x/net/{ipv4,ipv6}: multicastlistener_test.go assumes a specific port is available

[bcmills]

There are two tests in this file that assume that IPv6 port 1024 is available:

https://github.com/golang/net/blob/master/ipv6/multicastlistener_test.go#L73:

c1, err := net.ListenPacket("udp6", "[ff02::]:1024") // wildcard address with reusable port
if err != nil {
	t.Fatal(err)
}
defer c1.Close()

c2, err := net.ListenPacket("udp6", "[ff02::]:1024") // wildcard address with reusable port
if err != nil {
	t.Fatal(err)
}
defer c2.Close()

https://github.com/golang/net/blob/master/ipv6/multicastlistener_test.go#L140

c, err := net.ListenPacket("udp6", fmt.Sprintf("[%s%%%s]:1024", ip.String(), ifi.Name)) // unicast address with non-reusable port
if err != nil {
	t.Fatal(err)
}

That makes the test excessively fragile when run on a machine that may have open IPv6 connections.

[ianlancetaylor]

CC @mikioh

[gopherbot]

CL https://golang.org/cl/44770 mentions this issue.

[gopherbot]

CL https://golang.org/cl/44775 mentions this issue.