gh-85283: Add PyMem_RawMalloc() to the limited C API

[vstinner]

Add PyMem_RawMalloc(), PyMem_RawCalloc(), PyMem_RawRealloc() and PyMem_RawFree() to the limited C API.

These functions were added by Python 3.4 and are needed to port stdlib extensions to the limited C API, like grp and pwd.

• Issue: [C API] Convert a few stdlib extensions to the limited C API (PEP 384) #85283

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📚 Documentation preview 📚: https://cpython-previews--108570.org.readthedocs.build/

[vstinner]

See also What To Do About Custom Allocators and the GIL? discussion. I don't think that PyMem_RawMalloc() is causing any trouble here.

cc @ericsnowcurrently

[vstinner]

These functions (...) are needed to port stdlib extensions to the limited C API, like grp and pwd.

The alternative is to no add these functions to limited C API, and convert grp and pwd to libc malloc() and free(). The drawback is that tracemalloc will no longer be able to trace their memory allocation.

[encukou]

If nogil/mimalloc is merged, will there be any reason to use these instead of PyMem_Malloc/PyMem_Free?

[vstinner]

If nogil/mimalloc is merged, will there be any reason to use these instead of PyMem_Malloc/PyMem_Free?

I understood that with the nogil implementation:

• PyMem_RawMalloc() will still call malloc() as before, there is no change.
• PyMem_Malloc() will use minalloc, instead of pymalloc.
• PyMem_Malloc() no longer requires the caller to hold the GIL, since there is no GIL anymore in the nogil build.

For me, PyMem_RawMalloc() will still be different than PyMem_RawMalloc().

On Python 3.12 and older, PyMem_RawMalloc() must be used if the GIL is not held. Such case was not compatible with the limited C API, since PyMem_RawMalloc() is not part of Python 3.12 limited C API. Having PyMem_RawMalloc() in the limited C API may help compatibility with older Python versions (which use the regular API, not the limited API).

cc @colesbury

[vstinner]

Code search using regex PyMem_Raw on PyPI top 5,000 projects (at 2023-07-04).

The following 34 projects use PyMem_Raw*() functions:

• Cython (0.29.36)
• Pillow (10.0.0)
• biopython (1.81)
• bitstruct (8.17.0)
• catboost (1.2)
• cx_Freeze (6.15.2)
• ddtrace (1.15.1)
• dlib (19.24.2)
• fastobo (0.12.2)
• gnureadline (8.1.2)
• mariadb (1.1.6)
• memray (1.8.1)
• mpi4py (3.1.4)
• mujoco (2.3.6)
• numba (0.57.1)
• numpy (1.25.0)
• onnx (1.14.0)
• onnxoptimizer (0.3.13)
• onnxsim (0.4.33)
• orjson (3.9.1)
• osmium (3.6.0)
• osqp (0.6.3)
• praat-parselmouth (0.4.3)
• pyarmor (8.2.9)
• pybind11 (2.10.4)
• pygame (2.5.0)
• pyinstaller (5.13.0)
• pyjson5 (1.6.3)
• pylibmc (1.6.3)
• pyreadline (2.1)
• pyreadline3 (3.4.1)
• scs (3.2.3)
• typed_ast (1.5.5)
• uvloop (0.17.0)

[colesbury]

What @vstinner wrote about nogil is accurate, except that PyMem_Malloc still requires holding the GIL (or equivalent in --disable-gil builds) due to the way GC heap traversal is implemented. It's possible that could be relaxed in the future.

[vstinner]

What @vstinner wrote about nogil is accurate, except that PyMem_Malloc still requires holding the GIL (or equivalent in --disable-gil builds) due to the way GC heap traversal is implemented. It's possible that could be relaxed in the future.

The important information is that even with nogil, PyMem_RawMalloc() still fit an use case which is not fitted by PyMem_Malloc(). So it's still relevant to use PyMem_RawMalloc() in "nogil Python".

[ericsnowcurrently]

This shouldn't affect per-interpreter GIL since the only specific concern there is thread-safety.

Note that the docs now (3.12+) say that all 3 domains must be thread-safe. (This was discussed in a DPO thread and changed for gh-105766.) So does the rationale here apply to the other two memory domains (PyMem_Malloc(), `PyObject_Malloc(), etc.)?

As to my opinion about this change, I think we should be a bit more cautious about expanding the limited API until we have more clarity on where we're going with the C-API (hopefully at the core sprint). I'm particularly reluctant to add PyMem_Raw*() if it's just for the sake of converting a stdlib extension module to the limited API.

[ericsnowcurrently]

Note that the docs now (3.12+) say that all 3 domains must be thread-safe. (This was discussed in a DPO thread and changed for gh-105766.)

Just to clarify, the GIL must still be held when using the "mem" and "object" domains (PyMem_Malloc(), PyObject_Malloc(), etc.), since each GIL still protects that interpreter's pymalloc state. (Obviously that would change under no-gil.)

[vstinner]

As to my opinion about this change, I think we should be a bit more cautious about expanding the limited API until we have more clarity on where we're going with the C-API (hopefully at the core sprint). I'm particularly reluctant to add PyMem_Raw*() if it's just for the sake of converting a stdlib extension module to the limited API.

