Make asyncio debug offsets reachable from _PyRuntime and give them a cookie, version and size #160107

Feature or enhancement

I work on Datadog's Python profiler. I used AI to help write this and have reviewed it.

Proposal

Make the asyncio debug offsets reachable from _PyRuntime, and give them a header like _Py_DebugOffsets has:

• Add void *asyncio_debug_offsets to _PyRuntimeState and publish its offset in _Py_DebugOffsets.runtime_state. _asyncio sets it during module init; NULL means _asyncio isn't initialized yet.
• Start the asyncio table with a cookie, PY_VERSION_HEX and its size.
• _remote_debugging follows the pointer and checks the header. Keep the AsyncioDebug section for existing tools.

This changes both table layouts, so it's 3.16-only.

Why

1. Discovery. _Py_DebugOffsets sits at the start of _PyRuntime, which is exported. Nothing points to the asyncio table and _Py_AsyncioDebug isn't exported, so readers search loaded files by name for the AsyncioDebug section and parse ELF/Mach-O/PE headers from disk. The name filter already had to change once (_asyncio.cpython in 3.14, python after build _asyncio module as static module #136669).
2. Validation. The table has no cookie, version or size. _remote_debugging checks the cookie of each PyRuntime candidate, but takes the first AsyncioDebug section it finds.

How it would be used

_remote_debugging, and in-process samplers such as DataDog/dd-trace-py#19802, would read the pointer from _PyRuntime, check the header, and then read the table, with no section search.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

• Async Call-Stack Reconstruction #91048: added the asyncio debug offsets and the AsyncioDebug section
• Harden validation of remote debugging offset tables #148178: validation of the remote debug offset tables

Linked PRs

• gh-160107: Make AsyncioDebug reachable from _PyRuntime and add a header #160108