This will make it easier to start running the real cAdvisor alongside
Kubelet. This change is primarily no-op refactoring. The main behavioral
change is that we always create a cAdvisor interface and expect it to
always be available. When we make a request, if cAdvisor is not
connected the request fails with a connection error. This failure is
handled today as well.
This is a partial rollback of commit 6e6f465a36 ("Fix a crash for
kubelet when without EtcdClient") in which we used the `reflect` module
to inspect that the pointer stored inside the interface was `nil`, but
as pointed out by @lavalamp, the correct solution is to make the
function return the interface type, in which case a `return nil` will
return the interface nil and not a nil pointer that turns into a non-nil
value when coerced into an interface.
For more details, see http://golang.org/doc/faq#nil_error and the
discussion in PR #3356.
Tested by installing a kubelet built from head with this patch into a
containervm instance and confirming it did not crash on standalone.go.
Confirmed that by only removing the `reflect.IsNil()` comparison but not
changing the return type of `EtcdClientOrDie()` did indeed cause that
same crash, so changing the return type does indeed fix the issue.
Signed-off-by: Filipe Brandenburger <filbranden@google.com>
