Close canonical cleanup race windows

src/docforge/_fs_safety.py

@@ -14,6 +14,7 @@ from typing import Protocol, cast
 
 from .errors import DocForgeError
 
+RENAME_NOREPLACE = 1
 RENAME_EXCHANGE = 2
 
 
@@ -32,9 +33,13 @@ class _RenameAt2(Protocol):
     ) -> int: ...
 
 
-def rename_exchange_at(directory_fd: int, first: str, second: str) -> None:
-    """Atomically exchange two names inside one already bound directory."""
-
+def _rename_at2(
+    old_directory_fd: int,
+    old_name: str,
+    new_directory_fd: int,
+    new_name: str,
+    flags: int,
+) -> int:
     library = ctypes.CDLL(None, use_errno=True)
     try:
         rename_at2 = cast(_RenameAt2, library.renameat2)
@@ -53,20 +58,40 @@ def rename_exchange_at(directory_fd: int, first: str, second: str) -> None:
     rename_at2.restype = ctypes.c_int
     ctypes.set_errno(0)
     result = rename_at2(
-        directory_fd,
-        os.fsencode(first),
-        directory_fd,
-        os.fsencode(second),
-        RENAME_EXCHANGE,
+        old_directory_fd,
+        os.fsencode(old_name),
+        new_directory_fd,
+        os.fsencode(new_name),
+        flags,
     )
     if result == 0:
-        return
+        return 0
     error_number = ctypes.get_errno()
     if error_number in {errno.ENOSYS, errno.EINVAL, errno.EOPNOTSUPP}:
         raise DocForgeError(
             "atomic_exchange_unavailable",
             "Atomic exchange is unavailable on this filesystem",
         )
+    return error_number
+
+
+def rename_exchange_between_at(
+    first_directory_fd: int,
+    first: str,
+    second_directory_fd: int,
+    second: str,
+) -> None:
+    """Atomically exchange names between two bound directories on one filesystem."""
+
+    error_number = _rename_at2(
+        first_directory_fd,
+        first,
+        second_directory_fd,
+        second,
+        RENAME_EXCHANGE,
+    )
+    if error_number == 0:
+        return
     raise DocForgeError(
         "publication_failure",
         "Could not exchange atomic publication paths",
@@ -74,6 +99,38 @@ def rename_exchange_at(directory_fd: int, first: str, second: str) -> None:
     ) from OSError(error_number, os.strerror(error_number))
 
 
+def rename_exchange_at(directory_fd: int, first: str, second: str) -> None:
+    """Atomically exchange two names inside one already bound directory."""
+
+    rename_exchange_between_at(directory_fd, first, directory_fd, second)
+
+
+def rename_noreplace_between_at(
+    source_directory_fd: int,
+    source: str,
+    target_directory_fd: int,
+    target: str,
+) -> bool:
+    """Atomically move one name without replacing a target that appeared."""
+
+    error_number = _rename_at2(
+        source_directory_fd,
+        source,
+        target_directory_fd,
+        target,
+        RENAME_NOREPLACE,
+    )
+    if error_number == 0:
+        return True
+    if error_number == errno.EEXIST:
+        return False
+    raise DocForgeError(
+        "publication_failure",
+        "Could not move an atomic publication path without replacement",
+        error_number=error_number,
+    ) from OSError(error_number, os.strerror(error_number))
+
+
 def open_bound_directory(path: Path) -> int:
     """Open one real directory and bind its current inode for later operations."""