Source File
	response.go

Belonging Package
	net/http

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// HTTP Response reading and parsing.

package http

import (
	"bufio"
	"bytes"
	"crypto/tls"
	"errors"
	"fmt"
	"io"
	"net/textproto"
	"net/url"
	"strconv"
	"strings"

	"golang.org/x/net/http/httpguts"
)

var respExcludeHeader = map[string]bool{
	"Content-Length":    true,
	"Transfer-Encoding": true,
	"Trailer":           true,
}

// Response represents the response from an HTTP request.
//
// The [Client] and [Transport] return Responses from servers once
// the response headers have been received. The response body
// is streamed on demand as the Body field is read.
type Response struct {
	Status     string // e.g. "200 OK"
	StatusCode int    // e.g. 200
	Proto      string // e.g. "HTTP/1.0"
	ProtoMajor int    // e.g. 1
	ProtoMinor int    // e.g. 0

	// Header maps header keys to values. If the response had multiple
	// headers with the same key, they may be concatenated, with comma
	// delimiters.  (RFC 7230, section 3.2.2 requires that multiple headers
	// be semantically equivalent to a comma-delimited sequence.) When
	// Header values are duplicated by other fields in this struct (e.g.,
	// ContentLength, TransferEncoding, Trailer), the field values are
	// authoritative.
	//
	// Keys in the map are canonicalized (see CanonicalHeaderKey).
	Header Header

	// Body represents the response body.
	//
	// The response body is streamed on demand as the Body field
	// is read. If the network connection fails or the server
	// terminates the response, Body.Read calls return an error.
	//
	// The http Client and Transport guarantee that Body is always
	// non-nil, even on responses without a body or responses with
	// a zero-length body. It is the caller's responsibility to
	// close Body. The default HTTP client's Transport may not
	// reuse HTTP/1.x "keep-alive" TCP connections if the Body is
	// not read to completion and closed.
	//
	// The Body is automatically dechunked if the server replied
	// with a "chunked" Transfer-Encoding.
	//
	// As of Go 1.12, the Body will also implement io.Writer
	// on a successful "101 Switching Protocols" response,
	// as used by WebSockets and HTTP/2's "h2c" mode.
	Body io.ReadCloser

	// ContentLength records the length of the associated content. The
	// value -1 indicates that the length is unknown. Unless Request.Method
	// is "HEAD", values >= 0 indicate that the given number of bytes may
	// be read from Body.
	ContentLength int64

	// Contains transfer encodings from outer-most to inner-most. Value is
	// nil, means that "identity" encoding is used.
	TransferEncoding []string

	// Close records whether the header directed that the connection be
	// closed after reading Body. The value is advice for clients: neither
	// ReadResponse nor Response.Write ever closes a connection.
	Close bool

	// Uncompressed reports whether the response was sent compressed but
	// was decompressed by the http package. When true, reading from
	// Body yields the uncompressed content instead of the compressed
	// content actually set from the server, ContentLength is set to -1,
	// and the "Content-Length" and "Content-Encoding" fields are deleted
	// from the responseHeader. To get the original response from
	// the server, set Transport.DisableCompression to true.
	Uncompressed bool

	// Trailer maps trailer keys to values in the same
	// format as Header.
	//
	// The Trailer initially contains only nil values, one for
	// each key specified in the server's "Trailer" header
	// value. Those values are not added to Header.
	//
	// Trailer must not be accessed concurrently with Read calls
	// on the Body.
	//
	// After Body.Read has returned io.EOF, Trailer will contain
	// any trailer values sent by the server.
	Trailer Header

	// Request is the request that was sent to obtain this Response.
	// Request's Body is nil (having already been consumed).
	// This is only populated for Client requests.
	Request *Request

	// TLS contains information about the TLS connection on which the
	// response was received. It is nil for unencrypted responses.
	// The pointer is shared between responses and should not be
	// modified.
	TLS *tls.ConnectionState
}

