0.23.0: build breakage with zstd 1.5.7

[0-wiz-0]

When trying to compile release 0.23.0 having zstd 1.5.7 installed, I see

build/zstandard/_cffi.c: In function '_cffi_const_ZSTD_c_useBlockSplitter':
build/zstandard/_cffi.c:12168:12: error: 'ZSTD_c_useBlockSplitter' undeclared (first use in this function); did you mean 'ZSTD_c_maxBlockSize'?
12168 |   int n = (ZSTD_c_useBlockSplitter) <= 0;
      |            ^~~~~~~~~~~~~~~~~~~~~~~
      |            ZSTD_c_maxBlockSize

This parameter was removed in 1.5.7.

[hhoffstaette]

This was changed in https://github.com/facebook/zstd/pull/4180 - ZSTD_c_useBlockSplitter was an experimental parameter that never should have been used.

[jjerphan]

To understand why this fails, see: https://github.com/indygreg/python-zstandard/blob/0063333790a853360c816101511635865405834f/c-ext/backend_c.c#L137-L151.

[indygreg]

Fixed with the 0.24 release and the upgrade to 1.5.7.

This was an error because the cffi bindings parse the zstd c source code and turn all structs and defines into Python equivalent values. But we parse the bundled libffi. So the bundled zstd 1.5.6 generated a cffi reference to a symbol that wasn't present in the library being compiled against, leading to an error.

Nominally we'd have cffi parse whatever libzstd is being linked against or have an allow list of symbols to limit exposure to this failure. But that can be discussed in a separate issue.