gh-155295: Avoid crash de-instrumenting malformed code objects

[taegyunkim]

Prevent _PyCode_GetCode() from dereferencing missing monitoring data when a malformed code object contains INSTRUMENTED_LINE or INSTRUMENTED_INSTRUCTION.

Origin and AI assistance

This failure was initially identified while reviewing production crash logs available to Datadog. The investigation, reproducer, implementation, tests, and this PR write-up were produced primarily with GPT-5.6 Sol (high), with human direction, review, and local validation.

deopt_code() now validates the opcode-specific monitoring array before calling _Py_GetBaseCodeUnit(). It raises SystemError and leaves the bytecode cache empty when the code object's instrumentation state is inconsistent.

PyCode_GetCode() documents that it returns NULL with an exception set on error. Because the new consistency check adds a non-allocation failure path, the marshal writer now preserves an exception already set by _PyCode_GetCode() rather than replacing it with MemoryError.

The regression test uses CodeType.replace() to create the malformed state in a subprocess. Without the original fix, both instrumented opcodes terminate the subprocess with SIGSEGV. The test covers direct code.co_code access and marshal.dumps(code).

Validation:

• ./python -m test test_code -m test_co_code_with_invalid_monitoring_data --verbose
• ./python -m test -j3 test_code test_marshal test_dis
• ./python -m test -R 3:3 test_code -m test_co_code_with_invalid_monitoring_data
• PATH=/usr/bin:/bin:/usr/sbin:/sbin ./python -m test -j12: 50,379 tests executed, 470 test files passed, 3,449 tests skipped, no failures
• make patchcheck

• Issue: Code object de-instrumentation can dereference missing monitoring data #155295

[chatgpt-codex-connector]

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[taegyunkim]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

Reviewed commit: 6c92458775

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[taegyunkim]

https://www.githubstatus.com/incidents/qcvjkzcs7j74 incident going on with GH actions :(
GH actions are not running

[picnixz]

We don't want to avoid those crashes. We don't and won't protect against wrong instrumented code.

[taegyunkim]

Thanks for quick response!