Write --write-changes via a tempfile so interrupts cannot empty files

codespell_lib/_codespell.py

@@ -24,7 +24,9 @@
 import os
 import re
 import shlex
+import stat
 import sys
+import tempfile
 import textwrap
 from collections.abc import Iterable, Sequence
 from re import Match, Pattern
@@ -1309,13 +1311,40 @@ def parse_file(
                         f"  {cfilename}:{cline}: {cwrongword} ==> {crightword}",
                         file=sys.stderr,
                     )
-            with open(filename, "w", encoding=encoding, newline="") as f:
-                for _, _, lines in fragments:
-                    f.writelines(lines)
+            _write_file_atomically(filename, encoding, fragments)
 
     return bad_count
 
 
+def _write_file_atomically(
+    filename: str,
+    encoding: str,
+    fragments: Iterable[tuple[Any, Any, list[str]]],
+) -> None:
+    """Replace *filename* with a same-directory tempfile so a failed write
+    cannot leave the original file truncated to zero bytes.
+    """
+    directory = os.path.dirname(os.path.abspath(filename))
+    try:
+        original_mode = stat.S_IMODE(os.stat(filename).st_mode)
+    except OSError:
+        original_mode = None
+    fd, tmp_path = tempfile.mkstemp(prefix=".codespell-", suffix=".tmp", dir=directory)
+    try:
+        with os.fdopen(fd, "w", encoding=encoding, newline="") as tmp:
+            for _, _, lines in fragments:
+                tmp.writelines(lines)
+        if original_mode is not None:
+            os.chmod(tmp_path, original_mode)
+        os.replace(tmp_path, filename)
+    except Exception:
+        try:
+            os.unlink(tmp_path)
+        except OSError:
+            pass
+        raise
+
+
 def flatten_clean_comma_separated_arguments(
     arguments: Iterable[str],
 ) -> list[str]:

codespell_lib/tests/test_basic.py

@@ -3,6 +3,7 @@
 import os
 import os.path as op
 import re
+import stat
 import subprocess
 import sys
 from collections.abc import Generator
@@ -175,6 +176,40 @@ def test_basic(
     assert cs.main(tmp_path) == 0
 
 
+def test_write_changes_keeps_original_if_replace_fails(
+    tmp_path: Path,
+    capsys: pytest.CaptureFixture[str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """An interrupted --write-changes must not empty the original file."""
+    fname = tmp_path / "a.txt"
+    original = "this file has an abandonned word\n"
+    fname.write_text(original)
+
+    def fail_replace(src: str, dst: str) -> None:
+        raise InterruptedError
+
+    monkeypatch.setattr(os, "replace", fail_replace)
+    with pytest.raises(InterruptedError):
+        cs.main("-q", "16", "-w", fname)
+    assert fname.read_text() == original
+    assert list(tmp_path.glob(".codespell-*.tmp")) == []
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="POSIX permission bits")
+def test_write_changes_preserves_file_mode(
+    tmp_path: Path,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """Atomic --write-changes must not reset the original file mode."""
+    fname = tmp_path / "a.txt"
+    fname.write_text("this file has an abandonned word\n")
+    fname.chmod(0o640)
+    assert cs.main("-q", "16", "-w", fname) == 0
+    assert fname.read_text() == "this file has an abandoned word\n"
+    assert stat.S_IMODE(fname.stat().st_mode) == 0o640
+
+
 def test_write_changes_lists_changes(
     tmp_path: Path,
     capsys: pytest.CaptureFixture[str],