net: add sequential and RFC 6555-compliant TCP dialing.

dialSerial connects to a list of addresses in sequence.  If a
timeout is specified, then each address gets an equal fraction of the
remaining time, with a magic constant (2 seconds) to prevent
"dial a million addresses" from allotting zero time to each.

Normally, net.Dial passes the DNS stub resolver's output to dialSerial.
If an error occurs (like destination/port unreachable), it quickly skips
to the next address, but a blackhole in the network will cause the
connection to hang until the timeout elapses.  This is how UNIXy clients
traditionally behave, and is usually sufficient for non-broken networks.

The DualStack flag enables dialParallel, which implements Happy Eyeballs
by racing two dialSerial goroutines, giving the preferred family a
head start (300ms by default).  This allows clients to avoid long
timeouts when the network blackholes IPv4 xor IPv6.

Fixes #8453
Fixes #8455
Fixes #8847

Change-Id: Ie415809c9226a1f7342b0217dcdd8f224ae19058
Reviewed-on: https://go-review.googlesource.com/8768
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dial.go

@@ -21,6 +21,9 @@ type Dialer struct {
 	//
 	// The default is no timeout.
 	//
+	// When dialing a name with multiple IP addresses, the timeout
+	// may be divided between them.
+	//
 	// With or without a timeout, the operating system may impose
 	// its own earlier timeout. For instance, TCP timeouts are
 	// often around 3 minutes.
@@ -38,13 +41,17 @@ type Dialer struct {
 	// If nil, a local address is automatically chosen.
 	LocalAddr Addr
 
-	// DualStack allows a single dial to attempt to establish
+	// DualStack enables RFC 6555-compliant "Happy Eyeballs" dialing
-	// multiple IPv4 and IPv6 connections and to return the first
+	// when the network is "tcp" and the destination is a host name
-	// established connection when the network is "tcp" and the
+	// with both IPv4 and IPv6 addresses. This allows a client to
-	// destination is a host name that has multiple address family
+	// tolerate networks where one address family is silently broken.
-	// DNS records.
 	DualStack bool
 
+	// FallbackDelay specifies the length of time to wait before
+	// spawning a fallback connection, when DualStack is enabled.
+	// If zero, a default delay of 300ms is used.
+	FallbackDelay time.Duration
+
 	// KeepAlive specifies the keep-alive period for an active
 	// network connection.
 	// If zero, keep-alives are not enabled. Network protocols
@@ -54,11 +61,11 @@ type Dialer struct {
 
 // Return either now+Timeout or Deadline, whichever comes first.
 // Or zero, if neither is set.
-func (d *Dialer) deadline() time.Time {
+func (d *Dialer) deadline(now time.Time) time.Time {
 	if d.Timeout == 0 {
 		return d.Deadline
 	}
-	timeoutDeadline := time.Now().Add(d.Timeout)
+	timeoutDeadline := now.Add(d.Timeout)
 	if d.Deadline.IsZero() || timeoutDeadline.Before(d.Deadline) {
 		return timeoutDeadline
 	} else {
@@ -66,6 +73,39 @@ func (d *Dialer) deadline() time.Time {
 	}
 }
 
+// partialDeadline returns the deadline to use for a single address,
+// when multiple addresses are pending.
+func (d *Dialer) partialDeadline(now time.Time, addrsRemaining int) (time.Time, error) {
+	deadline := d.deadline(now)
+	if deadline.IsZero() {
+		return deadline, nil
+	}
+	timeRemaining := deadline.Sub(now)
+	if timeRemaining <= 0 {
+		return time.Time{}, errTimeout
+	}
+	// Tentatively allocate equal time to each remaining address.
+	timeout := timeRemaining / time.Duration(addrsRemaining)
+	// If the time per address is too short, steal from the end of the list.
+	const saneMinimum = 2 * time.Second
+	if timeout < saneMinimum {
+		if timeRemaining < saneMinimum {
+			timeout = timeRemaining
+		} else {
+			timeout = saneMinimum
+		}
+	}
+	return now.Add(timeout), nil
+}
+
+func (d *Dialer) fallbackDelay() time.Duration {
+	if d.FallbackDelay > 0 {
+		return d.FallbackDelay
+	} else {
+		return 300 * time.Millisecond
+	}
+}
+
 func parseNetwork(net string) (afnet string, proto int, err error) {
 	i := last(net, ':')
 	if i < 0 { // no colon
@@ -154,30 +194,44 @@ func DialTimeout(network, address string, timeout time.Duration) (Conn, error) {
 	return d.Dial(network, address)
 }
 
