gh-155742: Use PyBytesWriter in marshal

[vstinner]

Replace soft deprecated PyBytes_FromStringAndSize() and _PyBytes_Resize() with PyBytesWriter.

• Issue: Treat bytes objects as immutable in C code #155742

[serhiy-storchaka]

I do not think the current code is broken.

[vstinner]

I do not think the current code is broken.

I didn't say that the current code is broken. The PR only just avoids the soft deprecated PyBytes_FromStringAndSize() function.

I reworked the error handling.

@serhiy-storchaka: Please review the updated PR.

[vstinner]

I wrote a script to test manually this PR by injecting MemoryError at different places:

import marshal
import io
import _testcapi
obj = b'x' * (1024 * 1024)
file = io.BytesIO()
for i in range(10):
    try:
        try:
            _testcapi.set_nomemory(i)
            res = marshal.dump(obj, file)
        finally:
            _testcapi.remove_mem_hooks()
    except Exception as exc:
        print(f"marshal.dump failed: {exc!r}")
    else:
        print(f"{res=}")

Before, the code failed with an assertion error. With my latest change, the code works is all cases (always raise MemoryError as expected).

[vstinner]

Ah, I noticed that the PyMarshal C API is not tested by test_capi currently. So I wrote PR gh-156890 to add tests.

[vstinner]

I extracted the TYPE_STRING change: it does in fact fix an issue, using PyBytes_FromStringAndSize(str, n) allows getting 1-byte singletons: PR gh-157398.

[serhiy-storchaka]

I do not think we need this change. It looks to me like a code churn which makes the code more complicated.