(envtest) Reaper

A shim that wraps the envtest binaries (etcd, kube-apiserver) so they don't outlive the test process that started them.

reaper is symlinked as shims/etcd and shims/kube-apiserver (see packages.envtest-shim in flake.nix) and pointed at by TEST_ASSET_ETCD / TEST_ASSET_KUBE_APISERVER. When run, it execs the identically named binary from $KUBEBUILDER_ASSETS, forwarding stdio and its own exit code, and kills that child once its own parent exits. Without it, a go test run that dies without cleaning up (SIGKILL, a panic in the wrong place, ^C in some harnesses) leaves etcd and kube-apiserver processes behind.

Watching the parent is OS specific, so each watch_$GOOS.go implements watchParent. Both block, without polling, until the parent exits or the context is canceled:

• darwin: kqueue with EVFILT_PROC / NOTE_EXIT.
• linux: pidfd_open (Linux 5.3+), which yields an fd that becomes readable when the process exits, plus an eventfd to break out of poll on cancellation.

Note that PR_SET_PDEATHSIG is deliberately not used on linux: its notion of "parent" is the thread that created the process, and Go's runtime retires threads at will, which would kill the child while the test is still running.