gh-124397: Add free-threading support for iterators.

Doc/library/threading.rst

@@ -1436,3 +1436,155 @@ is equivalent to::
 Currently, :class:`Lock`, :class:`RLock`, :class:`Condition`,
 :class:`Semaphore`, and :class:`BoundedSemaphore` objects may be used as
 :keyword:`with` statement context managers.
+
+
+Iterator synchronization
+------------------------
+
+By default, Python iterators do not support concurrent access. Most iterators make
+no guarantees when accessed simultaneously from multiple threads. Generator
+iterators, for example, raise :exc:`ValueError` if one of their iterator methods
+is called while the generator is already executing. The tools in this section
+allow reliable concurrency support to be added to ordinary iterators and
+iterator-producing callables.
+
+The :class:`serialize_iterator` wrapper lets multiple threads share a single iterator and
+take turns consuming from it. While one thread is running ``__next__()``, the
+others block until the iterator becomes available. Each value produced by the
+underlying iterator is delivered to exactly one caller.
+
+The :func:`concurrent_tee` function lets multiple threads each receive the full
+stream of values from one underlying iterator. It creates independent iterators
+that all draw from the same source. Values are buffered until consumed by all
+of the derived iterators.
+
+.. class:: serialize_iterator(iterable)
+
+   Return an iterator wrapper that serializes concurrent calls to
+   :meth:`~iterator.__next__` using a lock.
+
+   If the wrapped iterator also defines :meth:`~generator.send`,
+   :meth:`~generator.throw`, or :meth:`~generator.close`, those calls
+   are serialized as well.
+
+   This makes it possible to share a single iterator, including a generator
+   iterator, between multiple threads. A lock assures that calls are handled
+   one at a time. No values are duplicated or skipped by the wrapper itself.
+   Each item from the underlying iterator is given to exactly one caller.
+
+   This wrapper does not copy or buffer values. Threads that call
+   :func:`next` while another thread is already advancing the iterator will
+   block until the active call completes.
+
+   Example:
+
+   .. code-block:: python
+
+      import threading
+
+      def count():
+          for i in range(5):
+              yield i
+
+      it = threading.serialize_iterator(count())
+
+      def worker():
+          for item in it:
+              print(threading.current_thread().name, item)
+
+      threads = [threading.Thread(target=worker) for _ in range(2)]
+      for thread in threads:
+          thread.start()
+      for thread in threads:
+          thread.join()
+
+   In this example, each number is printed exactly once, but the work is shared
+   between the two threads.
+
+   .. versionadded:: next
+
+
+.. function:: synchronized_iterator(func)
+
+   Wrap an iterator-producing callable so that each iterator it returns is
+   automatically passed through :class:`serialize_iterator`.
+
+   This is especially useful as a :term:`decorator` for generator functions,
+   allowing their generator-iterators to be consumed from multiple threads.
+
+   Example:
+
+   .. code-block:: python
+
+      import threading
+
+      @threading.synchronized_iterator
+      def counter():
+          i = 0
+          while True:
+              yield i
+              i += 1
+
+      it = counter()
+
+      def worker():
+          for _ in range(5):
+              print(next(it))
+
+      threads = [threading.Thread(target=worker) for _ in range(2)]
+      for thread in threads:
+          thread.start()
+      for thread in threads:
+          thread.join()
+
+   The returned wrapper preserves the metadata of *func*, such as its name and
+   wrapped function reference.
+
+   .. versionadded:: next
+
+
+.. function:: concurrent_tee(iterable, n=2)
+
+   Return *n* independent iterators from a single input *iterable*, with
+   guaranteed behavior when the derived iterators are consumed concurrently.
+
+   This function is similar to :func:`itertools.tee`, but is intended for cases
+   where the source iterator may feed consumers running in different threads.
+   Each returned iterator yields every value from the underlying iterable, in
+   the same order.
+
+   Internally, values are buffered until every derived iterator has consumed
+   them.
+
+   The returned iterators share the same underlying synchronization lock. Each
+   individual derived iterator is intended to be consumed by one thread at a
+   time. If a single derived iterator must itself be shared by multiple
+   threads, wrap it with :class:`serialize_iterator`.
+
+   If *n* is ``0``, return an empty tuple. If *n* is negative, raise
+   :exc:`ValueError`.
+
+   Example:
+
+   .. code-block:: python
+
+      import threading
+
+      source = (x**2 for x in range(5))
+      left, right = threading.concurrent_tee(source)
+
+      def consume(name, iterable):
+          for item in iterable:
+              print(name, item)
+
+      t1 = threading.Thread(target=consume, args=("left", left))
+      t2 = threading.Thread(target=consume, args=("right", right))
+      t1.start()
+      t2.start()
+      t1.join()
+      t2.join()
+
+   In this example, both consumer threads see the full sequence of squares
+   from a single generator expression.
+
+   .. versionadded:: next

Doc/whatsnew/3.15.rst

@@ -1158,6 +1158,16 @@ tarfile
   (Contributed by Christoph Walcher in :gh:`57911`.)
 
 
+threading
+---------
+
+* Added :class:`~threading.serialize_iterator`,
+  :func:`~threading.synchronized_iterator`,
+  and :func:`~threading.concurrent_tee` to support concurrent access to
+  generators and iterators.
+  (Contributed by Raymond Hettinger in :gh:`124397`.)
+
+
 timeit
 ------