Inconsistent `.extend()` behavior in bytearray

Bug report

Bug description:

Extending a bytearrary can behave very differently when extending via an iterable than when extending a list or array.array in the same way:

>>> from itertools import islice
>>> xs = [0, 1, 2]
>>> xs.extend(islice(xs, 12000))
>>> len(xs)
12003
>>> import array
>>> xs = array.array('Q', [0, 1, 2])
>>> xs.extend(islice(xs, 12000))
>>> len(xs)
12003

Tricky, but it's always worked this way, and is very convenient to extend a sequence with copies of itself. Note that the iterator picks up new elements of the sequence while they're being added.

bytearray doesn't work this way, though. It appears to capture the sequence's length just once at the start, and so can't do more than double the original length.

>>> xs = bytearray([0, 1, 2])
>>> xs.extend(islice(xs, 12000))
>>> len(xs)
6
>>> list(xs)
[0, 1, 2, 0, 1, 2]

Bumped into this when changing old code to switch from lists of small ints to bytearrays instead. Quite a head-scratcher to figure out what went wrong! ;-)

CPython versions tested on:

3.15, 3.14

Operating systems tested on:

No response

Linked PRs

• gh-145333
• gh-145637

Comienza reproduciendo el comportamiento de bytearray.extend con el ejemplo proporcionado de itertools.islice y, a continuación, revisa los PR enlazados gh-145333 y gh-145637 para conocer la dirección actual. Se considera terminado cuando el comportamiento se haya resuelto de forma coherente con los casos mostrados de list y array.array y se haya identificado en ese trabajo la cobertura de regresión relevante.