core/sqlite: extract immutable connection helper

use in bluemaestro/zotero modules

my/core/dataset.py

@@ -1,12 +1,14 @@
-from pathlib import Path
+from .common import assert_subpackage; assert_subpackage(__name__)
+
+from .common import PathIsh
+from .sqlite import sqlite_connect_immutable
+
 
 # TODO wonder if also need to open without WAL.. test this on read-only directory/db file
-def connect_readonly(db: Path):
+def connect_readonly(db: PathIsh):
     import dataset # type: ignore
     # see https://github.com/pudo/dataset/issues/136#issuecomment-128693122
     # todo not sure if mode=ro has any benefit, but it doesn't work on read-only filesystems
     # maybe it should autodetect readonly filesystems and apply this? not sure
-    import sqlite3
-    # https://www.sqlite.org/draft/uri.html#uriimmutable
-    creator = lambda: sqlite3.connect(f'file:{db}?immutable=1', uri=True)
+    creator = lambda: sqlite_connect_immutable(db)
     return dataset.connect('sqlite:///', engine_kwargs={'creator': creator})

my/core/sqlite.py

@@ -0,0 +1,25 @@
+from .common import assert_subpackage; assert_subpackage(__name__)
+
+import sqlite3
+
+from .common import PathIsh
+
+def sqlite_connect_immutable(db: PathIsh) -> sqlite3.Connection:
+    # https://www.sqlite.org/draft/uri.html#uriimmutable
+    return sqlite3.connect(f'file:{db}?immutable=1', uri=True)
+
+
+from pathlib import Path
+def test_sqlite_connect_immutable(tmp_path: Path) -> None:
+    db = str(tmp_path / 'db.sqlite')
+    with sqlite3.connect(db) as conn:
+        conn.execute('CREATE TABLE testtable (col)')
+
+    import pytest  # type: ignore
+    with pytest.raises(sqlite3.OperationalError, match='readonly database'):
+        with sqlite_connect_immutable(db) as conn:
+            conn.execute('DROP TABLE testtable')
+
+    # succeeds without immutable
+    with sqlite3.connect(db) as conn:
+        conn.execute('DROP TABLE testtable')