net: Resolve*([::]) gives IPv4 zero, not IPv6

[heschi]

https://go-review.googlesource.com/45088 changed the behavior such that ResolveUDPAddr("[::]") followed by DialUDP on the result dials IPv4 localhost, not IPv6 localhost, which then fails when IPv4 is not configured. This is because ResolveUDPAddr unconditionally prefers IPv4 addresses when they're present in the addrList returned by internetAddrList. Looking at the code, I expect ResolveTCPAddr will have the same problem.

package main
import (
	"net"
	"fmt"
	"os"
	"strings"
)

func main() {
	addr, err := net.ResolveUDPAddr("udp", "[::]:9999")
	if err != nil {
		panic(err)
	}
	conn, err := net.DialUDP("udp", nil, addr)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%s (%#v) %s (%#v)\n", addr, addr, conn.RemoteAddr(), conn.RemoteAddr())
	if strings.Contains(conn.RemoteAddr().String(), "127.0.0.1") {
		fmt.Printf("Fail\n")
		os.Exit(1)
	}
}

At d8a7990:
[::]:9999 (&net.UDPAddr{IP:net.IP{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}, Port:9999, Zone:""}) [::1]:9999 (&net.UDPAddr{IP:net.IP{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1}, Port:9999, Zone:""})

At 78cf0e5:

0.0.0.0:9999 (&net.UDPAddr{IP:net.IP{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xff, 0xff, 0x0, 0x0, 0x0, 0x0}, Port:9999, Zone:""}) 127.0.0.1:9999 (&net.UDPAddr{IP:net.IP{0x7f, 0x0, 0x0, 0x1}, Port:9999, Zone:""})

cc @bradfitz

[bradfitz]

This is because ResolveUDPAddr unconditionally prefers IPv4 addresses when they're present in the addrList returned by internetAddrList.

Wow, okay.

Thanks for finding that. That's certainly not documented at https://golang.org/pkg/net/#ResolveUDPAddr

This is causing breakages somewhere I assume?

[heschi]

That's just my read of the code, of course, but it seems pretty clear:
return addrs.first(isIPv4).(*UDPAddr), nil
where "isIPv4" is the predicate used to pick the preferred address from the list.

Yeah, a few internal breakages.

[gopherbot]

CL https://golang.org/cl/47554 mentions this issue.