Segfault / heap corruption when parsing bare & inside template parameter values (reproducible with PYTHONMALLOC=debug)

[hakan-persson]

Version: mwparserfromhell 0.7.2, Python 3.12, Linux x86_64

Minimal reproducer:

import mwparserfromhell
mwparserfromhell.parse("{{T|p=a & b}}")

Run with:

PYTHONMALLOC=debug python reproducer.py

Observed: Immediate segfault with guard-byte corruption (0x00 instead of 0xfd):

Debug memory block at address p=0x...: API '!'
    The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfd):
        at p-7: 0x00 *** OUCH
        ...
Fatal Python error: Segmentation fault
  File ".../mwparserfromhell/parser/__init__.py", line 84 in parse

Expected: Either a successful parse result or a Python exception — no memory corruption.

Notes:

• The crash is immediate and 100% reproducible with PYTHONMALLOC=debug, which debugpy enables automatically when launching under VS Code
• Without the debug allocator, the same corrupt write lands in unguarded memory and the process continues — no visible crash, but the heap is silently corrupted
• The trigger is a bare & (unencoded ampersand) as content inside a template parameter value; & does not trigger it