Retained messages are being dropped when connected via unix socket

[Kiaatix]

Mosquitto doesn't deliver all retained messages when connected via unix socket.

Tested with Mosquitto 2.0.20 with config

listener 1883
listener 0 /tmp/mosquitto.sock

Publish 5000 retained messages:

#!/bin/bash
for i in {1..5000}
do
    mosquitto_pub -t "test/$i/testmsg" -m "test $i" --retain
done

Connection over TCP works as expected:
mosquitto_sub -h 127.0.0.1 -t "test/#"
Output:

test 1
test 2
.
.
.
test 5000

But with
mosquitto_sub --unix /tmp/mosquitto.sock -t "test/#"
Output:

test 1
test 2
.
.
.
test 1462

with mosquitto.log entry
Outgoing messages are being dropped for client auto-C7D5C72D-ECFE-F6F4-AB79-0FDAC37A8969.

There should be no difference in message handling between connection types.

[sikmir]

Can't reproduce with 2.0.20.

[Kiaatix]

The problem only exists when connecting/subscribing after the messages have been published, i.e. when the broker delivers the retained messages. When the client is already subscribed while publishing, it works normally.

I was able to reproduce it on a Ubuntu 22.04 machine with mosquitto 2.0.11, on Ubuntu 24.04 with mosquitto 2.0.18 and still experiencing it with 2.0.20 from mosquitto-ppa on a fresh Ubuntu 24.04 VM.

My active config is:

persistence true
persistence_location /var/lib/mosquitto/
log_dest file /var/log/mosquitto/mosquitto.log
listener 1883
listener 0 /tmp/mosquitto.sock
allow_anonymous true

After publishing the messages, I connect mosquitto_sub over TCP and everything is delivered as expected with log:

1740761663: New connection from 127.0.0.1:40418 on port 1883.
1740761663: New client connected from 127.0.0.1:40418 as auto-5644E03B-6044-F541-7F7F-8901E2E4B862 (p2, c1, k60).
1740761665: Client auto-5644E03B-6044-F541-7F7F-8901E2E4B862 disconnected.

Then, when connectiong over unix socket, I'm receiving only ~1500 of the retained messages with log:

1740761680: New connection from /tmp/mosquitto.sock:0 on port 0.
1740761680: New client connected from /tmp/mosquitto.sock:0 as auto-D299BB41-637E-6CFC-1FDF-2717F9681972 (p2, c1, k60).
1740761680: Outgoing messages are being dropped for client auto-D299BB41-637E-6CFC-1FDF-2717F9681972.
1740761682: Client auto-D299BB41-637E-6CFC-1FDF-2717F9681972 disconnected

When i set max_queued_messages 6000, it works as expected, but as far as I know, message queuing only applies to QoS 1+2 and not for retained messages.