proposed fix for issue #67

docs/changelog.rst

@@ -1,5 +1,9 @@
 Changelog
 =========
+v3.10.5 (2023-03-25)
+--------------------
+- Add explicit error check as certain UNIX filesystems do not support flock. by :user:`jahrules`.
+
 v3.10.4 (2023-03-24)
 --------------------
 - Update os.open to preserve mode= for certain edge cases. by :user:`jahrules`.

src/filelock/_unix.py

@@ -2,6 +2,7 @@
 
 import os
 import sys
+from errno import ENOSYS
 from typing import cast
 
 from ._api import BaseFileLock
@@ -39,8 +40,10 @@ def _acquire(self) -> None:
                 pass  # This locked is not owned by this UID
             try:
                 fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
-            except OSError:
+            except OSError as exception:
                 os.close(fd)
+                if exception.errno == ENOSYS:  # NotImplemented error
+                    raise NotImplementedError("FileSystem does not appear to support flock; user SoftFileLock instead")
             else:
                 self._lock_file_fd = fd
 

tests/test_filelock.py

@@ -6,11 +6,12 @@
 import sys
 import threading
 from contextlib import contextmanager
+from errno import ENOSYS
 from inspect import getframeinfo, stack
 from pathlib import Path, PurePath
 from stat import S_IWGRP, S_IWOTH, S_IWUSR, filemode
 from types import TracebackType
-from typing import Callable, Iterator, Tuple, Type, Union
+from typing import TYPE_CHECKING, Callable, Iterator, Tuple, Type, Union
 
 import pytest
 from _pytest.logging import LogCaptureFixture
@@ -24,6 +25,9 @@
     WindowsFileLock,
 )
 
+if TYPE_CHECKING:
+    from _typeshed import HasFileno
+
 
 @pytest.mark.parametrize(
     ("lock_type", "path_type"),
@@ -494,3 +498,25 @@ def test_wrong_platform(tmp_path: Path) -> None:
         lock.acquire()
     with pytest.raises(NotImplementedError):
         lock._release()
+
+
+def test_flock_not_implemented_unix(tmp_path: Path) -> None:
+    if sys.platform == "win32":
+        pytest.skip("Windows filesystems support flock")
+
+    def dummy_flock(fd: int | HasFileno, operation: int) -> None:
+        raise OSError(ENOSYS, "mock error")
+        return fd, operation  # needed for strict type checker
+
+    import fcntl
+
+    lock_path = tmp_path / "a.lock"
+    _fcntl_flock = fcntl.flock
+    try:
+        fcntl.flock = dummy_flock
+        with pytest.raises(NotImplementedError):
+            with FileLock(str(lock_path)):
+                pass
+
+    finally:
+        fcntl.flock = _fcntl_flock

whitelist.txt

@@ -3,9 +3,11 @@ autodoc
 autosectionlabel
 caplog
 eacces
+enosys
 extlinks
 filelock
 filemode
+fileno
 frameinfo
 fspath
 getframeinfo
@@ -29,5 +31,6 @@ skipif
 stacklevel
 trunc
 typehints
+typeshed
 unlck
 win32