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

docs/changelog.rst

@@ -16,6 +16,7 @@ Unreleased
 - ``table.upsert()`` and ``table.upsert_all()`` now detect the primary key or compound primary key of an existing table, so the ``pk=`` argument is no longer required when upserting into a table that already has a primary key.
 - ``table.upsert()`` and ``table.upsert_all()`` now raise ``PrimaryKeyRequired`` if a record is missing a value for any primary key column, or has a value of ``None`` for one. Previously such records - which can never match an existing row - were quietly inserted as brand new rows, or triggered a confusing ``KeyError`` after the insert had already taken place.
 - ``db.table(table_name).insert({})`` can now be used to insert a row consisting entirely of default values into an existing table, using ``INSERT INTO ... DEFAULT VALUES``. (:issue:`759`)
+- ``db.enable_wal()`` and ``db.disable_wal()`` now raise a ``RuntimeError`` if called while a transaction is open. Previously they would silently commit the open transaction as a side effect of changing the journal mode, breaking the rollback guarantee of ``db.atomic()`` and of user-managed transactions.
 
 .. _v4_0rc1:
 

docs/python-api.rst

@@ -2702,6 +2702,8 @@ You can disable WAL mode using ``.disable_wal()``:
 
     Database("my_database.db").disable_wal()
 
+The journal mode can only be changed outside of a transaction. Calling either method while a transaction is open - inside a ``db.atomic()`` block, for example - raises a ``RuntimeError``, unless the database is already in the requested mode in which case the call is a no-op.
+
 You can check the current journal mode for a database using the ``journal_mode`` property:
 
 .. code-block:: python