+// dialContext holds common state for all dial operations.
+type dialContext struct {
+	Dialer
+	network, address string
+}
+
 // Dial connects to the address on the named network.
 //
 // See func Dial for a description of the network and address
 // parameters.
 func (d *Dialer) Dial(network, address string) (Conn, error) {
-	addrs, err := resolveAddrList("dial", network, address, d.deadline())
+	addrs, err := resolveAddrList("dial", network, address, d.deadline(time.Now()))
 	if err != nil {
 		return nil, &OpError{Op: "dial", Net: network, Source: nil, Addr: nil, Err: err}
 	}
-	var dialer func(deadline time.Time) (Conn, error)
+
+	ctx := &dialContext{
+		Dialer:  *d,
+		network: network,
+		address: address,
+	}
+
+	var primaries, fallbacks addrList
 	if d.DualStack && network == "tcp" {
-		primaries, fallbacks := addrs.partition(isIPv4)
+		primaries, fallbacks = addrs.partition(isIPv4)
-		if len(fallbacks) > 0 {
+	} else {
-			dialer = func(deadline time.Time) (Conn, error) {
+		primaries = addrs
-				return dialMulti(network, address, d.LocalAddr, addrList{primaries[0], fallbacks[0]}, deadline)
 	}
+
+	var c Conn
+	if len(fallbacks) == 0 {
+		// dialParallel can accept an empty fallbacks list,
+		// but this shortcut avoids the goroutine/channel overhead.
+		c, err = dialSerial(ctx, primaries, nil)
+	} else {
+		c, err = dialParallel(ctx, primaries, fallbacks)
 	}
-	}
+
-	if dialer == nil {
-		dialer = func(deadline time.Time) (Conn, error) {
-			return dialSingle(network, address, d.LocalAddr, addrs.first(isIPv4), deadline)
-		}
-	}
-	c, err := dial(network, addrs.first(isIPv4), dialer, d.deadline())
 	if d.KeepAlive > 0 && err == nil {
 		if tc, ok := c.(*TCPConn); ok {
 			setKeepAlive(tc.fd, true)
@@ -188,70 +242,135 @@ func (d *Dialer) Dial(network, address string) (Conn, error) {
 	return c, err
 }
 
