Bug report

Bug description:

Several methods from the C implementation of the itertools module are not yet safe to use under the free-threading build. In this issue we list several issues to be addressed. The issues below are discussed foritertools.product, but the issues are similar for the other classes.

• When iterating overproduct the result tuple is re-used when the reference count is 1. We can use the new_PyObject_IsUniquelyReferenced method to perform the check whether we can re-use the tuple. (this issue was also reported inenum_next and pairwise_next can result in tuple elements with zero reference count in free-threading build #121464)

• On the first invocation ofproduct a new result is constructed.

This is not thread-safe, as multiple threads could haveresult == NULL evaluate to true. We could move the construction of theproductobject.result to the constructor ofproduct. This does mean thatproduct will use more memory before the first invocation ofnext. This seems to be acceptable, as constructing aproduct without iterating over it seems rare in practice.
The tuple also needs to be filled with data. Forproduct it seems safe to do this in the constructor, as the data is coming
fromproductobject->pools which is a tuple of tuples. But forpairwise the data is coming from an iterable

which could be a generator. Reading data from the iterator before the first invocation ofpairwise_next seems like a behavior change we do not want to make.

An alternative is to use some kind of locking insideproduct_next, but the locking should not add any overhead in the common path otherwise the single-thread performance will suffer.

• In case iterables are exhausted some cleaning up is done. For example inpairwise_next at

This cleaning up is not safe in concurrent iteration. Instead we can defer the cleaning up untill the object itself is decallocated (this approach was used forreversed, seehttps://github.com/python/cpython/pull/120971/files#r1653313765)

• Actually constructing the new result requires some care as well. Even if we are fine with having funny results under concurrent iteration (see the discussionSequence iterator thread-safety #120496), the concurrent iteration should not corrupt the interpreter. For example this code is not safe:

If two threads both incrementindices[i] the check on line 2078 is never true end we end up indexingpool withPyTuple_GET_ITEM outside the bounds on line 2088. Here we could change the check intoindices[i] >= PyTuple_GET_SIZE(pool). That is equivalent for the single-threaded case, but does not lead to out-of-bounds indexing in the multi-threaded case (although it does lead to funny results!)

@rhettinger@colesbury Any input on the points above would be welcome.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-123471: Make concurrent iteration over itertools.pairwise safe under free-threading #123848
• Draft: gh-123471: Make concurrent iteration over itertools.pairwise safe under free-threading #125417
• gh-123471: make itertools.batched thread-safe #129416
• gh-123471: Make concurrent iteration over itertools.cycle safe under free-threading #131212
• gh-123471: Make concurrent iteration over itertools.repeat safe under free-threading #131247
• gh-123471: Make itertools.product and itertools.combinations thread-safe #132814
• gh-123471: Make itertools.chain thread-safe #135689