Initial commit

Add a work-in-progress demo chat server with TLS autodetection.

It needs to be perfected before I move on to writing an IRCd on
the foundations.  If history is of any indication, this might lead
towards a nice set of applications.

3 files changed, 369 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..796b96d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/build
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..c6b6b3e
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,12 @@
+Copyright (c) 2018, Přemysl Janouch <p@janouch.name>
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
diff --git a/prototypes/tls-autodetect.go b/prototypes/tls-autodetect.go
new file mode 100644
index 0000000..c58ed28
--- /dev/null
+++ b/prototypes/tls-autodetect.go
@@ -0,0 +1,356 @@
+//
+// Copyright (c) 2018, Přemysl Janouch <p@janouch.name>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+//
+
+// This is an example TLS-autodetecting chat server.
+//
+// You may connect to it either using:
+//  telnet localhost 1234
+// or
+//  openssl s_client -connect localhost:1234
+package main
+
+import (
+	"bufio"
+	"crypto/tls"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+)
+
+// --- Utilities ---------------------------------------------------------------
+
+// Trivial SSL/TLS autodetection. The first block of data returned by Recvfrom
+// must be at least three octets long for this to work reliably, but that should
+// not pose a problem in practice. We might try waiting for them.
+//
+//  SSL2:      1xxx xxxx | xxxx xxxx |    <1>
+//                (message length)  (client hello)
+//  SSL3/TLS:    <22>    |    <3>    | xxxx xxxx
+//            (handshake)|  (protocol version)
+//
+func detectTLS(sysconn syscall.RawConn) bool {
+	isTLS := false
+	sysconn.Read(func(fd uintptr) (done bool) {
+		var buf [3]byte
+		n, _, err := syscall.Recvfrom(int(fd), buf[:], syscall.MSG_PEEK)
+		switch {
+		case n == 3:
+			isTLS = buf[0]&0x80 != 0 && buf[2] == 1
+			fallthrough
+		case n == 2:
+			isTLS = buf[0] == 22 && buf[1] == 3
+		case n == 1:
+			isTLS = buf[0] == 22
+		case err == syscall.EAGAIN:
+			return false
+		}
+		return true
+	})
+	return isTLS
+}
+
+// --- Declarations ------------------------------------------------------------
+
+type connCloseWrite interface {
+	net.Conn
+	CloseWrite() error
+}
+
+type client struct {
+	transport  net.Conn       // underlying connection
+	tls        *tls.Conn      // TLS, if detected
+	conn       connCloseWrite // high-level connection
+	connReady  bool           // conn is safe to read from the main goroutine
+	inQ        []byte         // unprocessed input
+	outQ       []byte         // unprocessed output
+	writing    bool           // whether a writing goroutine is running
+	inShutdown bool           // whether we're closing connection
+}
+
+type readEvent struct {
+	client *client // client
+	data   []byte  // new data from the client
+	err    error   // read error
+}
+
+type writeEvent struct {
+	client  *client // client
+	written int     // amount of bytes written
+	err     error   // write error
+}
+
+var (
+	sigs   = make(chan os.Signal, 1)
+	conns  = make(chan net.Conn)
+	reads  = make(chan readEvent)
+	writes = make(chan writeEvent)
+
+	tlsConf       *tls.Config
+	clients       = make(map[*client]bool)
+	listener      net.Listener
+	inShutdown    bool
+	shutdownTimer <-chan time.Time
+)
+
+// --- Server ------------------------------------------------------------------
+
+// Broadcast to all /other/ clients (telnet-friendly, also in accordance to
+// the plan of extending this to an IRCd).
+func broadcast(line string, except *client) {
+	for c := range clients {
+		if c != except {
+			c.send(line)
+		}
+	}
+}
+
+func initiateShutdown() {
+	log.Println("shutting down")
+	if err := listener.Close(); err != nil {
+		log.Println(err)
+	}
+	for c := range clients {
+		c.kill()
+	}
+
+	shutdownTimer = time.After(3 * time.Second)
+	inShutdown = true
+}
+
+func forceShutdown(reason string) {
+	log.Printf("forced shutdown (%s)\n", reason)
+	for c := range clients {
+		c.destroy()
+	}
+}
+
+// --- Client ------------------------------------------------------------------
+
+func (c *client) send(line string) {
+	if !c.inShutdown {
+		c.outQ = append(c.outQ, (line + "\r\n")...)
+		c.flushOutQ()
+	}
+}
+
+func (c *client) shutdown() {
+	if c.inShutdown {
+		log.Println("client double shutdown")
+		return
+	}
+
+	// TODO: We must set a timer and destroy the client on timeout. Since we
+	//   have a central event loop, we probably need an event. Since we also
+	//   seem to need an event for TLS autodetection because of conn, we might
+	//   want to send an enumeration value.
+	c.inShutdown = true
+	c.conn.CloseWrite()
+}
+
+// Tear down the client connection, trying to do so in a graceful manner.
