The control channel's accepting thread dropped failed accepts with
incoming().flatten() and asked again at once. When accepting keeps
failing, for example while the process has no file descriptors left,
that loop keeps a core busy until it recovers.
Wait 100 ms after a failed accept, as the single-instance listener
already does. The loop goes through a small helper so the wait can be
tested without running out of file descriptors.
oracola91
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