Unbounded C recursion in `_elementtree.Element.__deepcopy__` causes causes stack overflow

[StanFromIreland]

Crash report

What happened?

Many thanks to Gil Portnoy (@dhkts1) for finding this.

---

import xml.etree.ElementTree as ET
import sys
import copy

root = ET.Element("r")
cur = root
for i in range(500_000):
    c = ET.Element("n")
    cur.append(c)
    cur = c

copy.deepcopy(root)  # SIGSEGV

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

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

No response

Linked PRs

• gh-148801: Fix unbound C recursion in Element.__deepcopy__() #148802