+func (c *client) kill() {
+	if c.connReady {
+		c.send("Goodbye")
+		c.shutdown()
+	} else {
+		c.destroy()
+	}
+}
+
+// Close the connection and forget about the client.
+func (c *client) destroy() {
+	// Try to send a "close notify" alert if the TLS object is ready,
+	// otherwise just tear down the transport.
+	if c.connReady {
+		_ = c.conn.Close()
+	} else {
+		_ = c.transport.Close()
+	}
+
+	delete(clients, c)
+}
+
+// Handle the results from trying to read from the client connection.
+func (c *client) onRead(data []byte, readErr error) {
+	c.inQ = append(c.inQ, data...)
+	for {
+		advance, token, _ := bufio.ScanLines(c.inQ, false /* atEOF */)
+		c.inQ = c.inQ[advance:]
+		if advance == 0 {
+			break
+		}
+
+		line := string(token)
+		fmt.Println(line)
+		broadcast(line, c)
+	}
+
+	// TODO: Inform the client about the inQ overrun in the farewell message.
+	if len(c.inQ) > 8192 {
+		c.kill()
+		return
+	}
+
+	if readErr == io.EOF {
+		// TODO: What if we're already in shutdown?
+		c.shutdown()
+	} else if readErr != nil {
+		log.Println(readErr)
+		c.destroy()
+	}
+}
+
+// Spawn a goroutine to flush the outQ if possible and necessary.  If the
+// connection is not ready yet, it needs to be retried as soon as it becomes.
+func (c *client) flushOutQ() {
+	if c.connReady && !c.writing {
+		go write(c, c.outQ)
+		c.writing = true
+	}
+}
+
+// Handle the results from trying to write to the client connection.
+func (c *client) onWrite(written int, writeErr error) {
+	c.outQ = c.outQ[written:]
+	c.writing = false
+
+	if writeErr != nil {
+		log.Println(writeErr)
+		c.destroy()
+	} else if len(c.outQ) > 0 {
+		c.flushOutQ()
+	} else if c.inShutdown {
+		c.destroy()
+	}
+}
+
+// --- Worker goroutines -------------------------------------------------------
+
+func accept(ln net.Listener) {
+	// TODO: Consider specific cases in error handling, some errors
+	//   are transitional while others are fatal.
+	for {
+		if conn, err := ln.Accept(); err != nil {
+			log.Println(err)
+		} else {
+			conns <- conn
+		}
+	}
+}
+
+func read(client *client) {
+	// TODO: Either here or elsewhere we need to set a timeout.
+
+	client.conn = client.transport.(connCloseWrite)
+	if sysconn, err := client.transport.(syscall.Conn).SyscallConn(); err != nil {
+		// This is just for the TLS detection and doesn't need to be fatal.
+		log.Println(err)
+	} else if detectTLS(sysconn) {
+		client.tls = tls.Server(client.transport, tlsConf)
+		client.conn = client.tls
+	}
+
+	// TODO: Signal the main goroutine that conn is ready. In fact, the upper
+	//   part could be mostly moved to the main goroutine and we'd only spawn
+	//   a thin wrapper around detectTLS, sending back {*client, bool}. Heck,
+	//   I could get rid of connReady.
+
+	// A new buffer is allocated each time we receive some bytes, because of
+	// thread-safety. Therefore the buffer shouldn't be too large, or we'd
+	// need to copy it each time into a precisely sized new buffer.
+	var err error
+	for err == nil {
+		var (
+			buf [512]byte
+			n   int
+		)
+		n, err = client.conn.Read(buf[:])
+		reads <- readEvent{client, buf[:n], err}
+	}
+}
+
+// Flush outQ, which is passed by parameter so that there are no data races.
+func write(client *client, data []byte) {
+	// We just write as much as we can, the main goroutine does the looping.
+	n, err := client.conn.Write(data)
+	writes <- writeEvent{client, n, err}
+}
+
+// --- Main --------------------------------------------------------------------
+
+func processOneEvent() {
+	select {
+	case <-sigs:
+		if inShutdown {
+			forceShutdown("requested by user")
+		} else {
+			initiateShutdown()
+		}
+
+	case <-shutdownTimer:
+		forceShutdown("timeout")
+
+	case conn := <-conns:
+		log.Println("accepted client connection")
+		c := &client{transport: conn}
+		clients[c] = true
+		go read(c)
+
+	case ev := <-reads:
+		log.Println("received data from client")
+		if _, ok := clients[ev.client]; ok {
+			ev.client.onRead(ev.data, ev.err)
+		}
+
+	case ev := <-writes:
+		log.Println("sent data to client")
+		if _, ok := clients[ev.client]; ok {
+			ev.client.onWrite(ev.written, ev.err)
+		}
+	}
+}
+
+func main() {
+	// Just deal with unexpected flags, we don't use any ourselves.
+	flag.Parse()
+
+	if len(flag.Args()) != 3 {
+		log.Fatalf("usage: %s KEY CERT ADDRESS\n", os.Args[0])
+	}
+
+	cert, err := tls.LoadX509KeyPair(flag.Arg(1), flag.Arg(0))
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	tlsConf = &tls.Config{Certificates: []tls.Certificate{cert}}
+	listener, err = net.Listen("tcp", flag.Arg(2))
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	go accept(listener)
+	signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)
+
+	for !inShutdown || len(clients) > 0 {
+		processOneEvent()
+	}
+}