func Serve(l net.Listener, handler http.Handler) error {
	if l == nil {
		var err error
		l, err = net.FileListener(os.Stdin)
		if err != nil {
			return err
		}
		defer l.Close()
	}
	if handler == nil {
		handler = http.DefaultServeMux
	}
	for {
		rw, err := l.Accept()
		if err != nil {
			return err
		}
		c := newChild(rw, handler)
		go c.serve()
	}
}
Serve accepts incoming FastCGI connections on the listener l, creating a new
goroutine for each. The goroutine reads requests and then calls handler
to reply to them.
If l is nil, Serve accepts connections from os.Stdin.
If handler is nil, http.DefaultServeMux is used.
package fcgi
import (
	"context"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"net"
	"net/http"
	"net/http/cgi"
	"os"
	"strings"
	"sync"
	"time"
)
type request struct {
	pw        *io.PipeWriter
	reqId     uint16
	params    map[string]string
	buf       [1024]byte
	rawParams []byte
	keepConn  bool
}
type envVarsContextKey struct{}
func newRequest(reqId uint16, flags uint8) *request {
	r := &request{
		reqId:    reqId,
		params:   map[string]string{},
		keepConn: flags&flagKeepConn != 0,
	}
	r.rawParams = r.buf[:0]
	return r
}
func (r *request) parseParams() {
	text := r.rawParams
	r.rawParams = nil
	for len(text) > 0 {
		keyLen, n := readSize(text)
		if n == 0 {
			return
		}
		text = text[n:]
		valLen, n := readSize(text)
		if n == 0 {
			return
		}
		text = text[n:]
		if int(keyLen)+int(valLen) > len(text) {
			return
		}
		key := readString(text, keyLen)
		text = text[keyLen:]
		val := readString(text, valLen)
		text = text[valLen:]
		r.params[key] = val
	}
}
type response struct {
	req         *request
	header      http.Header
	w           *bufWriter
	wroteHeader bool
}
func newResponse(c *child, req *request) *response {
	return &response{
		req:    req,
		header: http.Header{},
		w:      newWriter(c.conn, typeStdout, req.reqId),
	}
}
func (r *response) Header() http.Header {
	return r.header
}
func (r *response) Write(data []byte) (int, error) {
	if !r.wroteHeader {
		r.WriteHeader(http.StatusOK)
	}
	return r.w.Write(data)
}
func (r *response) WriteHeader(code int) {
	if r.wroteHeader {
		return
	}
	r.wroteHeader = true
	if code == http.StatusNotModified {
		r.header.Del("Content-Type")
		r.header.Del("Content-Length")
		r.header.Del("Transfer-Encoding")
	} else if r.header.Get("Content-Type") == "" {
		r.header.Set("Content-Type", "text/html; charset=utf-8")
	}
	if r.header.Get("Date") == "" {
		r.header.Set("Date", time.Now().UTC().Format(http.TimeFormat))
	}
	fmt.Fprintf(r.w, "Status: %d %s\r\n", code, http.StatusText(code))
	r.header.Write(r.w)
	r.w.WriteString("\r\n")
}
func (r *response) Flush() {
	if !r.wroteHeader {
		r.WriteHeader(http.StatusOK)
	}
	r.w.Flush()
}
func (r *response) Close() error {
	r.Flush()
	return r.w.Close()
}
type child struct {
	conn    *conn
	handler http.Handler
	mu       sync.Mutex           
	requests map[uint16]*request  
}
func newChild(rwc io.ReadWriteCloser, handler http.Handler) *child {
	return &child{
		conn:     newConn(rwc),
		handler:  handler,
		requests: make(map[uint16]*request),
	}
}
func (c *child) serve() {
	defer c.conn.Close()
	defer c.cleanUp()
	var rec record
	for {
		if err := rec.read(c.conn.rwc); err != nil {
			return
		}
		if err := c.handleRecord(&rec); err != nil {
			return
		}
	}
}
var errCloseConn = errors.New("fcgi: connection should be closed")
var emptyBody = ioutil.NopCloser(strings.NewReader(""))
var ErrRequestAborted = errors.New("fcgi: request aborted by web server")
var ErrConnClosed = errors.New("fcgi: connection to web server closed")
func (c *child) handleRecord(rec *record) error {
	c.mu.Lock()
	req, ok := c.requests[rec.h.Id]
	c.mu.Unlock()
	if !ok && rec.h.Type != typeBeginRequest && rec.h.Type != typeGetValues {
		return nil
	}
	switch rec.h.Type {
	case typeBeginRequest:
		if req != nil {
			return errors.New("fcgi: received ID that is already in-flight")
		}
		var br beginRequest
		if err := br.read(rec.content()); err != nil {
			return err
		}
		if br.role != roleResponder {
			c.conn.writeEndRequest(rec.h.Id, 0, statusUnknownRole)
			return nil
		}
		req = newRequest(rec.h.Id, br.flags)
		c.mu.Lock()
		c.requests[rec.h.Id] = req
		c.mu.Unlock()
		return nil
	case typeParams:
		if len(rec.content()) > 0 {
			req.rawParams = append(req.rawParams, rec.content()...)
