net: ensure ipv6 endpoints are tried first

When connecting to an endpoint that resolves to ipv4 and ipv6
addresses, ensure that ipv6 addresses are tried first, as per
happy eyeballs RFC.

Currently whether ipv4 or ipv6 addresses are tried first depends on
the dns resolution speed. If the A resolution returned first, it
would be first in the address list, and partition() uses the label
of the first result to build the primaries category.

Fixes #54928

Author: edevil

src/net/dial.go

@@ -421,6 +421,13 @@ func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn
 	var primaries, fallbacks addrList
 	if d.dualStack() && network == "tcp" {
 		primaries, fallbacks = addrs.partition(isIPv4)
+
+		// when fallbacks is not empty, ensure primaries
+		// are ipv6 addresses as per happy eyeballs spec
+		if len(fallbacks) > 0 && !isIPv4(fallbacks[0]) {
+			primaries, fallbacks = fallbacks, primaries
+		}
+
 	} else {
 		primaries = addrs
 	}