DB size quota enforcement (PR available)

[dstockton]

Would you be open to receiving a PR for quota (DB size) enforcement?
This adds the ability to switch connections into a delete & read-only (DELETE/DROP/VACUUM/etc) mode. Useful for several cases such as:

• preventing a single tenant filling disk space on a shared server
• serverless postgres offerings - especially where you want to limit your (cost) exposure
• scenarios where you don't have full administrative control of the DB server so can't (for example) use filesystem or postgres extension based quotas

I've been using this:
https://github.com/pgdogdev/pgdog/compare/main...dstockton:pgdog:feat/quota-enforcement?expand=1

[levkk]

I like it. However, I think this is better suited for a plugin.

Docs: https://docs.pgdog.dev/features/plugins/
Example (currently in prod): https://github.com/pgdogdev/pgdog/tree/main/plugins/pgdog-primary-only-tables

The plugin can be loaded at pgdog startup, connect to the DBs, poll pg_stat_database to get the DB sizes and start blocking non-delete queries once the quota is exceeded.

Plugins can do all of the above:

1. Have their own config: https://github.com/pgdogdev/helm#plugins
2. Run background jobs (with tokio)
3. Have initialization and shutdown hooks
4. See the entire query AST
5. Block queries: https://docsrs.pgdog.dev/pgdog_plugin/context/struct.Route.html#method.block

Let me know what you think, happy to chat about this further.