-// dialMulti attempts to establish connections to each destination of
+// dialParallel races two copies of dialSerial, giving the first a
-// the list of addresses. It will return the first established
+// head start. It returns the first established connection and
-// connection and close the other connections. Otherwise it returns
+// closes the others. Otherwise it returns an error from the first
-// error on the last attempt.
+// primary address.
-func dialMulti(net, addr string, la Addr, ras addrList, deadline time.Time) (Conn, error) {
+func dialParallel(ctx *dialContext, primaries, fallbacks addrList) (Conn, error) {
-	type racer struct {
+	results := make(chan dialResult) // unbuffered, so dialSerialAsync can detect race loss & cleanup
+	cancel := make(chan struct{})
+	defer close(cancel)
+
+	// Spawn the primary racer.
+	go dialSerialAsync(ctx, primaries, nil, cancel, results)
+
+	// Spawn the fallback racer.
+	fallbackTimer := time.NewTimer(ctx.fallbackDelay())
+	go dialSerialAsync(ctx, fallbacks, fallbackTimer, cancel, results)
+
+	var primaryErr error
+	for nracers := 2; nracers > 0; nracers-- {
+		res := <-results
+		// If we're still waiting for a connection, then hasten the delay.
+		// Otherwise, disable the Timer and let cancel take over.
+		if fallbackTimer.Stop() && res.error != nil {
+			fallbackTimer.Reset(0)
+		}
+		if res.error == nil {
+			return res.Conn, nil
+		}
+		if res.primary {
+			primaryErr = res.error
+		}
+	}
+	return nil, primaryErr
+}
+
+type dialResult struct {
 	Conn
 	error
+	primary bool
+}
+
+// dialSerialAsync runs dialSerial after some delay, and returns the
+// resulting connection through a channel. When racing two connections,
+// the primary goroutine uses a nil timer to omit the delay.
+func dialSerialAsync(ctx *dialContext, ras addrList, timer *time.Timer, cancel <-chan struct{}, results chan<- dialResult) {
+	if timer != nil {
+		// We're in the fallback goroutine; sleep before connecting.
+		select {
+		case <-timer.C:
+		case <-cancel:
+			return
 		}
-	// Sig controls the flow of dial results on lane. It passes a
+	}
-	// token to the next racer and also indicates the end of flow
+	c, err := dialSerial(ctx, ras, cancel)
-	// by using closed channel.
+	select {
-	sig := make(chan bool, 1)
+	case results <- dialResult{c, err, timer == nil}:
-	lane := make(chan racer, 1)
+		// We won the race.
-	for _, ra := range ras {
+	case <-cancel:
-		go func(ra Addr) {
+		// The other goroutine won the race.
-			c, err := dialSingle(net, addr, la, ra, deadline)
+		if c != nil {
-			if _, ok := <-sig; ok {
-				lane <- racer{c, err}
-			} else if err == nil {
-				// We have to return the resources
-				// that belong to the other
-				// connections here for avoiding
-				// unnecessary resource starvation.
 			c.Close()
 		}
-		}(ra)
 	}
-	defer close(sig)
+}
-	lastErr := errTimeout
+
-	nracers := len(ras)
+// dialSerial connects to a list of addresses in sequence, returning
-	for nracers > 0 {
+// either the first successful connection, or the first error.
-		sig <- true
+func dialSerial(ctx *dialContext, ras addrList, cancel <-chan struct{}) (Conn, error) {
-		racer := <-lane
+	var firstErr error // The error from the first address is most relevant.
-		if racer.error == nil {
+
-			return racer.Conn, nil
+	for i, ra := range ras {
+		select {
+		case <-cancel:
+			return nil, &OpError{Op: "dial", Net: ctx.network, Source: ctx.LocalAddr, Addr: ra, Err: errCanceled}
+		default:
 		}
-		lastErr = racer.error
+
-		nracers--
+		partialDeadline, err := ctx.partialDeadline(time.Now(), len(ras)-i)
+		if err != nil {
+			// Ran out of time.
+			if firstErr == nil {
+				firstErr = &OpError{Op: "dial", Net: ctx.network, Source: ctx.LocalAddr, Addr: ra, Err: err}
 			}
-	return nil, lastErr
+			break
+		}
+
+		// dialTCP does not support cancelation (see golang.org/issue/11225),
+		// so if cancel fires, we'll continue trying to connect until the next
+		// timeout, or return a spurious connection for the caller to close.
+		dialer := func(d time.Time) (Conn, error) {
+			return dialSingle(ctx, ra, d)
+		}
+		c, err := dial(ctx.network, ra, dialer, partialDeadline)
+		if err == nil {
+			return c, nil
+		}
+		if firstErr == nil {
+			firstErr = err
+		}
+	}
+
+	if firstErr == nil {
+		firstErr = &OpError{Op: "dial", Net: ctx.network, Source: nil, Addr: nil, Err: errMissingAddress}
+	}
+	return nil, firstErr
 }
 
