Fixed #16649 -- Refactored save_base logic

Model.save() will use UPDATE - if not updated - INSERT instead of
SELECT - if found UPDATE else INSERT. This should save a query when
updating, but will cost a little when inserting model with PK set.

Also fixed #17341 -- made sure .save() commits transactions only after
the whole model has been saved. This wasn't the case in model
inheritance situations.

The save_base implementation was refactored into multiple methods.
A typical chain for inherited save is:
save_base()
    _save_parents(self)
        for each parent:
            _save_parents(parent)
            _save_table(parent)
    _save_table(self)

4 files changed, 52 insertions, 5 deletions

diff --git a/tests/basic/tests.py b/tests/basic/tests.py
index 2de87a225f..a8005baca7 100644
--- a/tests/basic/tests.py
+++ b/tests/basic/tests.py
@@ -1,11 +1,13 @@
 from __future__ import absolute_import, unicode_literals
 
 from datetime import datetime
+import threading
 
 from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned, FieldError
+from django.db import connections, DEFAULT_DB_ALIAS
 from django.db.models.fields import Field, FieldDoesNotExist
 from django.db.models.query import QuerySet, EmptyQuerySet, ValuesListQuerySet
-from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
+from django.test import TestCase, TransactionTestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils import six
 from django.utils.translation import ugettext_lazy
 
@@ -690,4 +692,29 @@ class ModelTest(TestCase):
     def test_invalid_qs_list(self):
         qs = Article.objects.order_by('invalid_column')
         self.assertRaises(FieldError, list, qs)
-        self.assertRaises(FieldError, list, qs)
\ No newline at end of file
+        self.assertRaises(FieldError, list, qs)
+
+class ConcurrentSaveTests(TransactionTestCase):
+    @skipUnlessDBFeature('test_db_allows_multiple_connections')
+    def test_concurrent_delete_with_save(self):
+        """
+        Test fetching, deleting and finally saving an object - we should get
+        an insert in this case.
+        """
+        a = Article.objects.create(headline='foo', pub_date=datetime.now())
+        exceptions = []
+        def deleter():
+            try:
+                # Do not delete a directly - doing so alters its state.
+                Article.objects.filter(pk=a.pk).delete()
+                connections[DEFAULT_DB_ALIAS].commit_unless_managed()
+            except Exception as e:
+                exceptions.append(e)
+            finally:
+                connections[DEFAULT_DB_ALIAS].close()
+        self.assertEqual(len(exceptions), 0)
+        t = threading.Thread(target=deleter)
+        t.start()
+        t.join()
+        a.save()
+        self.assertEqual(Article.objects.get(pk=a.pk).headline, 'foo')
diff --git a/tests/model_inheritance/tests.py b/tests/model_inheritance/tests.py
index 62f521c07f..dc40d2d2e0 100644
--- a/tests/model_inheritance/tests.py
+++ b/tests/model_inheritance/tests.py
@@ -294,7 +294,7 @@ class ModelInheritanceTests(TestCase):
             rating=4,
             chef=c
         )
-        with self.assertNumQueries(6):
+        with self.assertNumQueries(3):
             ir.save()
 
     def test_update_parent_filtering(self):
diff --git a/tests/transactions_regress/models.py b/tests/transactions_regress/models.py
index a4b576c3ca..e09e81d93d 100644
--- a/tests/transactions_regress/models.py
+++ b/tests/transactions_regress/models.py
@@ -4,6 +4,8 @@ from django.db import models
 class Mod(models.Model):
     fld = models.IntegerField()
 
+class SubMod(Mod):
+    cnt = models.IntegerField(unique=True)
 
 class M2mA(models.Model):
     others = models.ManyToManyField('M2mB')
diff --git a/tests/transactions_regress/tests.py b/tests/transactions_regress/tests.py
index 142c09d3cf..e320f76169 100644
--- a/tests/transactions_regress/tests.py
+++ b/tests/transactions_regress/tests.py
@@ -1,6 +1,7 @@
 from __future__ import absolute_import
 
-from django.db import connection, connections, transaction, DEFAULT_DB_ALIAS, DatabaseError
+from django.db import (connection, connections, transaction, DEFAULT_DB_ALIAS, DatabaseError,
+                       IntegrityError)
 from django.db.transaction import commit_on_success, commit_manually, TransactionManagementError
 from django.test import TransactionTestCase, skipUnlessDBFeature
 from django.test.utils import override_settings
@@ -8,8 +9,25 @@ from django.utils.unittest import skipIf, skipUnless
 
 from transactions.tests import IgnorePendingDeprecationWarningsMixin
 
-from .models import Mod, M2mA, M2mB
+from .models import Mod, M2mA, M2mB, SubMod
 
+class ModelInheritanceTests(TransactionTestCase):
+    def test_save(self):
+        # First, create a SubMod, then try to save another with conflicting
+        # cnt field. The problem was that transactions were committed after
+        # every parent save when not in managed transaction. As the cnt
+        # conflict is in the second model, we can check if the first save
+        # was committed or not.
+        SubMod(fld=1, cnt=1).save()
+        # We should have committed the transaction for the above - assert this.
+        connection.rollback()
+        self.assertEqual(SubMod.objects.count(), 1)
+        try:
+            SubMod(fld=2, cnt=1).save()
+        except IntegrityError:
+            connection.rollback()
+        self.assertEqual(SubMod.objects.count(), 1)
+        self.assertEqual(Mod.objects.count(), 1)
 
 class TestTransactionClosing(IgnorePendingDeprecationWarningsMixin, TransactionTestCase):
     """