Fixed #3711, #6734, #12581 -- Bounded connection.queries.

Prevented unlimited memory consumption when running background tasks
with DEBUG=True.

Thanks Rob, Alex, Baptiste, and others.

tests/backends/tests.py

@@ -8,11 +8,13 @@ from decimal import Decimal
 import re
 import threading
 import unittest
+import warnings
 
 from django.conf import settings
 from django.core.management.color import no_style
 from django.db import (connection, connections, DEFAULT_DB_ALIAS,
-    DatabaseError, IntegrityError, transaction)
+    DatabaseError, IntegrityError, reset_queries, transaction)
+from django.db.backends import BaseDatabaseWrapper
 from django.db.backends.signals import connection_created
 from django.db.backends.postgresql_psycopg2 import version as pg_version
 from django.db.backends.utils import format_number, CursorWrapper
@@ -630,7 +632,7 @@ class BackendTestCase(TestCase):
         # to use ProgrammingError).
         with self.assertRaises(connection.features.closed_cursor_error_class):
             # cursor should be closed, so no queries should be possible.
-            cursor.execute("select 1")
+            cursor.execute("SELECT 1" + connection.features.bare_select_suffix)
 
     @unittest.skipUnless(connection.vendor == 'postgresql',
                          "Psycopg2 specific cursor.closed attribute needed")
@@ -666,6 +668,62 @@ class BackendTestCase(TestCase):
             except Exception:
                 pass
 
+    @override_settings(DEBUG=True)
+    def test_queries(self):
+        """
+        Test the documented API of connection.queries.
+        """
+        reset_queries()
+
+        with connection.cursor() as cursor:
+            cursor.execute("SELECT 1" + connection.features.bare_select_suffix)
+        self.assertEqual(1, len(connection.queries))
+
+        self.assertIsInstance(connection.queries, list)
+        self.assertIsInstance(connection.queries[0], dict)
+        self.assertItemsEqual(connection.queries[0].keys(), ['sql', 'time'])
+
+        reset_queries()
+        self.assertEqual(0, len(connection.queries))
+
+
+    # Unfortunately with sqlite3 the in-memory test database cannot be closed.
+    @skipUnlessDBFeature('test_db_allows_multiple_connections')
+    @override_settings(DEBUG=True)
+    def test_queries_limit(self):
+        """
+        Test that the backend doesn't store an unlimited number of queries.
+
+        Regression for #12581.
+        """
+        old_queries_limit = BaseDatabaseWrapper.queries_limit
+        BaseDatabaseWrapper.queries_limit = 3
+        new_connections = ConnectionHandler(settings.DATABASES)
+        new_connection = new_connections[DEFAULT_DB_ALIAS]
+
+        try:
+            with new_connection.cursor() as cursor:
+                cursor.execute("SELECT 1" + new_connection.features.bare_select_suffix)
+                cursor.execute("SELECT 2" + new_connection.features.bare_select_suffix)
+
+            with warnings.catch_warnings(record=True) as w:
+                self.assertEqual(2, len(new_connection.queries))
+                self.assertEqual(0, len(w))
+
+            with new_connection.cursor() as cursor:
+                cursor.execute("SELECT 3" + new_connection.features.bare_select_suffix)
+                cursor.execute("SELECT 4" + new_connection.features.bare_select_suffix)
+
+            with warnings.catch_warnings(record=True) as w:
+                self.assertEqual(3, len(new_connection.queries))
+                self.assertEqual(1, len(w))
+                self.assertEqual(str(w[0].message), "Limit for query logging "
+                    "exceeded, only the last 3 queries will be returned.")
+
+        finally:
+            BaseDatabaseWrapper.queries_limit = old_queries_limit
+            new_connection.close()
+
 
 # We don't make these tests conditional because that means we would need to
 # check and differentiate between: