intermittent failures when reuse_port is enabled

[Imre Rad]

I'm encountering connection issues to Dovecot when reuse_port is activated.
Some of the connections work fine, but about every 3rd of them does not
receive the banner after the TCP connection has established:

# nc -v 10.6.7.133 143
Connection to 10.6.7.133 143 port [tcp/imap2] succeeded!
* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE
STARTTLS AUTH=PLAIN AUTH=LOGIN] Hello.
^C

# nc -v 10.6.7.133 143
Connection to 10.6.7.133 143 port [tcp/imap2] succeeded!
* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE
STARTTLS AUTH=PLAIN AUTH=LOGIN] Hello.
^C

# nc -v 10.6.7.133 143
Connection to 10.6.7.133 143 port [tcp/imap2] succeeded!
^C

The relevant processes listening on this port:

# netstat -nap|grep 143
tcp        0      0 0.0.0.0:143             0.0.0.0:*               LISTEN
     15110/dovecot/imap-
tcp        0      0 0.0.0.0:143             0.0.0.0:*               LISTEN
     15103/dovecot/imap-
tcp        5      0 0.0.0.0:143             0.0.0.0:*               LISTEN
     15100/dovecot

After removing the reuse_port = yes configuration directive there is only
one single process listening (and the service is running without any
issues):

# netstat -nap|grep 143
tcp        0      0 0.0.0.0:143             0.0.0.0:*               LISTEN
     15292/dovecot

The test config was Linux 4.9.0-4-amd64 on debian-stretch and
dovecot-2.2.36.4 (compiled by me) running on a VM with 1 CPU core. If that
matters, dovecot was started in a docker container but using the hosts's
network namespace.

What am I missing?

Thanks,
Imre
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