child.go

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // This file implements CGI from the perspective of a child
     6  // process.
     7  
     8  package cgi
     9  
    10  import (
    11  	"bufio"
    12  	"crypto/tls"
    13  	"errors"
    14  	"fmt"
    15  	"io"
    16  	"net"
    17  	"net/http"
    18  	"net/url"
    19  	"os"
    20  	"strconv"
    21  	"strings"
    22  )
    23  
    24  // Request returns the HTTP request as represented in the current
    25  // environment. This assumes the current program is being run
    26  // by a web server in a CGI environment.
    27  // The returned Request's Body is populated, if applicable.
    28  func Request() (*http.Request, error) {
    29  	r, err := RequestFromMap(envMap(os.Environ()))
    30  	if err != nil {
    31  		return nil, err
    32  	}
    33  	if r.ContentLength > 0 {
    34  		r.Body = io.NopCloser(io.LimitReader(os.Stdin, r.ContentLength))
    35  	}
    36  	return r, nil
    37  }
    38  
    39  func envMap(env []string) map[string]string {
    40  	m := make(map[string]string)
    41  	for _, kv := range env {
    42  		if k, v, ok := strings.Cut(kv, "="); ok {
    43  			m[k] = v
    44  		}
    45  	}
    46  	return m
    47  }
    48  
    49  // RequestFromMap creates an [http.Request] from CGI variables.
    50  // The returned Request's Body field is not populated.
    51  func RequestFromMap(params map[string]string) (*http.Request, error) {
    52  	r := new(http.Request)
    53  	r.Method = params["REQUEST_METHOD"]
    54  	if r.Method == "" {
    55  		return nil, errors.New("cgi: no REQUEST_METHOD in environment")
    56  	}
    57  
    58  	r.Proto = params["SERVER_PROTOCOL"]
    59  	var ok bool
    60  	r.ProtoMajor, r.ProtoMinor, ok = http.ParseHTTPVersion(r.Proto)
    61  	if !ok {
    62  		return nil, errors.New("cgi: invalid SERVER_PROTOCOL version")
    63  	}
    64  
    65  	r.Close = true
    66  	r.Trailer = http.Header{}
    67  	r.Header = http.Header{}
    68  
    69  	r.Host = params["HTTP_HOST"]
    70  
    71  	if lenstr := params["CONTENT_LENGTH"]; lenstr != "" {
    72  		clen, err := strconv.ParseInt(lenstr, 10, 64)
    73  		if err != nil {
    74  			return nil, errors.New("cgi: bad CONTENT_LENGTH in environment: " + lenstr)
    75  		}
    76  		r.ContentLength = clen
    77  	}
    78  
    79  	if ct := params["CONTENT_TYPE"]; ct != "" {
    80  		r.Header.Set("Content-Type", ct)
    81  	}
    82  
    83  	// Copy "HTTP_FOO_BAR" variables to "Foo-Bar" Headers
    84  	for k, v := range params {
    85  		if k == "HTTP_HOST" {
    86  			continue
    87  		}
    88  		if after, found := strings.CutPrefix(k, "HTTP_"); found {
    89  			r.Header.Add(strings.ReplaceAll(after, "_", "-"), v)
    90  		}
    91  	}
    92  
    93  	uriStr := params["REQUEST_URI"]
    94  	if uriStr == "" {
    95  		// Fallback to SCRIPT_NAME, PATH_INFO and QUERY_STRING.
    96  		uriStr = params["SCRIPT_NAME"] + params["PATH_INFO"]
    97  		s := params["QUERY_STRING"]
    98  		if s != "" {
    99  			uriStr += "?" + s
   100  		}
   101  	}
   102  
   103  	// There's apparently a de-facto standard for this.
   104  	// https://web.archive.org/web/20170105004655/http://docstore.mik.ua/orelly/linux/cgi/ch03_02.htm#ch03-35636
   105  	if s := params["HTTPS"]; s == "on" || s == "ON" || s == "1" {
   106  		r.TLS = &tls.ConnectionState{HandshakeComplete: true}
   107  	}
   108  
   109  	if r.Host != "" {
   110  		// Hostname is provided, so we can reasonably construct a URL.