 // dialSingle attempts to establish and returns a single connection to
-// the destination address.
+// the destination address. This must be called through the OS-specific
-func dialSingle(net, addr string, la, ra Addr, deadline time.Time) (c Conn, err error) {
+// dial function, because some OSes don't implement the deadline feature.
+func dialSingle(ctx *dialContext, ra Addr, deadline time.Time) (c Conn, err error) {
+	la := ctx.LocalAddr
 	if la != nil && la.Network() != ra.Network() {
-		return nil, &OpError{Op: "dial", Net: net, Source: la, Addr: ra, Err: errors.New("mismatched local address type " + la.Network())}
+		return nil, &OpError{Op: "dial", Net: ctx.network, Source: la, Addr: ra, Err: errors.New("mismatched local address type " + la.Network())}
 	}
 	switch ra := ra.(type) {
 	case *TCPAddr:
 		la, _ := la.(*TCPAddr)
-		c, err = dialTCP(net, la, ra, deadline)
+		c, err = testHookDialTCP(ctx.network, la, ra, deadline)
 	case *UDPAddr:
 		la, _ := la.(*UDPAddr)
-		c, err = dialUDP(net, la, ra, deadline)
+		c, err = dialUDP(ctx.network, la, ra, deadline)
 	case *IPAddr:
 		la, _ := la.(*IPAddr)
-		c, err = dialIP(net, la, ra, deadline)
+		c, err = dialIP(ctx.network, la, ra, deadline)
 	case *UnixAddr:
 		la, _ := la.(*UnixAddr)
-		c, err = dialUnix(net, la, ra, deadline)
+		c, err = dialUnix(ctx.network, la, ra, deadline)
 	default:
-		return nil, &OpError{Op: "dial", Net: net, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: addr}}
+		return nil, &OpError{Op: "dial", Net: ctx.network, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: ctx.address}}
 	}
 	if err != nil {
 		return nil, err // c is non-nil interface containing nil pointer

src/net/dial_test.go

@@ -5,6 +5,7 @@
 package net
 
 import (
+	"io"
 	"net/internal/socktest"
 	"runtime"
 	"sync"
@@ -207,6 +208,360 @@ func TestDialerDualStackFDLeak(t *testing.T) {
 	}
 }
 
