Created commit_on_success_with_read_committed decorator.

This decorator executes the decorated function inside a
transaction with isolation level set to READ COMMITTED.

TNL-367

common/djangoapps/util/db.py

+"""
+Utility functions related to databases.
+"""
+from functools import wraps
+
+from django.db import connection, transaction
+
+
+def commit_on_success_with_read_committed(func):  # pylint: disable=invalid-name
+    """
+    Decorator which executes the decorated function inside a transaction with isolation level set to READ COMMITTED.
+
+    If the function returns a response the transaction is committed and if the function raises an exception the
+    transaction is rolled back.
+
+    Note: This only works on MySQL.
+    """
+    @wraps(func)
+    def wrapper(*args, **kwargs):  # pylint: disable=missing-docstring
+
+        if connection.vendor == 'mysql':
+            # The isolation level cannot be changed while a transaction is in progress. So we close any existing one.
+            if connection.transaction_state:
+                connection.commit()
+
+            # This will set the transaction isolation level to READ COMMITTED for the next transaction.
+            cursor = connection.cursor()
+            cursor.execute("SET TRANSACTION ISOLATION LEVEL READ COMMITTED")
+
+        with transaction.commit_on_success():
+            return func(*args, **kwargs)
+
+    return wrapper