Critical error hidden in logging type INFO

[matrixbot]

This issue has been migrated from #14367.

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Description

I was trying to add an event_persister worker via docker-compose using synapse_worker_event_persister1 as service name which also is the docker DNS record.
But apparently _ is not allowed for event_persister host's (which IMO is a bug on its on).

The error was only visible when showing "INFO" logs

Steps to reproduce

• add a generic worker in docker-compose.yaml with synapse_worker_event_persister1 as name
• reference it

instance_map:
    event_persister1:
        host: synapse_worker_event_persister1
        port: 8034

• send requests and get M_UNKNOWN: Failed to talk to event_persister1 process
• show docker logs and see nothing

version: 1

formatters:
  precise:
    format: '%(asctime)s - %(name)s - %(lineno)d - %(levelname)s - %(request)s - %(message)s'

handlers:


  console:
    class: logging.StreamHandler
    formatter: precise

loggers:
    synapse.storage.SQL:
        # beware: increasing this to DEBUG will make synapse log sensitive
        # information such as access tokens.
        level: INFO

root:
    level: WARN


    handlers: [console]


disable_existing_loggers: false

• change root.level to INFO to finally see error

Homeserver

self hosted

Synapse Version

1.70.1

Installation Method

Docker (matrixdotorg/synapse)

Platform

linux

Relevant log output

2022-11-04 14:19:14,311 - synapse.http.client - 455 - INFO - POST-5 - Error sending request to POST http://synapse_worker_event_persister1:8034/_synapse/replication/send_events/JfZHQlavDY: InvalidCodepoint Codepoint U+005F at position 8 of 'synapse_worker_event_persister1' not allowed

Anything else that would be useful to know?

I would guess https://github.com/matrix-org/synapse/blob/7bc110a19e6de0572b0c9513726d13298b45ced2/synapse/replication/http/_base.py#L298-L300 is the code throwing the error