Re: [PATCH] Add `verify-system` sslmode to use system CA pool for server cert

Huge +1 from me. On Azure we're already using public CAs to sign certificates for our managed postgres offerings[1][2]. Right now, our customers have to go to the hassle of downloading a specific root cert or finding their OS default location. Neither of these allow us to give users a simple copy-pastable connection string that uses secure settings. This would change this and make it much easier for our customers to use secure connections to their database.

1. From the rest of the thread it's not entirely clear to me why this patch goes for the sslrootcert=system approach, instead of changing what sslrootcert='' means when using verify-full. Like Tom Lane suggested, we could change it to try ~/.postgresql/root.crt and if that doesn't exist make it try the system store, instead of erroring out like it does now when ~/.postgresql/root.crt doesn't exist. This approach seems nicer to me, as it doesn't require introducing another special keyword. It would also remove the need for the changing of defaults depending on the value of sslrootcert. NOTE: For sslmode=verify-ca we should still error out if ~/.postgresql/root.crt doesn't exist, because as mentioned upthread it is trivial to get a cert from these CAs.

2. Should we allow the same approach with ssl_ca_file on the server side, for client cert validation?

[1]: https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/how-to-connect-tls-ssl

[2]: https://learn.microsoft.com/en-us/azure/cosmos-db/postgresql/howto-ssl-connection-security