ockam_vault_extern_error_t's definition/lack of documentation might encourage memory leaks.

[thomcc]

When filing #1561, I noticed an issue which is plausibly a bit more serious. Specifically, ockam_vault_extern_error_t::domain is allocated by CString::into_raw (which notes that it's C's responsibility to free https://github.com/ockam-network/ockam/blob/develop/implementations/rust/ockam/ockam_ffi/src/error.rs#L37), but no API to free resources managed by the ockam_vault_extern_error_t is provided, nor is the fact that it's needed documented.

Moreover, this field being a const char* parameter implies that it'd be invalid for the caller to free it. If I saw this in a C api, I'd probably expect that it's a pointer to a static error description string.

That said, fixing this is kind of a pain, and maybe it doesn't matter in practice, since errors should be rare — I'm only careful about noticing it since my background includes a lot of FFI work, including a very similar type.

[SanjoDeundiak]

Thanks @thomcc for submitting this issue. We used to have static strings for error domains, but it was impossible to use with our Actor model. More details here #967
Creating API for freeing memory is something that needs to be done and integrated into our ffi layer. If you're up to doing that, it would be great!

[etorreborre]

This was fixed with this PR: #2138