Avoid unnecessary NotShareableError creation in XIData FULL_FALLBACK

Proposal:

Motivation

The pickle fallback path in _PyObject_GetXIData() does unnecessary work on successful transfers of types that are not in the XIData registry, creating and discarding a NotShareableError that user code never sees.

This is a follow-up to gh-148072 / GH-148125, which cached pickle.dumps and pickle.loads per interpreter. That change removed module lookup overhead, but there is still avoidable cost in the fallback path itself.

This extra exception work adds measurable overhead to cross-interpreter transfers that rely on pickle fallback.

Approach

Check the XIData registry first with lookup_getdata() before calling _get_xidata().

• If the type is registered, keep using the existing path unchanged.
• If the type is not registered, skip _get_xidata() and go directly to the function/pickle fallback chain.

On total failure, raise the same NotShareableError as the existing code path, so external behavior stays the same.

Benchmark results

This removes unnecessary exception creation on a hot fallback path and gives a measurable speedup for transfers that rely on pickle fallback.

In local benchmarks on Apple M2, combined with GH-148125, this improved round-trip transfer times for mutable objects such as list and dict, especially for larger payloads.

Large-payload benchmark results (500 iterations, Apple M2)

Baseline is upstream/main after GH-148125 (pickle cache). "After" includes this change.

Data	Before	After	Speedup
list [1 x 100KB]	1.19ms	0.12ms	9.9x
list [1 x 1MB]	10.26ms	0.20ms	51x
list [10 x 100KB]	9.99ms	0.10ms	100x
dict {10 x 100KB}	10.41ms	0.26ms	40x

Related

• gh-148072 — Predecessor: pickle module caching (merged as GH-148125)
• gh-124694 — InterpreterPoolExecutor addition

Validation

Local testing passed:

• ./python -m test test_interpreters
• ./python -m test test_concurrent_futures

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:

• https://github.com/python/cpython/issues/148072

Linked PRs

• gh-148224

Start with _PyObject_GetXIData() and trace lookup_getdata() and _get_xidata() in the pickle fallback path. Review linked PR gh-148224, then run ./python -m test test_interpreters and ./python -m test test_concurrent_futures; done means preserving the existing NotShareableError behavior while avoiding unnecessary exception creation.