Debug mode: check correct HPyContext usage

[steve-s]

Based on WIP in https://github.com/hpyproject/hpy/pull/350 done with @NiRit100 at the HPy sprint

• the private data of debug context now contains a flag is_valid and all the context functions check this flag as the first thing.
• the private data of debug context is split to "shared" part and the is_valid flag
• we have a circular buffer of multiple copies of the debug context
• the trampoline (debug_ctx_CallRealFunctionFromTrampoline) takes next context from the buffer, sets is_valid=false for the previous context and is_valid=true for the context to be used (after the call the flags are reverted back)

[mattip]

This has a merge conflict. Is it ready for review?

[steve-s]

I've rebased the PR and it's ready for a review now.

[steve-s]

@antocuni would you have some spare cycles for a review :-)?

[mattip]

This has conflicts.

[steve-s]

Proactively rebased on top of https://github.com/hpyproject/hpy/pull/402 to streamline merging of both PRs

[mattip]

Could you describe under what cases the checks might fail? What is the scenario that would cause someone to get the wrong context?

[steve-s]

Could you describe under what cases the checks might fail? What is the scenario that would cause someone to get the wrong context?

What this aims to prevent is a situation when someone misunderstands the contract of HPy and stashes context into some global variable to read it later in a context of another new Python->HPy call. For example, in the test:

https://github.com/hpyproject/hpy/pull/363/files#diff-885e9f36d2a0124e4a50c595a7fac468a1ac9af3c5dea852646c1d52d5694346R37

on this line it uses global variable keep as HPyContext, but it should use the ctx argument.

[mattip]

Thanks @steve-s