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Add native libFuzzer fuzz harness for Python API, addresses #283

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Pivots away from atheris (which lacks Python 3.14 support) to a single
libFuzzer-linked executable that embeds CPython, registers mcrfpy, and
dispatches each iteration to a Python fuzz_one_input(data: bytes) function
loaded from tests/fuzz/fuzz_<target>.py by MCRF_FUZZ_TARGET env var.

libFuzzer instruments the C++ engine code where all #258-#278 bugs live;
Python drives the fuzzing logic via an in-house ByteStream replacement
for atheris.FuzzedDataProvider. Python-level exceptions are caught; only
ASan/UBSan signal real bugs.

CMake
- MCRF_FUZZER=ON builds mcrfpy_fuzz from all src/*.cpp except main.cpp
  plus tests/fuzz/fuzz_common.cpp, linked with -fsanitize=fuzzer,address,
  undefined. Asset+lib post-build copy added so the embedded interpreter
  finds its stdlib and default_font/default_texture load.

Makefile
- fuzz-build builds only mcrfpy_fuzz (fast iterate)
- fuzz loops over six targets setting MCRF_FUZZ_TARGET for each
- fuzz-long TARGET=x SECONDS=n for deep manual runs
- fuzz-repro TARGET=x CRASH=path for crash reproduction
- Shared ASAN_OPTIONS / PYTHONHOME env via FUZZ_ENV define

tests/fuzz
- fuzz_common.cpp: LLVMFuzzerInitialize bootstraps Python, imports target,
  resolves fuzz_one_input. LLVMFuzzerTestOneInput wraps bytes as PyBytes,
  calls target, swallows Python errors.
- fuzz_common.py: ByteStream byte consumer + safe_reset() + EXPECTED_EXCEPTIONS
- Six target stubs (grid_entity, property_types, anim_timer_scene,
  maps_procgen, fov, pathfinding_behavior) to be fleshed out in follow-up
- README with build/run/triage instructions

Verified end-to-end: make fuzz-build produces build-fuzz/mcrfpy_fuzz,
make fuzz FUZZ_SECONDS=3 ran all six targets (~2400-9800 exec/s each,
667-1883 coverage edges), make fuzz-repro loaded and replayed a corpus
input cleanly. No crashes from the stubs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
This commit is contained in:
John McCardle 2026-04-10 11:05:04 -04:00
commit 90a2945a9f
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