Make py_zstd_compress_mt2 thread-safe

[KowalskiThomas]

What is this PR?

This PR is a fix for a crash we witnessed in a service of ours. py_zstd_compress_mt2 used a single global compression context (at the module level), m_cctx, which is thus shared across all calls:

python-zstd/src/python-zstd.h

Line 130 in fec54dd

static ZSTD_CCtx* m_cctx;

python-zstd/src/python-zstd.c

Lines 279 to 281 in fec54dd

	return NULL;
	}
	ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, level);

Since ZSTD_CCtx is not thread-safe, and because compression releases the GIL, two Python threads calling these functions at the same time use the same context concurrently. This can corrupt the state and lead to out-of-bounds access inside zstd.

The crash was detected in this form:

ZSTD_compressBlock_doubleFast_noDict_generic   zstd/lib/compress/zstd_double_fast.c:185
ZSTD_compressBlock_doubleFast_noDict_4         zstd/lib/compress/zstd_double_fast.c:557
ZSTD_compressBlock_doubleFast                  zstd/lib/compress/zstd_double_fast.c:577
ZSTD_buildSeqStore                             zstd/lib/compress/zstd_compress.c:3420
ZSTD_compressBlock_internal                    zstd/lib/compress/zstd_compress.c:4399
ZSTD_compress_frameChunk                       zstd/lib/compress/zstd_compress.c:4646
ZSTD_compressContinue_internal                 zstd/lib/compress/zstd_compress.c:4834
ZSTD_compressEnd_public                        zstd/lib/compress/zstd_compress.c:5412
ZSTD_compressStream_generic                    zstd/lib/compress/zstd_compress.c:6151
ZSTD_compressStream2                           zstd/lib/compress/zstd_compress.c:6540
ZSTD_compressStream2_simpleArgs                zstd/lib/compress/zstd_compress.c:6561
ZSTD_compress2                                 zstd/lib/compress/zstd_compress.c:6581
py_zstd_compress_mt2                           src/python-zstd.c:276
_PyObject_MakeTpCall
_PyEval_EvalFrameDefault
_PyObject_MakeTpCall
_PyEval_EvalFrameDefault

Looking further into it, we found another race as well. When called with different level values, reset_cContext frees and recreates m_cctx. If one thread does this while another is still inside the GIL-released ZSTD_compress2, the latter compresses into a freed context.

Proposed fix

The fix proposed in this PR is to make py_zstd_compress_mt2 create and free its own ZSTD_CCtx on each call, instead of sharing the module-global one. This is the same pattern already used by py_zstd_compress_mt:

python-zstd/src/python-zstd.c

Lines 141 to 149 in fec54dd

	cctx = ZSTD_createCCtx();
	ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, level);
	ZSTD_CCtx_setParameter(cctx, ZSTD_c_nbWorkers, threads);
	Py_BEGIN_ALLOW_THREADS
	cSize = ZSTD_compress2(cctx, dest, (size_t)dest_size, source, (size_t)source_size);
	Py_END_ALLOW_THREADS
	ZSTD_freeCCtx(cctx);