test(controller): race condition in handleSigterm()

[vflaux]

What does it do ?

Fix a race condition between the signal handler setup and the signal send in the test for handleSigterm().

In the test, in rare case the SIGTERM signal could be send before the handleSigterm() calls signal.Notify().

In this case, the cancel function is never called.

The test always pass because it silently ignore this with the select case sig := <-sigChan.

I think we need to test that the signal received is a SIGTERM and that the cancel func has been called. Not one of the two conditions.

Motivation

This primarily concerns testing and ensuring correctness. There is most likely no issue when the controller run.

I could reproduce this with go test -timeout 30s -count 1000 -run ^TestHandleSigterm$ sigs.k8s.io/external-dns/controller after removing the sig := <-sigChan select case in the current code.

Found this when trying to understand this race condition report: https://prow.k8s.io/view/gs/kubernetes-ci-logs/pr-logs/pull/kubernetes-sigs_external-dns/5810/pull-external-dns-unit-test/1963995845443457024

Not sure if this would fix as I'm not able to reproduce it.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

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[vflaux]

Probably not needed if #5798 is merged.