Fixed #9055 -- Standardized behaviour of parameter escaping in db cursors

Previously, depending on the database backend or the cursor type,
you'd need to double the percent signs in the query before passing
it to cursor.execute. Now cursor.execute consistently need percent
doubling whenever params argument is not None (placeholder substitution
will happen).
Thanks Thomas Güttler for the report and Walter Doekes for his work
on the patch.

django/db/backends/__init__.py

@@ -815,6 +815,8 @@ class BaseDatabaseOperations(object):
         to_unicode = lambda s: force_text(s, strings_only=True, errors='replace')
         if isinstance(params, (list, tuple)):
             u_params = tuple(to_unicode(val) for val in params)
+        elif params is None:
+            u_params = ()
         else:
             u_params = dict((to_unicode(k), to_unicode(v)) for k, v in params.items())
 

django/db/backends/mysql/base.py

@@ -115,6 +115,7 @@ class CursorWrapper(object):
 
     def execute(self, query, args=None):
         try:
+            # args is None means no string interpolation
             return self.cursor.execute(query, args)
         except Database.OperationalError as e:
             # Map some error codes to IntegrityError, since they seem to be

django/db/backends/oracle/base.py

@@ -757,17 +757,18 @@ class FormatStylePlaceholderCursor(object):
         return [p.force_bytes for p in params]
 
     def execute(self, query, params=None):
-        if params is None:
-            params = []
-        else:
-            params = self._format_params(params)
-        args = [(':arg%d' % i) for i in range(len(params))]
         # cx_Oracle wants no trailing ';' for SQL statements.  For PL/SQL, it
         # it does want a trailing ';' but not a trailing '/'.  However, these
         # characters must be included in the original query in case the query
         # is being passed to SQL*Plus.
         if query.endswith(';') or query.endswith('/'):
             query = query[:-1]
+        if params is None:
+            params = []
+            query = convert_unicode(query, self.charset)
+        else:
+            params = self._format_params(params)
+            args = [(':arg%d' % i) for i in range(len(params))]
             query = convert_unicode(query % tuple(args), self.charset)
         self._guess_input_sizes([params])
         try:

django/db/backends/sqlite3/base.py

@@ -433,7 +433,9 @@ class SQLiteCursorWrapper(Database.Cursor):
     This fixes it -- but note that if you want to use a literal "%s" in a query,
     you'll need to use "%%s".
     """
-    def execute(self, query, params=()):
+    def execute(self, query, params=None):
+        if params is None:
+            return Database.Cursor.execute(self, query)
         query = self.convert_query(query)
         return Database.Cursor.execute(self, query, params)
 

django/db/backends/util.py

@@ -35,11 +35,14 @@ class CursorWrapper(object):
 
 class CursorDebugWrapper(CursorWrapper):
 
-    def execute(self, sql, params=()):
+    def execute(self, sql, params=None):
         self.db.set_dirty()
         start = time()
         try:
             with self.db.wrap_database_errors():
+                if params is None:
+                    # params default might be backend specific
+                    return self.cursor.execute(sql)
                 return self.cursor.execute(sql, params)
         finally:
             stop = time()

docs/topics/db/sql.txt

@@ -227,6 +227,12 @@ For example::
     were committed to the database. Since Django now defaults to database-level
     autocommit, this isn't necessary any longer.
 
+Note that if you want to include literal percent signs in the query, you have to
+double them in the case you are passing parameters::
+
+     cursor.execute("SELECT foo FROM bar WHERE baz = '30%'")
+     cursor.execute("SELECT foo FROM bar WHERE baz = '30%%' and id = %s", [self.id])
+
 If you are using :doc:`more than one database </topics/db/multi-db>`, you can
 use ``django.db.connections`` to obtain the connection (and cursor) for a
 specific database. ``django.db.connections`` is a dictionary-like

tests/backends/tests.py

@@ -361,18 +361,34 @@ class ConnectionCreatedSignalTest(TransactionTestCase):
 
 
 class EscapingChecks(TestCase):
+    """
+    All tests in this test case are also run with settings.DEBUG=True in
+    EscapingChecksDebug test case, to also test CursorDebugWrapper.
+    """
+    def test_paramless_no_escaping(self):
+        cursor = connection.cursor()
+        cursor.execute("SELECT '%s'")
+        self.assertEqual(cursor.fetchall()[0][0], '%s')
+
+    def test_parameter_escaping(self):
+        cursor = connection.cursor()
+        cursor.execute("SELECT '%%', %s", ('%d',))
+        self.assertEqual(cursor.fetchall()[0], ('%', '%d'))
 
     @unittest.skipUnless(connection.vendor == 'sqlite',
                          "This is a sqlite-specific issue")
-    def test_parameter_escaping(self):
+    def test_sqlite_parameter_escaping(self):
         #13648: '%s' escaping support for sqlite3
         cursor = connection.cursor()
-        response = cursor.execute(
-            "select strftime('%%s', date('now'))").fetchall()[0][0]
-        self.assertNotEqual(response, None)
+        cursor.execute("select strftime('%s', date('now'))")
+        response = cursor.fetchall()[0][0]
         # response should be an non-zero integer
         self.assertTrue(int(response))
 
+@override_settings(DEBUG=True)
+class EscapingChecksDebug(EscapingChecks):
+    pass
+
 
 class SqlliteAggregationTests(TestCase):
     """