feat(py): Expose dynamic rule condition validation

[RaduW]

Added dynamic rule condition validation function in sentry-relay.

#skip-changelog

Sorry, clicked the wrong button.

[jan-auer]

You can return RelayStr::new("") here. No allocation required.

[RaduW]

I know, but I don't like it.
If you insist I'll do it but here are my reasons not to:

What I want to do (in a future PR) is change the current RealyStr::new(s: &str) to:

   pub(crate) fn new(s: &'static str) -> RelayStr 

and

   pub(crate) fn unsafe unsafe_new(s: &str) -> RelayStr 

That also affects the implementation of impl<'a> From<&'a str> for RelayStr so I need to look
a bit more in detail at it.

Until then I consider the implementation of RealyStr::new as broken, it is unsafe and breaks the Rust
guarantees. It will happily compile something like:

pub fn bad_function() -> RelayStr {
    RelayStr::new(&format!("Hello {}", "Jan"))
}

[mitsuhiko]

@jan-auer i feel like we should revisit this string pattern. I remember we incorrectly deduced what the allocation / borrow behavior once before.

[jan-auer]

@mitsuhiko Right, we had bugs prior to Relay in libsymbolic. It's worth noting that the Python layer always copies the string anyway.

@RaduW you're right, the name new doesn't carry the implications the function has. It is worth noting that RelayStr is only to be used in an unsafe FFI layer, so the exposure is pretty low. Let's use RelayStr::default for now, which will inherently not have any borrowing problems.

To continue a discussion from Slack: There is not much code, and it is easy to enumerate all places where we use RelayStr::new:

• relay_data_category_name, which always returns static strings
• RelayStr::default, which can be considered an implementation detail of empty strings in RelayStr
• relay_str_from_cstr, which deals with pointers. It's also dead code
• relay_validate_pii_config, which can also be changed to RelayStr::default
• relay_convert_datascrubbing_config, which returns a static "{}".

To sum this up, we can address this much simpler by:

• Changing the signature to new(s: &'static str) -> Self (this is now inherently safe)
• Removing relay_str_from_cstr

[jan-auer]

Thanks, this is good to merge.

A side note on naming: One mentions "dynamic rule", the other "sampling". Given that both validate within the same structure, we could choose more similar names, such as validate_sampling_config and validate_sampling_condition.

[jan-auer]

Can you just make this an early return? For example:

    match serde_json::from_str::<SamplingConfig>((*value).as_str()) {
        Ok(config) => {
            for rule in config.rules {
                if !rule.condition.supported() {
                    return RelayStr::new("unsupported sampling rule");
                }
            }
            
            RelayStr::default()
        }
        Err(e) => RelayStr::from_string(e.to_string()),
    }

Note that I'm using new here as per discussion in #941 (comment).