I couldn't really find any documentation regarding how to setup TLS
communication between Synapse and Redis, so I looked through the source
code and found it. I figured I should go ahead and document it here.
These errors are harmless and are a long-standing issue that is just now
being logged, see https://github.com/element-hq/synapse/issues/19042
```
2025-10-10 15:30:00,026 - synapse.util.metrics - 330 - ERROR - notify_interested_services-0 - Metric named cache_lru_cache__matches_user_in_member_list_example.com already registered for server example.com
2025-10-10 16:30:00.167
2025-10-10 15:30:00,026 - synapse.util.metrics - 330 - ERROR - notify_interested_services-0 - Metric named cache_lru_cache_is_interested_in_room_example.com already registered for server example.com
2025-10-10 16:30:00.167
2025-10-10 15:30:00,025 - synapse.util.metrics - 330 - ERROR - notify_interested_services-0 - Metric named cache_lru_cache_is_interested_in_event_example.com already registered for server example.com
2025-10-10 16:29:15.560
2025-10-10 15:29:15,449 - synapse.util.metrics - 330 - ERROR - notify_interested_services_ephemeral-0 - Metric named cache_lru_cache__matches_user_in_member_list_example.com already registered for server example.com
2025-10-10 16:29:15.560
2025-10-10 15:29:15,449 - synapse.util.metrics - 330 - ERROR - notify_interested_services_ephemeral-0 - Metric named cache_lru_cache_is_interested_in_room_example.com already registered for server example.com
```
(more sane standard location for this sort of thing)
The one difference here is that previously, `start_doing_background_updates
()` only ran on the main Synapse instance. But since it now lives in
`start_background_tasks()`, it will run on the worker that supposed to
`run_background_tasks`. Doesn't seem like a problem though.
This means we
can move the open registration config validation from `setup()` to
`HomeServerConfig.validate_config()` (much more sane).
Spawning from looking at this area of code in
https://github.com/element-hq/synapse/pull/19015
### Background
As part of Element's plan to support a light form of vhosting (virtual
host) (multiple instances of Synapse in the same Python process), we're
currently diving into the details and implications of running multiple
instances of Synapse in the same Python process.
"Clean tenant provisioning" tracked internally by
https://github.com/element-hq/synapse-small-hosts/issues/221
### Partial startup problem
In the context of Synapse Pro for Small Hosts, since the Twisted reactor
is already running (from the `multi_synapse` shard process itself), when
provisioning a homeserver tenant, the `reactor.callWhenRunning(...)`
callbacks will be invoked immediately. This includes the Synapse's
[`start`](0615b64bb4/synapse/app/homeserver.py (L418-L429))
callback which sets up everything (including listeners, background
tasks, etc). If we encounter an error at this point, we are partially
setup but the exception will [bubble back to
us](8be122186b/multi_synapse/app/shard.py (L114-L121))
without us having a handle to the homeserver yet so we can't call
`hs.shutdown()` and clean everything up.
### What does this PR do?
Structures Synapse so we split creating the homeserver instance from
setting everything up. This way we have access to `hs` if anything goes
wrong during setup and can subsequently `hs.shutdown()` to clean
everything up.
This was originally removed in
https://github.com/element-hq/synapse/pull/18886 but it looks like it
snuck back in https://github.com/element-hq/synapse/pull/18828 during a
[bad
merge](4cd3d9172e).
Noticed while looking at Synapse setup and startup (just by happen
stance).
I don't think this has adverse effects on Synapse actually working and
`start_background_tasks()` can be called multiple times.
### Is there a good way to audit all of these merges?
As I would like to see the conflicts for each merge.
This works but it's still hard to notice anything is wrong:
```
git log --remerge-diff <commit-sha>
```
> shows the difference from mechanical merge result and the result that
is actually recorded in a merge commit
via
https://stackoverflow.com/questions/15277708/how-do-you-see-show-a-git-merge-conflict-resolution-that-was-done-given-a-mer/71181334#71181334
The following better. Specify the version range to the commit right
before the merge to the merge. And can even specify which file to look
at to make it more obvious with the hindsight we have now.
```
git log --remerge-diff <merge-commit-sha>~1..<merge-commit-sha> -- synapse/server.py
```
Example:
```
git log --remerge-diff 4cd3d9172ed7b87e509746851a376c861a27820e~1..4cd3d9172ed7b87e509746851a376c861a27820e -- synapse/server.py
```