   111  		rawurl := r.Host + uriStr
   112  		if r.TLS == nil {
   113  			rawurl = "http://" + rawurl
   114  		} else {
   115  			rawurl = "https://" + rawurl
   116  		}
   117  		url, err := url.Parse(rawurl)
   118  		if err != nil {
   119  			return nil, errors.New("cgi: failed to parse host and REQUEST_URI into a URL: " + rawurl)
   120  		}
   121  		r.URL = url
   122  	}
   123  	// Fallback logic if we don't have a Host header or the URL
   124  	// failed to parse
   125  	if r.URL == nil {
   126  		url, err := url.Parse(uriStr)
   127  		if err != nil {
   128  			return nil, errors.New("cgi: failed to parse REQUEST_URI into a URL: " + uriStr)
   129  		}
   130  		r.URL = url
   131  	}
   132  
   133  	// Request.RemoteAddr has its port set by Go's standard http
   134  	// server, so we do here too.
   135  	remotePort, _ := strconv.Atoi(params["REMOTE_PORT"]) // zero if unset or invalid
   136  	r.RemoteAddr = net.JoinHostPort(params["REMOTE_ADDR"], strconv.Itoa(remotePort))
   137  
   138  	return r, nil
   139  }
   140  
   141  // Serve executes the provided [Handler] on the currently active CGI
   142  // request, if any. If there's no current CGI environment
   143  // an error is returned. The provided handler may be nil to use
   144  // [http.DefaultServeMux].
   145  func Serve(handler http.Handler) error {
   146  	req, err := Request()
   147  	if err != nil {
   148  		return err
   149  	}
   150  	if req.Body == nil {
   151  		req.Body = http.NoBody
   152  	}
   153  	if handler == nil {
   154  		handler = http.DefaultServeMux
   155  	}
   156  	rw := &response{
   157  		req:    req,
   158  		header: make(http.Header),
   159  		bufw:   bufio.NewWriter(os.Stdout),
   160  	}
   161  	handler.ServeHTTP(rw, req)
   162  	rw.Write(nil) // make sure a response is sent
   163  	if err = rw.bufw.Flush(); err != nil {
   164  		return err
   165  	}
   166  	return nil
   167  }
   168  
   169  type response struct {
   170  	req            *http.Request
   171  	header         http.Header
   172  	code           int
   173  	wroteHeader    bool
   174  	wroteCGIHeader bool
   175  	bufw           *bufio.Writer
   176  }
   177  
   178  func (r *response) Flush() {
   179  	r.bufw.Flush()
   180  }
   181  
   182  func (r *response) Header() http.Header {
   183  	return r.header
   184  }
   185  
   186  func (r *response) Write(p []byte) (n int, err error) {
   187  	if !r.wroteHeader {
   188  		r.WriteHeader(http.StatusOK)
   189  	}
   190  	if !r.wroteCGIHeader {
   191  		r.writeCGIHeader(p)
   192  	}
   193  	return r.bufw.Write(p)
   194  }
   195  
   196  func (r *response) WriteHeader(code int) {
   197  	if r.wroteHeader {
   198  		// Note: explicitly using Stderr, as Stdout is our HTTP output.
   199  		fmt.Fprintf(os.Stderr, "CGI attempted to write header twice on request for %s", r.req.URL)
   200  		return
   201  	}
   202  	r.wroteHeader = true
   203  	r.code = code
   204  }
   205  
   206  // writeCGIHeader finalizes the header sent to the client and writes it to the output.
   207  // p is not written by writeHeader, but is the first chunk of the body
   208  // that will be written. It is sniffed for a Content-Type if none is
   209  // set explicitly.
   210  func (r *response) writeCGIHeader(p []byte) {
   211  	if r.wroteCGIHeader {
   212  		return
   213  	}
   214  	r.wroteCGIHeader = true
   215  	fmt.Fprintf(r.bufw, "Status: %d %s\r\n", r.code, http.StatusText(r.code))
   216  	if _, hasType := r.header["Content-Type"]; !hasType {
   217  		r.header.Set("Content-Type", http.DetectContentType(p))
   218  	}
   219  	r.header.Write(r.bufw)
   220  	r.bufw.WriteString("\r\n")
   221  	r.bufw.Flush()
   222  }
   223
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