+// Define a pair of blackholed (IPv4, IPv6) addresses, for which dialTCP is
+// expected to hang until the timeout elapses. These addresses are reserved
+// for benchmarking by RFC 6890.
+const (
+	slowDst4    = "192.18.0.254"
+	slowDst6    = "2001:2::254"
+	slowTimeout = 1 * time.Second
+)
+
+// In some environments, the slow IPs may be explicitly unreachable, and fail
+// more quickly than expected. This test hook prevents dialTCP from returning
+// before the deadline.
+func slowDialTCP(net string, laddr, raddr *TCPAddr, deadline time.Time) (*TCPConn, error) {
+	c, err := dialTCP(net, laddr, raddr, deadline)
+	if ParseIP(slowDst4).Equal(raddr.IP) || ParseIP(slowDst6).Equal(raddr.IP) {
+		time.Sleep(deadline.Sub(time.Now()))
+	}
+	return c, err
+}
+
+func dialClosedPort() time.Duration {
+	l, err := Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return 999 * time.Hour
+	}
+	addr := l.Addr().String()
+	l.Close()
+	// On OpenBSD, interference from TestSelfConnect is mysteriously
+	// causing the first attempt to hang for a few seconds, so we throw
+	// away the first result and keep the second.
+	for i := 1; ; i++ {
+		startTime := time.Now()
+		c, err := Dial("tcp", addr)
+		if err == nil {
+			c.Close()
+		}
+		elapsed := time.Now().Sub(startTime)
+		if i == 2 {
+			return elapsed
+		}
+	}
+}
+
+func TestDialParallel(t *testing.T) {
+	if testing.Short() || !*testExternal {
+		t.Skip("avoid external network")
+	}
+	if !supportsIPv4 || !supportsIPv6 {
+		t.Skip("both IPv4 and IPv6 are required")
+	}
+
+	// Determine the time required to dial a closed port.
+	// On Windows, this takes roughly 1 second, but other platforms
+	// are expected to be instantaneous.
+	closedPortDelay := dialClosedPort()
+	var expectClosedPortDelay time.Duration
+	if runtime.GOOS == "windows" {
+		expectClosedPortDelay = 1095 * time.Millisecond
+	} else {
+		expectClosedPortDelay = 95 * time.Millisecond
+	}
+	if closedPortDelay > expectClosedPortDelay {
+		t.Errorf("got %v; want <= %v", closedPortDelay, expectClosedPortDelay)
+	}
+
+	const instant time.Duration = 0
+	const fallbackDelay = 200 * time.Millisecond
+
+	// Some cases will run quickly when "connection refused" is fast,
+	// or trigger the fallbackDelay on Windows.  This value holds the
+	// lesser of the two delays.
+	var closedPortOrFallbackDelay time.Duration
+	if closedPortDelay < fallbackDelay {
+		closedPortOrFallbackDelay = closedPortDelay
+	} else {
+		closedPortOrFallbackDelay = fallbackDelay
+	}
+
+	origTestHookDialTCP := testHookDialTCP
+	defer func() { testHookDialTCP = origTestHookDialTCP }()
+	testHookDialTCP = slowDialTCP
+
+	nCopies := func(s string, n int) []string {
+		out := make([]string, n)
+		for i := 0; i < n; i++ {
+			out[i] = s
+		}
+		return out
+	}
+
+	var testCases = []struct {
+		primaries       []string
+		fallbacks       []string
+		teardownNetwork string
+		expectOk        bool
+		expectElapsed   time.Duration
+	}{
+		// These should just work on the first try.
+		{[]string{"127.0.0.1"}, []string{}, "", true, instant},
+		{[]string{"::1"}, []string{}, "", true, instant},
+		{[]string{"127.0.0.1", "::1"}, []string{slowDst6}, "tcp6", true, instant},
+		{[]string{"::1", "127.0.0.1"}, []string{slowDst4}, "tcp4", true, instant},
+		// Primary is slow; fallback should kick in.
+		{[]string{slowDst4}, []string{"::1"}, "", true, fallbackDelay},
+		// Skip a "connection refused" in the primary thread.
+		{[]string{"127.0.0.1", "::1"}, []string{}, "tcp4", true, closedPortDelay},
+		{[]string{"::1", "127.0.0.1"}, []string{}, "tcp6", true, closedPortDelay},
+		// Skip a "connection refused" in the fallback thread.
+		{[]string{slowDst4, slowDst6}, []string{"::1", "127.0.0.1"}, "tcp6", true, fallbackDelay + closedPortDelay},
+		// Primary refused, fallback without delay.
+		{[]string{"127.0.0.1"}, []string{"::1"}, "tcp4", true, closedPortOrFallbackDelay},
+		{[]string{"::1"}, []string{"127.0.0.1"}, "tcp6", true, closedPortOrFallbackDelay},
+		// Everything is refused.
+		{[]string{"127.0.0.1"}, []string{}, "tcp4", false, closedPortDelay},
+		// Nothing to do; fail instantly.
+		{[]string{}, []string{}, "", false, instant},
