A flaky test exposed a race condition where shutting down one server
instance broke the startup of the next instance when using the same
socket path. Commit 1325c2f8be removed the reuse of the same socket
path and thus avoided the issue.
But the real fix is to ensure that the listening socket is really
closed once Stop returns. Two solutions were proposed in
https://github.com/grpc/grpc-go/issues/1861:
- waiting for the goroutine to complete
- closing the socket
The former is done here because it's cleaner to not keep lingering
goroutines. While at it, the Stop methods are made idempotent (similar
to e.g. Close on a socket) and no longer crash when called without
prior Start.
Fixes https://github.com/kubernetes/kubernetes/issues/59488
fcbb64b93d