concurrent iteration. (This is prep work for enabling specialization ofFOR_ITER in free-threaded builds.) The basic premise is: - Iterating over a shared _iterable_ (list, tuple or range) should be safe,   not involve data races, and behave like iteration normally does. - Using a shared _iterator_ should not crash or involve data races, and   should only produce items regular iteration would produce. It is _not_   guaranteed to produce all items, or produce each item only once.Providing stronger guarantees is possible for some of these iterators, butit's not always straight-forward and can significantly hamper the commoncase. Since iterators in general aren't shared between threads, and it'ssimply impossible to concurrently use many iterators (like generators),better to make sharing iterators without explicit synchronization clearlywrong.Specific issues fixed in order to make the tests pass: - List iteration could occasionally crash when a shared list wasn't already   marked as shared when reallocated. - Tuple iteration could occasionally crash when the iterator's reference to   the tuple was cleared on exhaustion. Like with list iteration, in   free-threaded builds we can't safely and efficiently clear the iterator's   reference to the iterable (doing it safely would mean extra, slow   refcount operations), so just keep the iterable reference around. - Fast range iterators (for integers that fit in C longs) shared between   threads would sometimes produce values past the end of the range, because   the iterators use two bits of state that we can't efficiently update   atomically. Rewriting the iterators to have a single bit of state is   possible, but probably means more math for each iteration and may not be   worth it. - Long range iterators (for other numbers) shared between threads would   crash catastrophically in a variety of ways. This now uses a critical   section. Rewriting this to be more efficient is probably possible, but   since it deals with arbitrary Python objects it's difficult to get right.There seem to be no more exising races in list_get_item_ref, so drop it fromthe tsan suppression list.

actually correct and the real problem was an incorrect assert. The fast pathstill contains notionally unsafe uses of memcpy/memmove, so addlist_get_item_ref back to the TSan suppressions file.

iterators, and fix build failures because labels can't technically be at theend of compound statements (a statement must follow the label, even if it'sempty).

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…len andtupleiter_setstate appropriately relaxed.

iterator test to accept the current behaviour of the fast range iterator andavoid testing the (very unsafe) long range iterator, use threading.Barrierwhere appropriate, and add comments to the unsafe uses of memcpy/memmove infree-threaded builds.

relaxed atomics. (These are not performance-sensitive parts of the code, andthis lets us detect races elsewhere involving the iterators.)

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