net: enable RFC 6555 Fast Fallback by default

The Dialer.DualStack field is now meaningless and documented as
deprecated.

To disable fallback, set FallbackDelay to a negative value.

Fixes #22225

Change-Id: Icc212fe07bb69d7651ab81e539b8b3e3d3372fa9
Reviewed-on: https://go-review.googlesource.com/c/146659Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/dial.go

@@ -44,16 +44,23 @@ type Dialer struct {
 	// If nil, a local address is automatically chosen.
 	LocalAddr Addr
 
-	// DualStack enables RFC 6555-compliant "Happy Eyeballs"
-	// dialing when the network is "tcp" and the host in the
-	// address parameter resolves to both IPv4 and IPv6 addresses.
-	// This allows a client to tolerate networks where one address
-	// family is silently broken.
+	// DualStack previously enabled RFC 6555 Fast Fallback
+	// support, also known as "Happy Eyeballs", in which IPv4 is
+	// tried soon if IPv6 appears to be misconfigured and
+	// hanging.
+	//
+	// Deprecated: Fast Fallback is enabled by default. To
+	// disable, set FallbackDelay to a negative value.
 	DualStack bool
 
 	// FallbackDelay specifies the length of time to wait before
-	// spawning a fallback connection, when DualStack is enabled.
+	// spawning a RFC 6555 Fast Fallback connection. That is, this
+	// is the amount of time to wait for IPv6 to succeed before
+	// assuming that IPv6 is misconfigured and falling back to
+	// IPv4.
+	//
 	// If zero, a default delay of 300ms is used.
+	// A negative value disables Fast Fallback support.
 	FallbackDelay time.Duration
 
 	// KeepAlive specifies the keep-alive period for an active
@@ -81,6 +88,8 @@ type Dialer struct {
 	Control func(network, address string, c syscall.RawConn) error
 }
 
+func (d *Dialer) dualStack() bool { return d.FallbackDelay >= 0 }
+
 func minNonzeroTime(a, b time.Time) time.Time {
 	if a.IsZero() {
 		return b
@@ -393,7 +402,7 @@ func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn
 	}
 
 	var primaries, fallbacks addrList
-	if d.DualStack && network == "tcp" {
+	if d.dualStack() && network == "tcp" {
 		primaries, fallbacks = addrs.partition(isIPv4)
 	} else {
 		primaries = addrs