Add a default cshim feature flag

[eeeebbbbrrrr]

This adds a cshim feature flag to pgx, enabled by default, and pushes it down to pgx-pg-sys and pgx-tests.

With this turned off we lose low-level access to a few things:

• pgx_list* functions
• pgx_planner_rt_fetch()
• pgx_SpinLock* functions

From pgx' perspective, this means a few modules are just not available without cshim:

• hooks (uses PgList)
• list (impl for PgList which uses pgx_list* functions)
• namespace (uses PgList)
• spinlock (uses pgx_SpinLock* functions)

This feature flag will hopefully make it easier to cross-compile pgx with only a small bit of compromises. This feature flag was initially designed with plrust user functions in mind, and they won't need access to any of the above.

[eeeebbbbrrrr]

I think this is ready for merge.

[thomcc]

Strictly speaking this should probably be a cshim feature enabled by default (enabling a cargo feature is never supposed to remove functionality).

But in practice I don't think this matters much, and it might be a hassle to disable otherwise.

[eeeebbbbrrrr]

Strictly speaking this should probably be a cshim feature enabled by default (enabling a cargo feature is never supposed to remove functionality).

But in practice I don't think this matters much, and it might be a hassle to disable otherwise.

Hmm. I think that would be incredibly invasive, code wise. Would that not mean basically putting a #[cfg] on nearly everything, or otherwise a major reorg?

[thomcc]

I think it would mostly be a matter of changing #[cfg(not(feature = "no-cshim"))] to #[cfg(feature = "cshim")], no?

[thomcc]

That said this isn't really a feature we expect users to enable, so it's probably fine.

[eeeebbbbrrrr]

#[cfg(feature = "cshim")]

Would it not mean adding this to everything that currently doesn't have this feature flag at all? Am I missing something.

[workingjubilee]

@eeeebbbbrrrr No, it would involve adding the feature flag only to things enabled specifically by the cshim. Currently the flag strategy is a "double negative": "if the no-cshim feature is NOT enabled, then include this code". Thus it's fine to simply reduce the double negatives to positives. Then, we'd add default = ["cshim"] to the Cargo.toml.

[eeeebbbbrrrr]

default = ["cshim"]

Do we like that? Sometimes using cargo across the entire pgx repo (cargo test --all) requires setting specific features and also specifying --no-default-features.

I can do it this way, of course, if it's preferred and otherwise idiomatic.

[workingjubilee]

Yeah, I don't mind setting features like that. We can prevent settings creep by making sure at least one feature includes all default features that aren't a Postgres version.

[eeeebbbbrrrr]

Okay, I'll rearrange this.

[eeeebbbbrrrr]

I don't think this PR is going to survive in this state after our discord discussions around cargo test. I'm switching gears to work on that, then I'll come back to this.

[eeeebbbbrrrr]

This PR is slightly complicated by rust-lang/cargo#4753.

The "solution", if you can call it that, is for our pgx-tests/Cargo.toml to not have a default = [ "cshim" ] feature. And if we want to run the tests with the cshim turned on, we'll have to specify it explicitly.

9646bdf does that, assuming it passes CI.

[eeeebbbbrrrr]

I'm content with this now. Merging.