Fixed #29015 -- Added an exception if the PostgreSQL database name is too long.

django/db/backends/postgresql/base.py

@@ -149,6 +149,12 @@ class DatabaseWrapper(BaseDatabaseWrapper):
             raise ImproperlyConfigured(
                 "settings.DATABASES is improperly configured. "
                 "Please supply the NAME value.")
+        if len(settings_dict['NAME'] or '') > self.ops.max_name_length():
+            raise ImproperlyConfigured(
+                'Database names longer than %d characters are not supported by '
+                'PostgreSQL. Supply a shorter NAME in settings.DATABASES.'
+                % self.ops.max_name_length()
+            )
         conn_params = {
             'database': settings_dict['NAME'] or 'postgres',
             **settings_dict['OPTIONS'],

tests/backends/postgresql/tests.py

@@ -2,6 +2,7 @@ import unittest
 import warnings
 from unittest import mock
 
+from django.core.exceptions import ImproperlyConfigured
 from django.db import DatabaseError, connection, connections
 from django.test import TestCase
 
@@ -39,6 +40,18 @@ class Tests(TestCase):
         self.assertEqual(len(w), 1)
         self.assertEqual(w[0].message.__class__, RuntimeWarning)
 
+    def test_database_name_too_long(self):
+        from django.db.backends.postgresql.base import DatabaseWrapper
+        settings = connection.settings_dict.copy()
+        max_name_length = connection.ops.max_name_length()
+        settings['NAME'] = 'a' + (max_name_length * 'a')
+        msg = (
+            'Database names longer than %d characters are not supported by '
+            'PostgreSQL. Supply a shorter NAME in settings.DATABASES.'
+        ) % max_name_length
+        with self.assertRaisesMessage(ImproperlyConfigured, msg):
+            DatabaseWrapper(settings).get_connection_params()
+
     def test_connect_and_rollback(self):
         """
         PostgreSQL shouldn't roll back SET TIME ZONE, even if the first