obfs4proxy not killed when unused

Seems like Tor does not successfuly kill obfs4proxy when it's no longer used. It's unclear why. Maybe obfs4proxy never receives the SIGTERM, or never notices that the stdin is closed?

Experiment:

Startup Tor Browser using default obfs4 bridges.
Check that obfs4proxy process has spawned (ps -fax | grep obfs)
Reconfigure Tor Browser to not use bridges at all.
Check that obfs4proxy process is still there...

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