Missing recursion checks building long lazy import alias can segfault #159288

Crash report

Bug: When lazily importing a long dotted named module using an alias, nested placeholders are used for each name to build the full chain recursively. If the dotted module name is long enough (import a.b.c.d.etc as foo), it's possible to overflow the C stack and segfault.

Severity: It's likely that these use cases would rarely be hit, but they do segfault when using lazy imports and they do not with eager imports, so I think it's still worth a small fix.

examples

Both tests leverage threading to reduce the available stack size.

Resolving from repr:

import threading
exec("lazy import " + ".".join(["m"] * 10_000) + " as x")
threading.stack_size(256 * 1024)
t = threading.Thread(target=repr, args=(globals()["x"],))
t.start(); t.join()

Segmentation fault (core dumped)

Resolving via a function call:

import threading
exec("lazy import " + ".".join(["m"] * 10_000) + " as x")
def f():
    x
threading.stack_size(256 * 1024)
t = threading.Thread(target=f)
t.start(); t.join()

Segmentation fault (core dumped)

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

No response

Linked PRs

• gh-159288: Avoid segfault by adding missing recursion checks when walking lazy import chains #159289