CLUSTER_DOMAIN is ignored when /etc/resolv.conf is missing #3417

/kind bug
/area networking

Expected behavior

network.GetClusterDomainName() and network.GetServiceHostname() return the value of the CLUSTER_DOMAIN environment variable when it is set, including when /etc/resolv.conf does not exist. #2092 added the variable for code that resolves cluster-local URLs from outside the cluster without access to /etc/resolv.conf, and #3283 made it take precedence over /etc/resolv.conf.

Actual behavior

When /etc/resolv.conf cannot be opened, GetClusterDomainName() returns cluster.local even if CLUSTER_DOMAIN is set. The os.Open error path in GetClusterDomainName returns before getClusterDomainName, which is the only place that reads the variable:

once.Do(func() {
	f, err := os.Open(resolverFileName)
	if err != nil {
		return
	}
	defer f.Close()
	domainName = getClusterDomainName(f)
})

Steps to reproduce the problem

On a machine without /etc/resolv.conf, run:

os.Setenv("CLUSTER_DOMAIN", "mesh.net")
fmt.Println(network.GetClusterDomainName()) // prints cluster.local, expected mesh.net

Additional info

Knative version: knative.dev/pkg main (7b6dcbb).