pathlib fast-copy can silently produce an empty destination after a zero-byte syscall #159395

Bug description

The fast-copy helpers in pathlib._os treat an initial zero-byte result from os.copy_file_range() or os.sendfile() as normal EOF, even when no data was copied.

Some filesystems can return zero for an unsuccessful fast copy. The existing shutil._fastcopy_copy_file_range() code already guards against this case by falling back when zero is returned before any data has been copied (see Lib/shutil.py and the Linux filesystem discussion).

A nonempty Path.copy() source can therefore produce an empty destination without raising an exception. This also affects Path.move() across devices: its fallback copies the source and then deletes it, so the zero-byte result could cause data loss.

Reproducer (Linux, current main)

The example uses fault injection to simulate a zero-byte syscall result. It does not establish that the issue occurs on every filesystem.

from pathlib import Path
from tempfile import TemporaryDirectory
from unittest.mock import patch
import pathlib._os

with TemporaryDirectory() as directory:
    source = Path(directory, "source")
    target = Path(directory, "target")
    source.write_bytes(b"important data")

    with (
        patch("pathlib._os._ficlone", None),
        patch("pathlib._os._fcopyfile", None),
        patch("pathlib._os.os.copy_file_range", return_value=0),
        patch("pathlib._os.os.sendfile", return_value=0),
    ):
        source.copy(target)

    print("source:", source.read_bytes())
    print("target:", target.read_bytes())
    # Before the fix, target is b''.

The proposed fix treats zero bytes copied on the first fast-copy call as a reason to try the next mechanism, eventually using buffered copying if needed. A zero-byte return after some data was copied still ends the fast-copy loop. A truly empty source remains supported.

Environment and validation

• Affects the implementation in CPython main (also relevant to versions with Path.copy()).
• The behavior was reproduced with the extracted fast-copy logic on Python 3.13 under Linux and mocked syscall results.
• The complete CPython development-branch test suite has not yet been run locally.

Linked PRs

• gh-159395: Fall back after an initial zero-byte pathlib fast copy #159396