net: DialTimeout

Fixes #240

R=adg, dsymonds, rsc, r, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5491062

Author: bradfitz

src/pkg/net/dial.go

@@ -4,6 +4,10 @@
 
 package net
 
+import (
+	"time"
+)
+
 func resolveNetAddr(op, net, addr string) (a Addr, err error) {
 	if addr == "" {
 		return nil, &OpError{op, net, nil, errMissingAddress}
@@ -42,11 +46,15 @@ func resolveNetAddr(op, net, addr string) (a Addr, err error) {
 //	Dial("tcp", "google.com:80")
 //	Dial("tcp", "[de:ad:be:ef::ca:fe]:80")
 //
-func Dial(net, addr string) (c Conn, err error) {
+func Dial(net, addr string) (Conn, error) {
 	addri, err := resolveNetAddr("dial", net, addr)
 	if err != nil {
 		return nil, err
 	}
+	return dialAddr(net, addr, addri)
+}
+
+func dialAddr(net, addr string, addri Addr) (c Conn, err error) {
 	switch ra := addri.(type) {
 	case *TCPAddr:
 		c, err = DialTCP(net, nil, ra)
@@ -65,6 +73,62 @@ func Dial(net, addr string) (c Conn, err error) {
 	return
 }
 
+// DialTimeout acts like Dial but takes a timeout.
+// The timeout includes name resolution, if required.
+func DialTimeout(net, addr string, timeout time.Duration) (Conn, error) {
+	// TODO(bradfitz): the timeout should be pushed down into the
+	// net package's event loop, so on timeout to dead hosts we
+	// don't have a goroutine sticking around for the default of
+	// ~3 minutes.
+	t := time.NewTimer(timeout)
+	defer t.Stop()
+	type pair struct {
+		Conn
+		error
+	}
+	ch := make(chan pair, 1)
+	resolvedAddr := make(chan Addr, 1)
+	go func() {
+		addri, err := resolveNetAddr("dial", net, addr)
+		if err != nil {
+			ch <- pair{nil, err}
+			return
+		}
+		resolvedAddr <- addri // in case we need it for OpError
+		c, err := dialAddr(net, addr, addri)
+		ch <- pair{c, err}
+	}()
+	select {
+	case <-t.C:
+		// Try to use the real Addr in our OpError, if we resolved it
+		// before the timeout. Otherwise we just use stringAddr.
+		var addri Addr
+		select {
+		case a := <-resolvedAddr:
+			addri = a
+		default:
+			addri = &stringAddr{net, addr}
+		}
+		err := &OpError{
+			Op:   "dial",
+			Net:  net,
+			Addr: addri,
+			Err:  &timeoutError{},
+		}
+		return nil, err
+	case p := <-ch:
+		return p.Conn, p.error
+	}
+	panic("unreachable")
+}
+
+type stringAddr struct {
+	net, addr string
+}
+
+func (a stringAddr) Network() string { return a.net }
+func (a stringAddr) String() string  { return a.addr }
+
 // Listen announces on the local network address laddr.
 // The network string net must be a stream-oriented
 // network: "tcp", "tcp4", "tcp6", or "unix", or "unixpacket".

src/pkg/net/dial_test.go

@@ -0,0 +1,88 @@
+// Copyright 2011 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.
+
+package net
+
+import (
+	"runtime"
+	"testing"
+	"time"
+)
+
+func newLocalListener(t *testing.T) Listener {
+	ln, err := Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		ln, err = Listen("tcp6", "[::1]:0")
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	return ln
+}
+
+func TestDialTimeout(t *testing.T) {
+	ln := newLocalListener(t)
+	defer ln.Close()
+
+	errc := make(chan error)
+
+	const SOMAXCONN = 0x80 // copied from syscall, but not always available
+	const numConns = SOMAXCONN + 10
+
+	// TODO(bradfitz): It's hard to test this in a portable
+	// way. This is unforunate, but works for now.
+	switch runtime.GOOS {
+	case "linux":
+		// The kernel will start accepting TCP connections before userspace
+		// gets a chance to not accept them, so fire off a bunch to fill up
+		// the kernel's backlog.  Then we test we get a failure after that.
+		for i := 0; i < numConns; i++ {
+			go func() {
+				_, err := DialTimeout("tcp", ln.Addr().String(), 200*time.Millisecond)
+				errc <- err
+			}()
+		}
+	case "darwin":
+		// At least OS X 10.7 seems to accept any number of
+		// connections, ignoring listen's backlog, so resort
+		// to connecting to a hopefully-dead 127/8 address.
+		go func() {
+			_, err := DialTimeout("tcp", "127.0.71.111:80", 200*time.Millisecond)
+			errc <- err
+		}()
+	default:
+		// TODO(bradfitz): this probably doesn't work on
+		// Windows? SOMAXCONN is huge there.  I'm not sure how
+		// listen works there.
+		// OpenBSD may have a reject route to 10/8.
+		// FreeBSD likely works, but is untested.
+		t.Logf("skipping test on %q; untested.", runtime.GOOS)
+		return
+	}
+
+	connected := 0
+	for {
+		select {
+		case <-time.After(15 * time.Second):
+			t.Fatal("too slow")
+		case err := <-errc:
+			if err == nil {
+				connected++
+				if connected == numConns {
+					t.Fatal("all connections connected; expected some to time out")
+				}
+			} else {
+				terr, ok := err.(timeout)
+				if !ok {
+					t.Fatalf("got error %q; want error with timeout interface", err)
+				}
+				if !terr.Timeout() {
+					t.Fatalf("got error %q; not a timeout", err)
+				}
+				// Pass. We saw a timeout error.
+				return
+			}
+		}
+	}
+}