- Move `register_start` (calls `os._exit(1)`) out of `setup` (our
composable function)
- We want to avoid `exit(...)` because we use these composable functions
in Synapse Pro for small hosts where we have multiple Synapse instances
running in the same process. We don't want a problem from one homeserver
tenant causing the entire Python process to exit and affect all of the
other homeserver tenants.
- Continuation of https://github.com/element-hq/synapse/pull/19116
- Align our app entrypoints: `homeserver` (main), `generic_worker`
(worker), and `admin_cmd`
### 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) (c.f
Synapse Pro for small hosts), 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/48
a7107458c6