Fix default_transaction_isolation on system/database level

[fantix]

client GucSource (backend connection parameter) has higher priority than database or configuration file (system). Now that default_transaction_isolation is exposed, it has to be removed from the backend connection parameters. Instead, it now becomes a permanent system config that always has a value.

This breaks remote backends without config file access unless they are preconfigured with default_transaction_isolation = serializable system-wide.

Refs #8276

• Add test

[msullivan]

Call this out as a HACK in the comments.
Some day we may want this to be a general mechanism?
(Not today.)

[fantix]

Yeah, now that I'm thinking more about it, maybe we should always reset the system backend settings to their Gel defaults explicitly, as we cannot rely on the backend defaults, like users could put anything in postgresql.conf and run Gel with --backend-dsn.

[msullivan]

Actually wait, I don't fully understand; when does default_transaction_isolation get set, in the normal flow?

[fantix]

Actually wait, I don't fully understand; when does default_transaction_isolation get set, in the normal flow?

During bootstrap, backend settings that have defaults are applied to the PG config file?

[msullivan]

Actually wait, I don't fully understand; when does default_transaction_isolation get set, in the normal flow?

During bootstrap, backend settings that have defaults are applied to the PG config file?

Roger, got it. I had forgotten that step but on reading the code it seems familiar.

[msullivan]

Wait --- cloud doesn't have configfile access, right?