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/8768Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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")
 )