Plugin compatibility checking uses non-ABI-stable types

[Nemo157]

Describe the bug

When loading a plugin the loader and plugin assume that the zenoh_plugin_trait::Compatibility type is ABI-stable enough to pass between arbitrary versions to then verify if they are compatible.

This type is itself repr(C), but it contains repr(Rust) types:

https://github.com/eclipse-zenoh/zenoh/blob/2f955cc8749a8451d193eb4b4060e46e30165804/plugins/zenoh-plugin-trait/src/compatibility.rs#L57-L66

There's a few potentially unstable type layouts here and in sub-types:

• &'static str: still undefined, it may be e.g. (len, ptr) or (ptr, len)
• Option<RustcVersion>/Option<PluginStructVersion>: both of these sub-structs, while being repr(C), contain multiple fields with niches, the Option discriminant may be packed in different niches, or kept as a separate tag field. If stored as a tag field the byte value used may be different in different compilations.

To reproduce

Probably requires compiling with different compiler versions with different niche heuristics and/or -Zbuild-std/-Zrandomize-layout. I'm not certain there are existing compilers that have different enough behavior to trigger these problems, but they're at least allowed by the current specifications.

System info

irrelevant

[milyin]

Agree with you. This is confirmed here https://users.rust-lang.org/t/repr-c-on-nested-structs/110654 This should be fixed together with this task I suppose: https://github.com/eclipse-zenoh/zenoh/issues/1745