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net: add sequential and RFC 6555-compliant TCP dialing.
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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 <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>
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Paul Marks committed Jun 16, 2015
1 parent 12b05bf commit 0d8366e
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Showing 4 changed files with 566 additions and 87 deletions.
249 changes: 184 additions & 65 deletions src/net/dial.go
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand All @@ -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
// multiple IPv4 and IPv6 connections and to return the first
// established connection when the network is "tcp" and the
// destination is a host name that has multiple address family
// DNS records.
// DualStack enables RFC 6555-compliant "Happy Eyeballs" dialing
// when the network is "tcp" and the destination is a host name
// with both IPv4 and IPv6 addresses. This allows a client to
// tolerate networks where one address family is silently broken.
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
Expand All @@ -54,18 +61,51 @@ 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 {
return d.Deadline
}
}

// 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
Expand Down Expand Up @@ -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)
if len(fallbacks) > 0 {
dialer = func(deadline time.Time) (Conn, error) {
return dialMulti(network, address, d.LocalAddr, addrList{primaries[0], fallbacks[0]}, deadline)
}
}
primaries, fallbacks = addrs.partition(isIPv4)
} else {
primaries = addrs
}
if dialer == nil {
dialer = func(deadline time.Time) (Conn, error) {
return dialSingle(network, address, d.LocalAddr, addrs.first(isIPv4), 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)
}
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)
Expand All @@ -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
// the list of addresses. It will return the first established
// connection and close the other connections. Otherwise it returns
// error on the last attempt.
func dialMulti(net, addr string, la Addr, ras addrList, deadline time.Time) (Conn, error) {
type racer struct {
Conn
error
// dialParallel races two copies of dialSerial, giving the first a
// head start. It returns the first established connection and
// closes the others. Otherwise it returns an error from the first
// primary address.
func dialParallel(ctx *dialContext, primaries, fallbacks addrList) (Conn, error) {
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
// by using closed channel.
sig := make(chan bool, 1)
lane := make(chan racer, 1)
for _, ra := range ras {
go func(ra Addr) {
c, err := dialSingle(net, addr, la, ra, deadline)
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)
c, err := dialSerial(ctx, ras, cancel)
select {
case results <- dialResult{c, err, timer == nil}:
// We won the race.
case <-cancel:
// The other goroutine won the race.
if c != nil {
c.Close()
}
}
defer close(sig)
lastErr := errTimeout
nracers := len(ras)
for nracers > 0 {
sig <- true
racer := <-lane
if racer.error == nil {
return racer.Conn, nil
}

// dialSerial connects to a list of addresses in sequence, returning
// either the first successful connection, or the first error.
func dialSerial(ctx *dialContext, ras addrList, cancel <-chan struct{}) (Conn, error) {
var firstErr error // The error from the first address is most relevant.

for i, ra := range ras {
select {
case <-cancel:
return nil, &OpError{Op: "dial", Net: ctx.network, Source: ctx.LocalAddr, Addr: ra, Err: errCanceled}
default:
}

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}
}
break
}
lastErr = racer.error
nracers--

// 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, lastErr
return nil, firstErr
}

// dialSingle attempts to establish and returns a single connection to
// the destination address.
func dialSingle(net, addr string, la, ra Addr, deadline time.Time) (c Conn, err error) {
// the destination address. This must be called through the OS-specific
// 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
Expand Down
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