[1.8.x] Refs #24299 -- Made contenttypes migrations signal handler more robust.

Backport of d392c1e150a46aca72e37263c44c034d92a79f79 from master

2 files changed, 38 insertions, 13 deletions

diff --git a/django/contrib/contenttypes/models.py b/django/contrib/contenttypes/models.py
index 5205ec80f9..2aadf80027 100644
--- a/django/contrib/contenttypes/models.py
+++ b/django/contrib/contenttypes/models.py
@@ -4,7 +4,7 @@ import warnings
 
 from django.apps import apps
 from django.db import models
-from django.db.utils import OperationalError, ProgrammingError
+from django.db.utils import IntegrityError, OperationalError, ProgrammingError
 from django.utils.deprecation import RemovedInDjango20Warning
 from django.utils.encoding import force_text, python_2_unicode_compatible
 from django.utils.translation import ugettext_lazy as _
@@ -59,10 +59,18 @@ class ContentTypeManager(models.Manager):
         # The ContentType entry was not found in the cache, therefore we
         # proceed to load or create it.
         try:
-            # We start with get() and not get_or_create() in order to use
-            # the db_for_read (see #20401).
-            ct = self.get(app_label=opts.app_label, model=opts.model_name)
-        except (OperationalError, ProgrammingError):
+            try:
+                # We start with get() and not get_or_create() in order to use
+                # the db_for_read (see #20401).
+                ct = self.get(app_label=opts.app_label, model=opts.model_name)
+            except self.model.DoesNotExist:
+                # Not found in the database; we proceed to create it.  This time we
+                # use get_or_create to take care of any race conditions.
+                ct, created = self.get_or_create(
+                    app_label=opts.app_label,
+                    model=opts.model_name,
+                )
+        except (OperationalError, ProgrammingError, IntegrityError):
             # It's possible to migrate a single app before contenttypes,
             # as it's not a required initial dependency (it's contrib!)
             # Have a nice error for this.
@@ -70,13 +78,6 @@ class ContentTypeManager(models.Manager):
                 "Error creating new content types. Please make sure contenttypes "
                 "is migrated before trying to migrate apps individually."
             )
-        except self.model.DoesNotExist:
-            # Not found in the database; we proceed to create it.  This time we
-            # use get_or_create to take care of any race conditions.
-            ct, created = self.get_or_create(
-                app_label=opts.app_label,
-                model=opts.model_name,
-            )
         self._add_to_cache(self.db, ct)
         return ct
 
diff --git a/django/contrib/contenttypes/tests/tests.py b/django/contrib/contenttypes/tests/tests.py
index 8e4dae3b37..eb436b6a64 100644
--- a/django/contrib/contenttypes/tests/tests.py
+++ b/django/contrib/contenttypes/tests/tests.py
@@ -5,8 +5,9 @@ import warnings
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.contenttypes.views import shortcut
 from django.contrib.sites.shortcuts import get_current_site
+from django.db.utils import IntegrityError, OperationalError, ProgrammingError
 from django.http import Http404, HttpRequest
-from django.test import TestCase, override_settings
+from django.test import TestCase, mock, override_settings
 from django.utils import six
 
 from .models import (
@@ -264,3 +265,26 @@ class ContentTypesTests(TestCase):
             "ContentType.name field doesn't exist any longer. Please remove it from your code."
         )
         self.assertTrue(ContentType.objects.filter(model='OldModel').exists())
+
+    @mock.patch('django.contrib.contenttypes.models.ContentTypeManager.get_or_create')
+    @mock.patch('django.contrib.contenttypes.models.ContentTypeManager.get')
+    def test_message_if_get_for_model_fails(self, mocked_get, mocked_get_or_create):
+        """
+        Check that `RuntimeError` with nice error message is raised if
+        `get_for_model` fails because of database errors.
+        """
+
+        def _test_message(mocked_method):
+            for ExceptionClass in (IntegrityError, OperationalError, ProgrammingError):
+                mocked_method.side_effect = ExceptionClass
+                with self.assertRaisesMessage(
+                    RuntimeError,
+                    "Error creating new content types. Please make sure contenttypes "
+                    "is migrated before trying to migrate apps individually."
+                ):
+                    ContentType.objects.get_for_model(ContentType)
+
+        _test_message(mocked_get)
+
+        mocked_get.side_effect = ContentType.DoesNotExist
+        _test_message(mocked_get_or_create)