Fixed #26351 -- Added MySQL check to warn about strict mode option

Thanks Adam Chainz for the initial implementation in django-mysql.
Thanks Adam Chainz, Tim Graham, and Shai Berger for the reviews.

django/db/backends/mysql/validation.py

 from django.core import checks
 from django.db.backends.base.validation import BaseDatabaseValidation
+from django.utils.version import get_docs_version
 
 
 class DatabaseValidation(BaseDatabaseValidation):
+    def check(self, **kwargs):
+        issues = super(DatabaseValidation, self).check(**kwargs)
+        issues.extend(self._check_sql_mode(**kwargs))
+        return issues
+
+    def _check_sql_mode(self, **kwargs):
+        with self.connection.cursor() as cursor:
+            cursor.execute("SELECT @@sql_mode")
+            sql_mode = cursor.fetchone()
+        modes = set(sql_mode[0].split(','))
+        if not (modes & {'STRICT_TRANS_TABLES', 'STRICT_ALL_TABLES'}):
+            return [checks.Warning(
+                "MySQL Strict Mode is not set for database connection '%s'" % self.connection.alias,
+                hint="MySQL's Strict Mode fixes many data integrity problems in MySQL, "
+                     "such as data truncation upon insertion, by escalating warnings into "
+                     "errors. It is strongly recommended you activate it. See: "
+                     "https://docs.djangoproject.com/en/%s/ref/databases/#mysql-sql-mode"
+                     % (get_docs_version(),),
+                id='mysql.W002',
+            )]
+        return []
+
     def check_field(self, field, **kwargs):
         """
         MySQL has the following field length restriction:

docs/ref/checks.txt

@@ -592,6 +592,8 @@ If you're using MySQL, the following checks will be performed:
 
 * **mysql.E001**: MySQL does not allow unique ``CharField``\s to have a
   ``max_length`` > 255.
+* **mysql.W002**: MySQL Strict Mode is not set for database connection
+  '<alias>'. See also :ref:`mysql-sql-mode`.
 
 Templates
 ---------

tests/check_framework/test_database.py

@@ -31,3 +31,28 @@ class DatabaseCheckTests(TestCase):
         with mock.patch('django.db.backends.base.validation.BaseDatabaseValidation.check') as mocked_check:
             run_checks(tags=[Tags.database])
             self.assertTrue(mocked_check.called)
+
+    @unittest.skipUnless(connection.vendor == 'mysql', 'Test only for MySQL')
+    def test_mysql_strict_mode(self):
+        good_sql_modes = [
+            'STRICT_TRANS_TABLES,STRICT_ALL_TABLES',
+            'STRICT_TRANS_TABLES',
+            'STRICT_ALL_TABLES',
+        ]
+        for response in good_sql_modes:
+            with mock.patch(
+                'django.db.backends.utils.CursorWrapper.fetchone', create=True,
+                return_value=(response,)
+            ):
+                self.assertEqual(self.func(None), [])
+
+        bad_sql_modes = ['', 'WHATEVER']
+        for response in bad_sql_modes:
+            with mock.patch(
+                'django.db.backends.utils.CursorWrapper.fetchone', create=True,
+                return_value=(response,)
+            ):
+                # One warning for each database alias
+                result = self.func(None)
+                self.assertEqual(len(result), 2)
+                self.assertEqual([r.id for r in result], ['mysql.W002', 'mysql.W002'])