Fixed #24742 -- Made runserver.check_migrations ignore read-only databases

Thanks Luis Del Giudice for the report, and Aymeric Augustin and Markus
Holtermann for the reviews.

django/core/management/commands/runserver.py

@@ -12,6 +12,7 @@ from django.core.exceptions import ImproperlyConfigured
 from django.core.management.base import BaseCommand, CommandError
 from django.core.servers.basehttp import get_internal_wsgi_application, run
 from django.db import DEFAULT_DB_ALIAS, connections
+from django.db.migrations.exceptions import MigrationSchemaMissing
 from django.db.migrations.executor import MigrationExecutor
 from django.utils import autoreload, six
 from django.utils.encoding import force_text, get_system_encoding
@@ -109,10 +110,7 @@ class Command(BaseCommand):
 
         self.stdout.write("Performing system checks...\n\n")
         self.check(display_num_errors=True)
-        try:
+        self.check_migrations()
-            self.check_migrations()
-        except ImproperlyConfigured:
-            pass
         now = datetime.now().strftime('%B %d, %Y - %X')
         if six.PY2:
             now = now.decode(get_system_encoding())
@@ -157,7 +155,17 @@ class Command(BaseCommand):
         Checks to see if the set of migrations on disk matches the
         migrations in the database. Prints a warning if they don't match.
         """
-        executor = MigrationExecutor(connections[DEFAULT_DB_ALIAS])
+        try:
+            executor = MigrationExecutor(connections[DEFAULT_DB_ALIAS])
+        except ImproperlyConfigured:
+            # No databases are configured (or the dummy one)
+            return
+        except MigrationSchemaMissing:
+            self.stdout.write(self.style.NOTICE(
+                "\nNot checking migrations as it is not possible to access/create the django_migrations table."
+            ))
+            return
+
         plan = executor.migration_plan(executor.loader.graph.leaf_nodes())
         if plan:
             self.stdout.write(self.style.NOTICE(

django/db/migrations/exceptions.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+from django.db.utils import DatabaseError
 from django.utils.encoding import python_2_unicode_compatible
 
 
@@ -53,3 +54,7 @@ class NodeNotFoundError(LookupError):
 
     def __repr__(self):
         return "NodeNotFoundError(%r)" % self.node
+
+
+class MigrationSchemaMissing(DatabaseError):
+    pass

django/db/migrations/recorder.py

@@ -2,9 +2,12 @@ from __future__ import unicode_literals
 
 from django.apps.registry import Apps
 from django.db import models
+from django.db.utils import DatabaseError
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.timezone import now
 
+from .exceptions import MigrationSchemaMissing
+
 
 class MigrationRecorder(object):
     """
@@ -49,8 +52,11 @@ class MigrationRecorder(object):
         if self.Migration._meta.db_table in self.connection.introspection.table_names(self.connection.cursor()):
             return
         # Make the table
-        with self.connection.schema_editor() as editor:
+        try:
-            editor.create_model(self.Migration)
+            with self.connection.schema_editor() as editor:
+                editor.create_model(self.Migration)
+        except DatabaseError as exc:
+            raise MigrationSchemaMissing("Unable to create the django_migrations table (%s)" % exc)
 
     def applied_migrations(self):
         """

tests/admin_scripts/tests.py

@@ -22,6 +22,9 @@ from django.conf import settings
 from django.core.management import (
     BaseCommand, CommandError, call_command, color,
 )
+from django.db import ConnectionHandler
+from django.db.migrations.exceptions import MigrationSchemaMissing
+from django.db.migrations.recorder import MigrationRecorder
 from django.test import LiveServerTestCase, TestCase, mock, override_settings
 from django.test.runner import DiscoverRunner
 from django.utils._os import npath, upath
@@ -1247,7 +1250,8 @@ class ManageRunserver(AdminScriptTestCase):
         def monkey_run(*args, **options):
             return
 
-        self.cmd = Command()
+        self.output = StringIO()
+        self.cmd = Command(stdout=self.output)
         self.cmd.run = monkey_run
 
     def assertServerSettings(self, addr, port, ipv6=None, raw_ipv6=False):
@@ -1298,6 +1302,26 @@ class ManageRunserver(AdminScriptTestCase):
         self.cmd.handle(addrport="deadbeef:7654")
         self.assertServerSettings('deadbeef', '7654')
 
+    def test_no_database(self):
+        """
+        Ensure runserver.check_migrations doesn't choke on empty DATABASES.
+        """
+        tested_connections = ConnectionHandler({})
+        with mock.patch('django.core.management.commands.runserver.connections', new=tested_connections):
+            self.cmd.check_migrations()
+
+    def test_readonly_database(self):
+        """
+        Ensure runserver.check_migrations doesn't choke when a database is read-only
+        (with possibly no django_migrations table).
+        """
+        with mock.patch.object(
+                MigrationRecorder, 'ensure_schema',
+                side_effect=MigrationSchemaMissing()):
+            self.cmd.check_migrations()
+        # Check a warning is emitted
+        self.assertIn("Not checking migrations", self.output.getvalue())
+
 
 class ManageRunserverEmptyAllowedHosts(AdminScriptTestCase):
     def setUp(self):