// Cookies parses and returns the cookies set in the Set-Cookie headers.
func (r *Response) Cookies() []*Cookie {
	return readSetCookies(r.Header)
}

// ErrNoLocation is returned by the [Response.Location] method
// when no Location header is present.
var ErrNoLocation = errors.New("http: no Location header in response")

// Location returns the URL of the response's "Location" header,
// if present. Relative redirects are resolved relative to
// [Response.Request]. [ErrNoLocation] is returned if no
// Location header is present.
func (r *Response) Location() (*url.URL, error) {
	lv := r.Header.Get("Location")
	if lv == "" {
		return nil, ErrNoLocation
	}
	if r.Request != nil && r.Request.URL != nil {
		return r.Request.URL.Parse(lv)
	}
	return url.Parse(lv)
}

// ReadResponse reads and returns an HTTP response from r.
// The req parameter optionally specifies the [Request] that corresponds
// to this [Response]. If nil, a GET request is assumed.
// Clients must call resp.Body.Close when finished reading resp.Body.
// After that call, clients can inspect resp.Trailer to find key/value
// pairs included in the response trailer.
func ReadResponse(r *bufio.Reader, req *Request) (*Response, error) {
	tp := textproto.NewReader(r)
	resp := &Response{
		Request: req,
	}

	// Parse the first line of the response.
	line, err := tp.ReadLine()
	if err != nil {
		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		}
		return nil, err
	}
	proto, status, ok := strings.Cut(line, " ")
	if !ok {
		return nil, badStringError("malformed HTTP response", line)
	}
	resp.Proto = proto
	resp.Status = strings.TrimLeft(status, " ")

	statusCode, _, _ := strings.Cut(resp.Status, " ")
	if len(statusCode) != 3 {
		return nil, badStringError("malformed HTTP status code", statusCode)
	}
	resp.StatusCode, err = strconv.Atoi(statusCode)
	if err != nil || resp.StatusCode < 0 {
		return nil, badStringError("malformed HTTP status code", statusCode)
	}
	if resp.ProtoMajor, resp.ProtoMinor, ok = ParseHTTPVersion(resp.Proto); !ok {
		return nil, badStringError("malformed HTTP version", resp.Proto)
	}

	// Parse the response headers.
	mimeHeader, err := tp.ReadMIMEHeader()
	if err != nil {
		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		}
		return nil, err
	}
	resp.Header = Header(mimeHeader)

	fixPragmaCacheControl(resp.Header)

	err = readTransfer(resp, r)
	if err != nil {
		return nil, err
	}

	return resp, nil
}

// RFC 7234, section 5.4: Should treat
//
//	Pragma: no-cache
//
// like
//
//	Cache-Control: no-cache
func fixPragmaCacheControl(header Header) {
	if hp, ok := header["Pragma"]; ok && len(hp) > 0 && hp[0] == "no-cache" {
		if _, presentcc := header["Cache-Control"]; !presentcc {
			header["Cache-Control"] = []string{"no-cache"}
		}
	}
}

// ProtoAtLeast reports whether the HTTP protocol used
// in the response is at least major.minor.
func (r *Response) ProtoAtLeast(major, minor int) bool {
	return r.ProtoMajor > major ||
		r.ProtoMajor == major && r.ProtoMinor >= minor
}

// Write writes r to w in the HTTP/1.x server response format,
// including the status line, headers, body, and optional trailer.
//
// This method consults the following fields of the response r:
//
//	StatusCode
//	ProtoMajor
//	ProtoMinor
//	Request.Method
//	TransferEncoding
//	Trailer
//	Body
//	ContentLength
//	Header, values for non-canonical keys will have unpredictable behavior
//
// The Response Body is closed after it is sent.
func (r *Response) Write(w io.Writer) error {
	// Status line
	text := r.Status
	if text == "" {
		text = StatusText(r.StatusCode)
		if text == "" {
			text = "status code " + strconv.Itoa(r.StatusCode)
		}
	} else {
		// Just to reduce stutter, if user set r.Status to "200 OK" and StatusCode to 200.
		// Not important.
		text = strings.TrimPrefix(text, strconv.Itoa(r.StatusCode)+" ")
	}

	if _, err := fmt.Fprintf(w, "HTTP/%d.%d %03d %s\r\n", r.ProtoMajor, r.ProtoMinor, r.StatusCode, text); err != nil {
		return err
	}