My long term goal doesn't concern the Python stdlib extensions, but just converting all C extensions towards the limited C API.

Cython uses PyMem_RawMalloc() and has a build mode which targets the limited C API: define the CYTHON_LIMITED_API macro in your Cython code.

A quick search on PyPI top 5,000 projects tells me that 113 projects use PyMem_RawMalloc(). On these 113 projects, 79 use PyMem_Malloc() in code generated by Cython. I used the command:

$ ./search_pypi_top.py PYPI-2023-07-04/ '(PyMem_RawMalloc|PyMem_RawCalloc|PyMem_RawRealloc|PyMem_RawFree)' --cython -o raw_malloc -q

Affected projects (113):

• ConfigSpace (0.7.1)
• Cython (0.29.36)
• DoubleMetaphone (1.1)
• Pillow (10.0.0)
• PyWavelets (1.4.1)
• Theano-PyMC (1.1.2)
• aesara (2.9.1)
• affinegap (1.12)
• aiohttp (3.8.4)
• aiokafka (0.8.1)
• arch (6.1.0)
• asyncpg (0.27.0)
• av (10.0.0)
• bezier (2021.2.12)
• biopython (1.81)
• bitstruct (8.17.0)
• catboost (1.2)
• causalml (0.13.0)
• cchardet (2.1.7)
• cityhash (0.4.7)
• clickhouse-driver (0.2.6)
• cx_Freeze (6.15.2)
• cyksuid (2.0.2)
• cytoolz (0.12.1)
• ddtrace (1.15.1)
• debugpy (1.6.7)
• dependency-injector (4.41.0)
• diffq (0.2.4)
• dlib (19.24.2)
• dtaidistance (2.3.10)
• econml (0.14.1)
• editdistance (0.6.2)
• fastavro (1.7.4)
• fastdtw (0.3.4)
• fastobo (0.12.2)
• fastparquet (2023.7.0)
• frozenlist (1.3.3)
• fuzzysearch (0.7.3)
• gensim (4.3.1)
• gluonnlp (0.10.0)
• gnureadline (8.1.2)
• grapheme (0.6.0)
• grpcio (1.56.0)
• grpcio-tools (1.56.0)
• hidapi (0.14.0)
• httptools (0.5.0)
• imagecodecs (2023.3.16)
• insightface (0.7.3)
• jq (1.4.1)
• lightfm (1.17)
• lupa (2.0)
• lxml (4.9.2)
• mariadb (1.1.6)
• marisa-trie (0.8.0)
• memray (1.8.1)
• mojimoji (0.0.12)
• mpi4py (3.1.4)
• msgpack (1.0.5)
• mujoco (2.3.6)
• numba (0.57.1)
• numcodecs (0.11.0)
• numpy (1.25.0)
• onnx (1.14.0)
• onnxoptimizer (0.3.13)
• onnxsim (0.4.33)
• orjson (3.9.1)
• osmium (3.6.0)
• osqp (0.6.3)
• peewee (3.16.2)
• plyvel (1.5.0)
• praat-parselmouth (0.4.3)
• ptvsd (4.3.2)
• pyarmor (8.2.9)
• pybedtools (0.9.0)
• pybind11 (2.10.4)
• pycapnp (1.3.0)
• pycld3 (0.22)
• pyclipper-1.3.0.post4
• pydevd (2.9.6)
• pygame (2.5.0)
• pyinstaller (5.13.0)
• pyjson5 (1.6.3)
• pylibmc (1.6.3)
• pymoo (0.6.0.1)
• pypcap (1.3.0)
• pyreadline (2.1)
• pyreadline3 (3.4.1)
• pyreadstat (1.2.2)
• pysam (0.21.0)
• pysimdjson (5.0.2)
• python-crfsuite (0.9.9)
• pyworld (0.3.3)
• pyzmq (25.1.0)
• quadprog (0.1.11)
• recordclass (0.19)
• ruamel.yaml.clib (0.2.7)
• ruptures (1.1.8)
• sasl (0.3.1)
• scikit-surprise (1.1.3)
• scs (3.2.3)
• spacy (3.5.4)
• srsly (2.4.6)
• ssh-python (1.0.0)
• ssh2-python (1.0.0)
• statsmodels (0.14.0)
• thriftpy2 (0.4.16)
• tslearn (0.6.0)
• typed_ast (1.5.5)
• uamqp (1.6.4)
• uvloop (0.17.0)
• versioneer (0.28)
• wordcloud (1.9.2)
• yarl (1.9.2)

[vstinner]

@encukou@colesbury@ericsnowcurrently: I'm sorry, I didn't get your opinion about adding these 4 functions to the limited C API. Do you approve this change? Or are you against it?

[ericsnowcurrently]

For just PyMem_RawMalloc(), I don't see a problem, but I'm not particularly invested in the decision. 😄

(In general, however, I think we should slow down on moving things to the limited API until after the discussions at the core sprint next month. I'm not saying that should block this PR though.)

[colesbury]

Seems fine to me too. (To be clear, I don't think this affects PEP 703 one way or the other.)

[vstinner]

I rebased my PR on the main branch.

[vstinner]

I merged my PR, thanks for reviews. It should help many projects to use the limited C API.