r_string(), r_byte() and PyMarshal_ReadLastObjectFromFile() did not check ferror() nor call PyErr_CheckSignals(), so a genuine I/O error or a read interrupted by Ctrl-C was reported as EOFError, as if the file had simply ended.

>>> import _testcapi, os
>>> os.mkdir('d')
>>> _testcapi.pymarshal_read_object_from_file('d')
EOFError: EOF read where object expected      # before
IsADirectoryError: [Errno 21] Is a directory  # after

The writing side had the same gap: w_flush() and w_string() discarded the result of fwrite(), so a failed or interrupted write to a FILE* was ignored. PyMarshal_WriteObjectToFile() also never turned the recorded WFERR_* code into an exception, so even an unmarshallable value was ignored.

>>> import _testcapi, marshal
>>> _testcapi.pymarshal_write_object_to_file(b'x' * 100000, '/dev/full', marshal.version)
                                             # before: no error
OSError: [Errno 28] No space left on device  # after
>>> _testcapi.pymarshal_write_object_to_file(object(), 'f', marshal.version)
                                             # before: no error
ValueError: unmarshallable object            # after

The WFERR_* to exception mapping is factored out of PyMarshal_WriteObjectToString() into w_set_error(), and all three writers now use it. This removes the note above PyMarshal_WriteLongToFile() saying that it never sets an exception and that doing so "should be regarded as an API-breaking change": detecting write errors makes it set one, which is what its documentation has promised since gh-105184.

The documentation of these functions listed only EOFError, ValueError and TypeError, which was inaccurate even before this change (MemoryError is raised when the read buffer cannot be allocated), so the per-function lists are replaced by one list for the whole section.

• Issue: marshal reports EOFError for I/O errors when reading from a FILE* #155907