Align itertools source and callback lifetimes - #9721

Summary

Align itertools callback state and source ownership with CPython 3.15, rebased onto main 3d0f6fc659b02733722ec658eb2be7c5a9288c68.

• Read accumulate's running total after source callbacks and release arithmetic operands before publishing the replacement total.
• Release converted non-exact-tuple starmap containers before calling the function; retain exact tuples through the call.
• Detach cycle's exhausted source after publishing replay state and preserve the outer call's replay index across finalizer reentry.
• Clear pairwise sources and previous items in the appropriate first/second-input failure order.
• Detach nonfinal exhausted zip_longest sources while retaining the final/error source and protecting the active count with its optional slots.

State locks

Cycle's source and saved items now share its existing PyRwLock<CycleState>. Pairwise's source and old item share its existing PyRwLock<PairwiseState>. This removes both additional source mutexes while retaining typed PyIter fields. No extra PyObjectCell/QSBR mechanism is introduced.

ZipLongestState keeps one mutex because optional slots and the active count must be updated consistently. Callback snapshots are owned; callbacks and finalizing releases occur outside locks. Atomic cycle-index and pairwise-terminal publication are retained. Independent static concurrency review found no new lock-order or callback-under-lock issue. Concurrent next calls may still interleave; performance under contention was not benchmarked.

Validation

For 3cb2363d7551eeb5c0aabcac0512f9284050e1bf:

• Normal commit hooks, rustfmt and diff checks passed.
• Workspace tests and all-target Clippy with -D warnings passed.
• Separate C-API tests (149 passed, 2 ignored) and Clippy with -D warnings passed.
• Release test_itertools: 137 tests run, 22 skipped, passed.
• Existing cycle/zip_longest threading tests: 2 passed.
• Full changed itertools snippet, cycle threading snippet and focused pytest wrapper: passed.
• Added finalizer-reentry/source-release regressions fail on the preserved pre-PR binary and pass with this revision. Existing Lib tests cover pairwise callback reentry.

CPython 3.15.0 source was checked. No CPython 3.15 executable was available for this validation pass; supplemental CPython 3.12 native-next pairwise probes passed. Other platforms and hosted CI remain separate checks.

Limits

Cycle follows ordinary CPython source-release behavior; CPython's free-threaded build deliberately retains that source. Cached result-tuple reuse, generic tuple-conversion failure cleanup, and batched are not addressed.

A separate pre-existing iterator-protocol mismatch remains: Python-defined __next__ StopIteration exceptions are normalized by PyIterReturn::from_pyresult, discarding the original exception/traceback early. This can change source-finalizer ordering relative to CPython. The same exception-identity loss reproduces before this PR. Complete lifetime equivalence is not claimed; repairing raw-versus-normalized next semantics belongs in the shared iterator protocol rather than a mutex workaround here.

AI-assisted contribution by OpenAI Codex, disclosed in commit trailers.