+		// Connecting to tons of addresses should not trip the deadline.
+		{nCopies("::1", 1000), []string{}, "", true, instant},
+	}
+
+	handler := func(dss *dualStackServer, ln Listener) {
+		for {
+			c, err := ln.Accept()
+			if err != nil {
+				return
+			}
+			c.Close()
+		}
+	}
+
+	// Convert a list of IP strings into TCPAddrs.
+	makeAddrs := func(ips []string, port string) addrList {
+		var out addrList
+		for _, ip := range ips {
+			addr, err := ResolveTCPAddr("tcp", JoinHostPort(ip, port))
+			if err != nil {
+				t.Fatal(err)
+			}
+			out = append(out, addr)
+		}
+		return out
+	}
+
+	for i, tt := range testCases {
+		dss, err := newDualStackServer([]streamListener{
+			{network: "tcp4", address: "127.0.0.1"},
+			{network: "tcp6", address: "::1"},
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer dss.teardown()
+		if err := dss.buildup(handler); err != nil {
+			t.Fatal(err)
+		}
+		if tt.teardownNetwork != "" {
+			// Destroy one of the listening sockets, creating an unreachable port.
+			dss.teardownNetwork(tt.teardownNetwork)
+		}
+
+		primaries := makeAddrs(tt.primaries, dss.port)
+		fallbacks := makeAddrs(tt.fallbacks, dss.port)
+		ctx := &dialContext{
+			Dialer: Dialer{
+				FallbackDelay: fallbackDelay,
+				Timeout:       slowTimeout,
+			},
+			network: "tcp",
+			address: "?",
+		}
+		startTime := time.Now()
+		c, err := dialParallel(ctx, primaries, fallbacks)
+		elapsed := time.Now().Sub(startTime)
+
+		if c != nil {
+			c.Close()
+		}
+
+		if tt.expectOk && err != nil {
+			t.Errorf("#%d: got %v; want nil", i, err)
+		} else if !tt.expectOk && err == nil {
+			t.Errorf("#%d: got nil; want non-nil", i)
+		}
+
+		expectElapsedMin := tt.expectElapsed - 95*time.Millisecond
+		expectElapsedMax := tt.expectElapsed + 95*time.Millisecond
+		if !(elapsed >= expectElapsedMin) {
+			t.Errorf("#%d: got %v; want >= %v", i, elapsed, expectElapsedMin)
+		} else if !(elapsed <= expectElapsedMax) {
+			t.Errorf("#%d: got %v; want <= %v", i, elapsed, expectElapsedMax)
+		}
+	}
+	// Wait for any slowDst4/slowDst6 connections to timeout.
+	time.Sleep(slowTimeout * 3 / 2)
+}
+
+func lookupSlowFast(fn func(string) ([]IPAddr, error), host string) ([]IPAddr, error) {
+	switch host {
+	case "slow6loopback4":
+		// Returns a slow IPv6 address, and a local IPv4 address.
+		return []IPAddr{
+			{IP: ParseIP(slowDst6)},
+			{IP: ParseIP("127.0.0.1")},
+		}, nil
+	default:
+		return fn(host)
+	}
+}
+
+func TestDialerFallbackDelay(t *testing.T) {
+	if testing.Short() || !*testExternal {
+		t.Skip("avoid external network")
+	}
+	if !supportsIPv4 || !supportsIPv6 {
+		t.Skip("both IPv4 and IPv6 are required")
+	}
+
+	origTestHookLookupIP := testHookLookupIP
+	defer func() { testHookLookupIP = origTestHookLookupIP }()
+	testHookLookupIP = lookupSlowFast
+
+	origTestHookDialTCP := testHookDialTCP
+	defer func() { testHookDialTCP = origTestHookDialTCP }()
+	testHookDialTCP = slowDialTCP
+
+	var testCases = []struct {
+		dualstack     bool
+		delay         time.Duration
+		expectElapsed time.Duration
+	}{
+		// Use a very brief delay, which should fallback immediately.
+		{true, 1 * time.Nanosecond, 0},
+		// Use a 200ms explicit timeout.
+		{true, 200 * time.Millisecond, 200 * time.Millisecond},
+		// The default is 300ms.
+		{true, 0, 300 * time.Millisecond},
+		// This case is last, in order to wait for hanging slowDst6 connections.
+		{false, 0, slowTimeout},
+	}
+
+	handler := func(dss *dualStackServer, ln Listener) {
+		for {
+			c, err := ln.Accept()
+			if err != nil {
+				return
+			}
+			c.Close()
+		}
+	}
+	dss, err := newDualStackServer([]streamListener{
+		{network: "tcp", address: "127.0.0.1"},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer dss.teardown()
+	if err := dss.buildup(handler); err != nil {
+		t.Fatal(err)
+	}
+
+	for i, tt := range testCases {
+		d := &Dialer{DualStack: tt.dualstack, FallbackDelay: tt.delay, Timeout: slowTimeout}
+
+		startTime := time.Now()
+		c, err := d.Dial("tcp", JoinHostPort("slow6loopback4", dss.port))
+		elapsed := time.Now().Sub(startTime)
+		if err == nil {
+			c.Close()
