`/sync` randomly tightloops

This issue has been migrated from [#15824](https://github.com/matrix-org/synapse/issues/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

```shell
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.