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A Go web server from scratch using syscalls
package main
// Simple, single-threaded server using system calls instead of the net library.
//
// Omitted features from the go net package:
//
// - TLS
// - Most error checking
// - Only supports bodies that close, no persistent or chunked connections
// - Redirects
// - Deadlines and cancellation
// - Non-blocking sockets
import (
"bufio"
"errors"
"flag"
"io"
"log"
"net"
"net/textproto"
"os"
"strconv"
"strings"
"syscall"
)
// netSocket is a file descriptor for a system socket.
type netSocket struct {
// System file descriptor.
fd int
}
func (ns netSocket) Read(p []byte) (int, error) {
if len(p) == 0 {
return 0, nil
}
n, err := syscall.Read(ns.fd, p)
if err != nil {
n = 0
}
return n, err
}
func (ns netSocket) Write(p []byte) (int, error) {
n, err := syscall.Write(ns.fd, p)
if err != nil {
n = 0
}
return n, err
}
// Creates a new netSocket for the next pending connection request.
func (ns *netSocket) Accept() (*netSocket, error) {
// syscall.ForkLock doc states lock not needed for blocking accept.
nfd, _, err := syscall.Accept(ns.fd)
if err == nil {
syscall.CloseOnExec(nfd)
}
if err != nil {
return nil, err
}
return &netSocket{nfd}, nil
}
func (ns *netSocket) Close() error {
return syscall.Close(ns.fd)
}
// Creates a new socket file descriptor, binds it and listens on it.
func newNetSocket(ip net.IP, port int) (*netSocket, error) {
// ForkLock docs state that socket syscall requires the lock.
syscall.ForkLock.Lock()
// AF_INET = Address Family for IPv4
// SOCK_STREAM = virtual circuit service
// 0: the protocol for SOCK_STREAM, there's only 1.
fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_STREAM, 0)
if err != nil {
return nil, os.NewSyscallError("socket", err)
}
syscall.ForkLock.Unlock()
// Allow reuse of recently-used addresses.
if err = syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1); err != nil {
syscall.Close(fd)
return nil, os.NewSyscallError("setsockopt", err)
}
// Bind the socket to a port
sa := &syscall.SockaddrInet4{Port: port}
copy(sa.Addr[:], ip)
if err = syscall.Bind(fd, sa); err != nil {
return nil, os.NewSyscallError("bind", err)
}
// Listen for incoming connections.
if err = syscall.Listen(fd, syscall.SOMAXCONN); err != nil {
return nil, os.NewSyscallError("listen", err)
}
return &netSocket{fd: fd}, nil
}
type request struct {
method string // GET, POST, etc.
header textproto.MIMEHeader
body []byte
uri string // The raw URI from the request
proto string // "HTTP/1.1"
}
func parseRequest(c *netSocket) (*request, error) {
b := bufio.NewReader(*c)
tp := textproto.NewReader(b)
req := new(request)
// First line: parse "GET /index.html HTTP/1.0"
var s string
s, _ = tp.ReadLine()
sp := strings.Split(s, " ")
req.method, req.uri, req.proto = sp[0], sp[1], sp[2]
// Parse headers
mimeHeader, _ := tp.ReadMIMEHeader()
req.header = mimeHeader
// Parse body
if req.method == "GET" || req.method == "HEAD" {
return req, nil
}
if len(req.header["Content-Length"]) == 0 {
return nil, errors.New("no content length")
}
length, err := strconv.Atoi(req.header["Content-Length"][0])
if err != nil {
return nil, err
}
body := make([]byte, length)
if _, err = io.ReadFull(b, body); err != nil {
return nil, err
}
req.body = body
return req, nil
}
func main() {
ipFlag := flag.String("ip_addr", "127.0.0.1", "The IP address to use")
portFlag := flag.Int("port", 8080, "The port to use.")
flag.Parse()
ip := net.ParseIP(*ipFlag)
port := *portFlag
socket, err := newNetSocket(ip, port)
defer socket.Close()
if err != nil {
panic(err)
}
log.Print("===============")
log.Print("Server Started!")
log.Print("===============")
log.Print()
log.Printf("addr: http://%s:%d", ip, port)
for {
// Block until incoming connection
rw, e := socket.Accept()
log.Print()
log.Print()
log.Printf("Incoming connection")
if e != nil {
panic(e)
}
// Read request
log.Print("Reading request")
req, err := parseRequest(rw)
log.Print("request: ", req)
if err != nil {
panic(err)
}
// Write response
log.Print("Writing response")
io.WriteString(rw, "HTTP/1.1 200 OK\r\n"+
"Content-Type: text/html; charset=utf-8\r\n"+
"Content-Length: 20\r\n"+
"\r\n"+
"<h1>hello world</h1>")
if err != nil {
log.Print(err.Error())
continue
}
}
}
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