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2022-11-09T12:10:25.615967Z
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V 699
tests: make sniffer more robust

The Sniffer class is often used by test tools such as pft_ping to verify
that packets actually get sent where they're expected.

It starts a background thread to capture packets, but this thread needs
some time to start, leading to intermittent test failures when the
capture doesn't start before the relevant packet is sent.

Add a semaphore to ensure the Sniffer constructor doesn't return until
the capture is actually running.

PR:		260461
MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 67557372dfef87eb94681a97a4157198efe21992)

Git Hash:   97967fc8e63fb9ce65efe62cb87f89e45711dd3c
Git Author: kp@FreeBSD.org
END