`ZstdCompressionWriter()` constructs successfully but leads to `segmentation fault`

I've been fuzzing Python C extension modules for a small research project.
I found a sanitizer issue which is a null-pointer dereference in ZstdCompressionWriter_memory_size.

I reproduced it with the binary wheel from a plain pip install zstandard.
The process terminates with SIGSEGV there as well.

I'm not sure whether zero-argument construction of this type is considered supported (calling ZstdCompressionWriter() with no arguments),
but since it currently terminates the interpreter rather than raising a Python exception, I thought it was worth reporting.

Versions

zstandard 0.25.0, cext backend, CPython 3.12, Linux x86_64.

Reproducer

import zstandard

w = zstandard.ZstdCompressionWriter()   # succeeds
w.memory_size()                         # SIGSEGV

Five calls across the two writer types behave the same way, each from a fresh direct construction:

Sanitizer build

Same call, zstandard built with -fsanitize=address,undefined:

c-ext/compressionwriter.c:64:65: runtime error:
    member access within null pointer of type 'ZstdCompressor'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
    c-ext/compressionwriter.c:64:65

AddressSanitizer: SEGV on unknown address 0x000000000020
    #0 ZstdCompressionWriter_memory_size  c-ext/compressionwriter.c:64:65

ZstdDecompressionWriter.memory_size() reports the equivalent at
c-ext/decompressionwriter.c:50:67.

Expected behavior

I think that direct construction should be rejected with a Python exception, as it is by the CFFI backend.

Under PYTHON_ZSTANDARD_IMPORT_POLICY=cffi the same construction is refused outright:

TypeError: ZstdCompressionWriter.__init__() missing 5 required positional
arguments: 'compressor', 'writer', 'source_size', 'write_size',
and 'write_return_read'

Actual behavior

The C extension permits construction with no arguments, leaving internal fields NULL.
Several methods dereference those fields and terminate the process with SIGSEGV.

Although these objects are normally obtained through stream_writer(), I wonder whether the C extension should reject zero-argument construction, as the CFFI backend already does, rather than produce an uninitialized object.

Trước tiên, hãy chạy Python reproducer, sau đó kiểm tra c-ext/compressionwriter.c:64 và c-ext/decompressionwriter.c:50 cùng với các đường dẫn khởi tạo writer. So sánh hành vi của C extension với CFFI backend đối với việc khởi tạo không có đối số. Hoàn thành khi việc khởi tạo trực tiếp không còn để lại các trường nội bộ NULL có thể truy cập và tạo ra một Python exception thay vì gây ra segfault.