			return nil
		}
		req.parseParams()
		return nil
	case typeStdin:
		content := rec.content()
		if req.pw == nil {
			var body io.ReadCloser
			if len(content) > 0 {
				body, req.pw = io.Pipe()
			} else {
				body = emptyBody
			}
			go c.serveRequest(req, body)
		}
		if len(content) > 0 {
			req.pw.Write(content)
		} else if req.pw != nil {
			req.pw.Close()
		}
		return nil
	case typeGetValues:
		values := map[string]string{"FCGI_MPXS_CONNS": "1"}
		c.conn.writePairs(typeGetValuesResult, 0, values)
		return nil
	case typeData:
		return nil
	case typeAbortRequest:
		c.mu.Lock()
		delete(c.requests, rec.h.Id)
		c.mu.Unlock()
		c.conn.writeEndRequest(rec.h.Id, 0, statusRequestComplete)
		if req.pw != nil {
			req.pw.CloseWithError(ErrRequestAborted)
		}
		if !req.keepConn {
			return errCloseConn
		}
		return nil
	default:
		b := make([]byte, 8)
		b[0] = byte(rec.h.Type)
		c.conn.writeRecord(typeUnknownType, 0, b)
		return nil
	}
}
func filterOutUsedEnvVars(envVars map[string]string) map[string]string {
	withoutUsedEnvVars := make(map[string]string)
	for k, v := range envVars {
		if addFastCGIEnvToContext(k) {
			withoutUsedEnvVars[k] = v
		}
	}
	return withoutUsedEnvVars
}
func (c *child) serveRequest(req *request, body io.ReadCloser) {
	r := newResponse(c, req)
	httpReq, err := cgi.RequestFromMap(req.params)
	if err != nil {
		r.WriteHeader(http.StatusInternalServerError)
		c.conn.writeRecord(typeStderr, req.reqId, []byte(err.Error()))
	} else {
		httpReq.Body = body
		withoutUsedEnvVars := filterOutUsedEnvVars(req.params)
		envVarCtx := context.WithValue(httpReq.Context(), envVarsContextKey{}, withoutUsedEnvVars)
		httpReq = httpReq.WithContext(envVarCtx)
		c.handler.ServeHTTP(r, httpReq)
	}
	r.Close()
	c.mu.Lock()
	delete(c.requests, req.reqId)
	c.mu.Unlock()
	c.conn.writeEndRequest(req.reqId, 0, statusRequestComplete)
	io.CopyN(ioutil.Discard, body, 100<<20)
	body.Close()
	if !req.keepConn {
		c.conn.Close()
	}
}
func (c *child) cleanUp() {
	c.mu.Lock()
	defer c.mu.Unlock()
	for _, req := range c.requests {
		if req.pw != nil {
			req.pw.CloseWithError(ErrConnClosed)
		}
	}
}