	// Clone it, so we can modify r1 as needed.
	r1 := new(Response)
	*r1 = *r
	if r1.ContentLength == 0 && r1.Body != nil {
		// Is it actually 0 length? Or just unknown?
		var buf [1]byte
		n, err := r1.Body.Read(buf[:])
		if err != nil && err != io.EOF {
			return err
		}
		if n == 0 {
			// Reset it to a known zero reader, in case underlying one
			// is unhappy being read repeatedly.
			r1.Body = NoBody
		} else {
			r1.ContentLength = -1
			r1.Body = struct {
				io.Reader
				io.Closer
			}{
				io.MultiReader(bytes.NewReader(buf[:1]), r.Body),
				r.Body,
			}
		}
	}
	// If we're sending a non-chunked HTTP/1.1 response without a
	// content-length, the only way to do that is the old HTTP/1.0
	// way, by noting the EOF with a connection close, so we need
	// to set Close.
	if r1.ContentLength == -1 && !r1.Close && r1.ProtoAtLeast(1, 1) && !chunked(r1.TransferEncoding) && !r1.Uncompressed {
		r1.Close = true
	}

	// Process Body,ContentLength,Close,Trailer
	tw, err := newTransferWriter(r1)
	if err != nil {
		return err
	}
	err = tw.writeHeader(w, nil)
	if err != nil {
		return err
	}

	// Rest of header
	err = r.Header.WriteSubset(w, respExcludeHeader)
	if err != nil {
		return err
	}

	// contentLengthAlreadySent may have been already sent for
	// POST/PUT requests, even if zero length. See Issue 8180.
	contentLengthAlreadySent := tw.shouldSendContentLength()
	if r1.ContentLength == 0 && !chunked(r1.TransferEncoding) && !contentLengthAlreadySent && bodyAllowedForStatus(r.StatusCode) {
		if _, err := io.WriteString(w, "Content-Length: 0\r\n"); err != nil {
			return err
		}
	}

	// End-of-header
	if _, err := io.WriteString(w, "\r\n"); err != nil {
		return err
	}

	// Write body and trailer
	err = tw.writeBody(w)
	if err != nil {
		return err
	}

	// Success
	return nil
}

func (r *Response) closeBody() {
	if r.Body != nil {
		r.Body.Close()
	}
}

// bodyIsWritable reports whether the Body supports writing. The
// Transport returns Writable bodies for 101 Switching Protocols
// responses.
// The Transport uses this method to determine whether a persistent
// connection is done being managed from its perspective. Once we
// return a writable response body to a user, the net/http package is
// done managing that connection.
func (r *Response) bodyIsWritable() bool {
	_, ok := r.Body.(io.Writer)
	return ok
}

// isProtocolSwitch reports whether the response code and header
// indicate a successful protocol upgrade response.
func (r *Response) isProtocolSwitch() bool {
	return isProtocolSwitchResponse(r.StatusCode, r.Header)
}

// isProtocolSwitchResponse reports whether the response code and
// response header indicate a successful protocol upgrade response.
func isProtocolSwitchResponse(code int, h Header) bool {
	return code == StatusSwitchingProtocols && isProtocolSwitchHeader(h)
}

// isProtocolSwitchHeader reports whether the request or response header
// is for a protocol switch.
func isProtocolSwitchHeader(h Header) bool {
	return h.Get("Upgrade") != "" &&
		httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade")
}

The pages are generated with Golds v0.6.9-preview. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
Please follow @Go100and1 (reachable from the left QR code) to get the latest news of Golds.