defer func() {
                b.closed = true
        }()
        if b.hdr == nil && b.closing {
                // no trailer and closing the connection next.
                // no point in reading to EOF.
                return nil
        }

        // In a server request, don't continue reading from the client
        // if we've already hit the maximum body size set by the
        // handler. If this is set, that also means the TCP connection
        // is about to be closed, so getting to the next HTTP request
        // in the stream is not necessary.
        if b.res != nil && b.res.requestBodyLimitHit {
                return nil
        }

        // Fully consume the body, which will also lead to us reading
        // the trailer headers after the body, if present.
        if _, err := io.Copy(ioutil.Discard, b); err != nil {
                return err
        }
        return nil
}