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/146659
Reviewed-by: Ian Lance Taylor <iant@golang.org>

diff --git a/src/net/dial.go b/src/net/dial.go
index b1a5ca7cd53f58d3c6f62b780bbdb16e715b50f7..76dcdc164cabd0f98813c386b1a71a0f704f6ba7 100644 (file)
--- a/src/net/dial.go
+++ b/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