`/sync` randomly tightloops

[matrixbot]

This issue has been migrated from #15824.

---

Description

In my test environment for matrix-media-repo I've got a localhost homeserver running on sqlite in docker which suddenly returns immediately to /sync requests. This causes Element to dutifully send the next request, to which Synapse responds immediately, and repeat.

Steps to reproduce

Unclear.

Homeserver

localhost

Synapse Version

v1.85.2

Installation Method

Docker (matrixdotorg/synapse)

Database

sqlite3

Workers

Single process

Platform

Running in Docker.

Configuration

# Configuration file for Synapse.
#
# This is a YAML file: see [1] for a quick introduction. Note in particular
# that *indentation is important*: all the elements of a list or dictionary
# should have the same indentation.
#
# [1] https://docs.ansible.com/ansible/latest/reference_appendices/YAMLSyntax.html
#
# For more information on how to configure Synapse, including a complete accounting of
# each option, go to docs/usage/configuration/config_documentation.md or
# https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html
server_name: "localhost"
pid_file: /data/homeserver.pid
listeners:
  - port: 8008
    tls: false
    type: http
    x_forwarded: true
    resources:
      - names: [client, federation]
        compress: false
database:
  name: sqlite3
  args:
    database: /data/homeserver.db
log_config: "/data/localhost.log.config"
media_store_path: /data/media_store
registration_shared_secret: "redacted, but it's public anyways"
report_stats: false
macaroon_secret_key: "redacted, but it's public anyways"
form_secret: "redacted, but it's public anyways"
signing_key_path: "/data/localhost.signing.key"

# Project-specific settings (not part of Synapse defaults)
enable_media_repo: false
enable_registration: true
enable_registration_without_verification: true
trusted_key_servers: []

# vim:ft=yaml

Relevant log output

media_repo_nginx    | 172.19.0.1 - - [24/Jun/2023:03:24:45 +0000] "GET /_matrix/client/r0/sync?filter=2&timeout=30000&since=s122_30870_0_15_114_1_4_30_0_1 HTTP/1.1" 200 295 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Safari/537.36" "-"
media_repo_synapse  | 2023-06-24 03:24:45,058 - synapse.access.http.8008 - 461 - INFO - GET-13264 - ::ffff:172.19.0.2 - 8008 - {@travis:localhost} Processe
d request: 0.005sec/0.000sec (0.001sec, 0.000sec) (0.001sec/0.003sec/4) 283B 200 "GET /_matrix/client/r0/sync?filter=2&timeout=30000&since=s122_30870_0_15_114_1_4_30_0_1 HTTP/1.0" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Safari/537.36" [0 dbevts]      
media_repo_nginx    | 172.19.0.1 - - [24/Jun/2023:03:24:45 +0000] "GET /_matrix/client/r0/sync?filter=2&timeout=30000&since=s122_30870_0_15_114_1_4_30_0_1 HTTP/1.1" 200 295 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Safari/537.36" "-"
media_repo_synapse  | 2023-06-24 03:24:45,093 - synapse.access.http.8008 - 461 - INFO - GET-13265 - ::ffff:172.19.0.2 - 8008 - {@travis:localhost} Processe
d request: 0.005sec/0.000sec (0.001sec, 0.000sec) (0.001sec/0.003sec/4) 283B 200 "GET /_matrix/client/r0/sync?filter=2&timeout=30000&since=s122_30870_0_15_114_1_4_30_0_1 HTTP/1.0" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Safari/537.36" [0 dbevts]

Anything else that would be useful to know?

Restarting the Docker Compose stack can sometimes help, but it eventually tightloops again.