package nhttp

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"golang.org/x/net/http/httpguts"
	"io"
	_http "net/http"
	"net/textproto"
	"strconv"
	"sync"
)

type transferReader struct {
	// Input
	Header        _http.Header
	StatusCode    int
	RequestMethod string
	ProtoMajor    int
	ProtoMinor    int
	// Output
	Body          io.ReadCloser
	ContentLength int64
	Chunked       bool
	Close         bool
	Trailer       _http.Header
}

// bodyLocked is an 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, errors.New("nhttp: invalid Read on closed Body")
	}
	return bl.b.readLocked(p)
}

// 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          any           // 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
}

func (b *body) Read(p []byte) (n int, err error) {
	b.mu.Lock()
	defer b.mu.Unlock()
	if b.closed {
		return 0, errors.New("nhttp: invalid Read on closed Body")
	}
	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
}

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 EOF (and trailers), so we can re-use this connection.
		if lr, ok := b.src.(*io.LimitedReader); ok && lr.N > 256<<10 {
			// 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(io.Discard, bodyLocked{b}, 256<<10)
			if err == io.EOF {
				err = nil
			}
			if n == 256<<10 {
				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(io.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
}

// 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 _http.Header, removeCloseHeader bool) bool {
	if major < 1 {
		return true
	}

	conv := header["Connection"]
	hasClose := httpguts.HeaderValuesContainsToken(conv, "close")
	if major == 1 && minor == 0 {
		return hasClose || !httpguts.HeaderValuesContainsToken(conv, "keep-alive")
	}

	if hasClose && removeCloseHeader {
		header.Del("Connection")
	}

	return hasClose
}

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

	// Unify input
	isResponse := false
	switch rr := msg.(type) {
	case *_http.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 *_http.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: chunked, and overriding Content-Length.
	if err := t.parseTransferEncoding(); err != nil {
		return err
	}

	realLength, err := fixLength(isResponse, t.StatusCode, t.RequestMethod, t.Header, t.Chunked)
	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.Chunked)
	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 7230, section 3.3.
	switch msg.(type) {
	case *_http.Response:
		if realLength == -1 && !t.Chunked && 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 t.Chunked:
		if t.RequestMethod == "HEAD" || !bodyAllowedForStatus(t.StatusCode) {
			t.Body = NoBody
		} else {
			t.Body = &body{src: 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 *_http.Request:
		rr.Body = t.Body
		rr.ContentLength = t.ContentLength
		if t.Chunked {
			rr.TransferEncoding = []string{"chunked"}
		}
		rr.Close = t.Close
		rr.Trailer = t.Trailer
	case *_http.Response:
		rr.Body = t.Body
		rr.ContentLength = t.ContentLength
		if t.Chunked {
			rr.TransferEncoding = []string{"chunked"}
		}
		rr.Close = t.Close
		rr.Trailer = t.Trailer
	}

	return nil
}

// parseTransferEncoding sets t.Chunked based on the Transfer-Encoding header.
func (t *transferReader) parseTransferEncoding() 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
	}

	// Like nginx, we only support a single Transfer-Encoding header field, and
	// only if set to "chunked". This is one of the most security sensitive
	// surfaces in HTTP/1.1 due to the risk of request smuggling, so we keep it
	// strict and simple.
	if len(raw) != 1 {
		return errors.New(fmt.Sprintf("too many transfer encodings: %q", raw))
	}
	if !EqualFold(raw[0], "chunked") {
		return errors.New(fmt.Sprintf("unsupported transfer encoding: %q", raw[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.Chunked = true
	return nil
}

func (t *transferReader) protoAtLeast(m, n int) bool {
	return t.ProtoMajor > m || (t.ProtoMajor == m && t.ProtoMinor >= n)
}

// Determine the expected body length, using RFC 7230 Section 3.3. 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 _http.Header, chunked bool) (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 := textproto.TrimString(contentLens[0])
		for _, ct := range contentLens[1:] {
			if first != textproto.TrimString(ct) {
				return 0, fmt.Errorf("nhttp: 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 requestMethod == "HEAD" {
		// 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("nhttp: 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 {
		return -1, nil
	}

	// Logic based on Content-Length
	var cl string
	if len(contentLens) == 1 {
		cl = textproto.TrimString(contentLens[0])
	}
	if cl != "" {
		n, err := parseContentLength(cl)
		if err != nil {
			return -1, err
		}
		return n, nil
	}
	header.Del("Content-Length")

	if isRequest {
		// RFC 7230 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
}

func parseContentLength(cl string) (int64, error) {
	cl = textproto.TrimString(cl)
	if cl == "" {
		return -1, nil
	}
	n, err := strconv.ParseUint(cl, 10, 63)
	if err != nil {
		return 0, errors.New("bad Content-Length")
	}
	return int64(n), nil

}

// Parse the trailer header
func fixTrailer(header _http.Header, chunked bool) (_http.Header, error) {
	vv, ok := header["Trailer"]
	if !ok {
		return nil, nil
	}
	if !chunked {
		// Trailer and no chunking:
		// this is an invalid use case for trailer header.
		// Nevertheless, no error will be returned and we
		// let users decide if this is a valid HTTP message.
		// The Trailer header will be kept in Response.Header
		// but not populate Response.Trailer.
		// See issue #27197.
		return nil, nil
	}
	header.Del("Trailer")

	trailer := make(_http.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 = errors.New("bad trailer key")
					return
				}
			}
			trailer[key] = nil
		})
	}
	if err != nil {
		return nil, err
	}
	if len(trailer) == 0 {
		return nil, nil
	}
	return trailer, nil
}

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 errTrailerEOF = errors.New("nhttp: 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("nhttp: 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 *_http.Request:
		mergeSetHeader(&rr.Trailer, _http.Header(hdr))
	case *_http.Response:
		mergeSetHeader(&rr.Trailer, _http.Header(hdr))
	}
	return nil
}

func mergeSetHeader(dst *_http.Header, src _http.Header) {
	if *dst == nil {
		*dst = src
		return
	}
	for k, vv := range src {
		(*dst)[k] = vv
	}
}

var (
	suppressedHeaders304    = []string{"Content-Type", "Content-Length", "Transfer-Encoding"}
	suppressedHeadersNoBody = []string{"Content-Length", "Transfer-Encoding"}
	excludedHeadersNoBody   = map[string]bool{"Content-Length": true, "Transfer-Encoding": true}
)

func suppressedHeaders(status int) []string {
	switch {
	case status == 304:
		// RFC 7232 section 4.1
		return suppressedHeaders304
	case !bodyAllowedForStatus(status):
		return suppressedHeadersNoBody
	}
	return nil
}