Iterating F("pk") can hang

[Jacob Walls]

#35603 solved one of the two scenarios that could lead to an infinite hang when F() expressions fall back to "old-style" iteration protocols via __getitem__(). It solved it for containment checks by throwing from __contains__. It didn't solve it for iteration, so we encountered an infinite hang in #37291.

When reviewing the fix for #37291, I began to suggest addressing the root cause by throwing an error from F.__iter__() until I realized that it would change the return value of hasattr(expr, "__iter__"). We have several uses in core like that.

What we should do, I think, is set F.__iter__ = None, and then update core (and add release notes) that code should update from:

if hasattr(expr, "__iter__"):

to:

if getattr(expr, "__iter__", None):

...so that the pythonic way of declaring no-iteration can be read statically.

[Simon Charette]

Setting F.__iter__ = None seems like a good way to go about this but I think we should use isinstance(expr, Iterable) instead of the getattr / hasattr call?

​https://docs.python.org/3/library/collections.abc.html#collections.abc.Iterable