+		} else if tt.dualstack {
+			t.Error(err)
+		}
+		expectMin := tt.expectElapsed - 1*time.Millisecond
+		expectMax := tt.expectElapsed + 95*time.Millisecond
+		if !(elapsed >= expectMin) {
+			t.Errorf("#%d: got %v; want >= %v", i, elapsed, expectMin)
+		}
+		if !(elapsed <= expectMax) {
+			t.Errorf("#%d: got %v; want <= %v", i, elapsed, expectMax)
+		}
+	}
+}
+
+func TestDialSerialAsyncSpuriousConnection(t *testing.T) {
+	ln, err := newLocalListener("tcp")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+
+	ctx := &dialContext{
+		network: "tcp",
+		address: "?",
+	}
+
+	results := make(chan dialResult)
+	cancel := make(chan struct{})
+
+	// Spawn a connection in the background.
+	go dialSerialAsync(ctx, addrList{ln.Addr()}, nil, cancel, results)
+
+	// Receive it at the server.
+	c, err := ln.Accept()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	// Tell dialSerialAsync that someone else won the race.
+	close(cancel)
+
+	// The connection should close itself, without sending data.
+	c.SetReadDeadline(time.Now().Add(1 * time.Second))
+	var b [1]byte
+	if _, err := c.Read(b[:]); err != io.EOF {
+		t.Errorf("got %v; want %v", err, io.EOF)
+	}
+}
+
+func TestDialerPartialDeadline(t *testing.T) {
+	now := time.Date(2000, time.January, 1, 0, 0, 0, 0, time.UTC)
+	var testCases = []struct {
+		now            time.Time
+		deadline       time.Time
+		addrs          int
+		expectDeadline time.Time
+		expectErr      error
+	}{
+		// Regular division.
+		{now, now.Add(12 * time.Second), 1, now.Add(12 * time.Second), nil},
+		{now, now.Add(12 * time.Second), 2, now.Add(6 * time.Second), nil},
+		{now, now.Add(12 * time.Second), 3, now.Add(4 * time.Second), nil},
+		// Bump against the 2-second sane minimum.
+		{now, now.Add(12 * time.Second), 999, now.Add(2 * time.Second), nil},
+		// Total available is now below the sane minimum.
+		{now, now.Add(1900 * time.Millisecond), 999, now.Add(1900 * time.Millisecond), nil},
+		// Null deadline.
+		{now, noDeadline, 1, noDeadline, nil},
+		// Step the clock forward and cross the deadline.
+		{now.Add(-1 * time.Millisecond), now, 1, now, nil},
+		{now.Add(0 * time.Millisecond), now, 1, noDeadline, errTimeout},
+		{now.Add(1 * time.Millisecond), now, 1, noDeadline, errTimeout},
+	}
+	for i, tt := range testCases {
+		d := Dialer{Deadline: tt.deadline}
+		deadline, err := d.partialDeadline(tt.now, tt.addrs)
+		if err != tt.expectErr {
+			t.Errorf("#%d: got %v; want %v", i, err, tt.expectErr)
+		}
+		if deadline != tt.expectDeadline {
+			t.Errorf("#%d: got %v; want %v", i, deadline, tt.expectDeadline)
+		}
+	}
+}
+
 func TestDialerLocalAddr(t *testing.T) {
 	ch := make(chan error, 1)
 	handler := func(ls *localServer, ln Listener) {
@@ -262,6 +617,9 @@ func TestDialerDualStack(t *testing.T) {
 			c.Close()
 		}
 	}
+
+	const T = 100 * time.Millisecond
+	for _, dualstack := range []bool{false, true} {
 		dss, err := newDualStackServer([]streamListener{
 			{network: "tcp4", address: "127.0.0.1"},
 			{network: "tcp6", address: "::1"},
@@ -274,8 +632,7 @@ func TestDialerDualStack(t *testing.T) {
 			t.Fatal(err)
 		}
 
-	const T = 100 * time.Millisecond
+		d := &Dialer{DualStack: dualstack, Timeout: T}
-	d := &Dialer{DualStack: true, Timeout: T}
 		for range dss.lns {
 			c, err := d.Dial("tcp", JoinHostPort("localhost", dss.port))
 			if err != nil {
@@ -290,6 +647,7 @@ func TestDialerDualStack(t *testing.T) {
 			}
 			c.Close()
 		}
+	}
 	time.Sleep(2 * T) // wait for the dial racers to stop
 }
 

src/net/hook.go

@@ -5,6 +5,7 @@
 package net
 
 var (
+	testHookDialTCP      = dialTCP
 	testHookHostsPath    = "/etc/hosts"
 	testHookLookupIP     = func(fn func(string) ([]IPAddr, error), host string) ([]IPAddr, error) { return fn(host) }
 	testHookSetKeepAlive = func() {}

src/net/net.go

@@ -321,6 +321,7 @@ var (
 
 	// For both read and write operations.
 	errTimeout          error = &timeoutError{}
+	errCanceled               = errors.New("operation was canceled")
 	errClosing                = errors.New("use of closed network connection")
 	ErrWriteToConnected       = errors.New("use of WriteTo with pre-connected connection")
 )