ENH Better error detection for permission problems

Previously, if a lock could not be created due to missing permissions
(or other issues), jug would treat this identically to failing to lock
because a lock already existed.

Now, it specifically looks for FileExistsError (introduced in Python
3.3) to detect a locked version

ChangeLog

@@ -1,6 +1,7 @@
 Unreleased
 	* Tasklet: Support for lambda functions in Tasklets
 	* pack: bugfix when using local imports
+	* jug: better error detection for permission problems
 
 Version 2.3.1 Sun 5 November 2023 by luispedro
 	* jug: update for Python 3.12

jug/backends/file_store.py

@@ -485,7 +485,7 @@ def get(self):
             F.write('Lock created on {0}\n'.format(datetime.now().strftime('%Y-%m-%d (%Hh%M.%S)')))
             F.close()
             return True
-        except OSError:
+        except FileExistsError:
             return False
 
     def release(self):

jug/tests/test_file_store.py

@@ -2,6 +2,8 @@
 from jug.backends.file_store import file_store
 from jug.hash import hash_one
 
+import pytest
+import os
 from io import BytesIO
 
 def test_encode_decode_empty_string():
@@ -84,3 +86,16 @@ def test_pack_cleanup(tmpdir):
 
     fs = file_store(tmpdir)
     assert len(list(fs.list())) == 2
+
+
+def test_lock_permissions(tmpdir):
+    tmpdir = str(tmpdir)
+    os.makedirs(tmpdir, exist_ok=True)
+    fs = file_store(tmpdir)
+    lock = fs.getlock(hash_one('k0'))
+    assert lock.get()
+    lock.release()
+    os.chmod(tmpdir + '/locks', 0o444)
+    with pytest.raises(PermissionError):
+        lock.get()
+