enable_wal() and disable_wal() refuse to run inside a transaction

Changing the journal mode assigns conn.isolation_level, which
commits any open transaction as a side effect - silently breaking
the rollback guarantee of atomic() blocks and of user-managed
transactions. Both methods now raise RuntimeError if a transaction
is open. Calling them when the database is already in the requested
mode remains a no-op.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UnLnhsH25Nnv7LHhekUfPd

tests/test_wal.py

@@ -21,3 +21,39 @@ def test_enable_disable_wal(db_path_tmpdir):
     db.disable_wal()
     assert "delete" == db.journal_mode
     assert "test.db-wal" not in [f.basename for f in tmpdir.listdir()]
+
+
+def test_enable_wal_inside_transaction_raises(db_path_tmpdir):
+    db, path, tmpdir = db_path_tmpdir
+    db["test"].insert({"id": 1}, pk="id")
+    with pytest.raises(RuntimeError):
+        with db.atomic():
+            db["test"].insert({"id": 2}, pk="id")
+            db.enable_wal()
+    # The atomic() block must have rolled back cleanly and the
+    # journal mode must be unchanged
+    assert db.journal_mode == "delete"
+    assert [r["id"] for r in db["test"].rows] == [1]
+
+
+def test_disable_wal_inside_transaction_raises(db_path_tmpdir):
+    db, path, tmpdir = db_path_tmpdir
+    db.enable_wal()
+    db["test"].insert({"id": 1}, pk="id")
+    with pytest.raises(RuntimeError):
+        with db.atomic():
+            db["test"].insert({"id": 2}, pk="id")
+            db.disable_wal()
+    assert db.journal_mode == "wal"
+    assert [r["id"] for r in db["test"].rows] == [1]
+
+
+def test_enable_wal_noop_inside_transaction_is_allowed(db_path_tmpdir):
+    # Calling enable_wal() when WAL is already enabled is a no-op,
+    # so it is fine inside a transaction
+    db, path, tmpdir = db_path_tmpdir
+    db.enable_wal()
+    with db.atomic():
+        db["test"].insert({"id": 1}, pk="id")
+        db.enable_wal()
+    assert [r["id"] for r in db["test"].rows] == [1]