Very basic locking mechanism for quotas

src/pretixbase/migrations/0008_quota_locked.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0007_auto_20150212_0939'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='quota',
+            name='locked',
+            field=models.DateTimeField(null=True, blank=True),
+            preserve_default=True,
+        ),
+    ]

src/pretixbase/models.py

@@ -1,6 +1,7 @@
 from itertools import product
 import copy
 import uuid
+import time
 
 from django.db import models
 from django.conf import settings
@@ -1041,6 +1042,10 @@ class Quota(Versionable):
         blank=True,
         verbose_name=_("Variations")
     )
+    locked = models.DateTimeField(
+        null=True, blank=True
+    )
+    locked_here = False
 
     class Meta:
         verbose_name = _("Quota")
@@ -1105,12 +1110,31 @@ class Quota(Versionable):
 
         return Quota.AVAILABILITY_OK, self.size - paid_orders - pending_valid_orders - valid_cart_positions
 
+    class LockTimeoutException(Exception):
+        pass
+
     def lock(self):
         """
         Issue a lock on this quota so nobody can take tickets from this quota until
-        you release the lock
+        you release the lock.
+
+        Raises an Quota.LockTimeoutException if the quota is locked every time we
+        try to obtain a lock.
         """
-        pass
+        retries = 5
+        for i in range(retries):
+            dt = now()
+            updated = Quota.objects.current.filter(
+                identity=self.identity, locked__isnull=True,
+                version_end_date__isnull=True
+            ).update(
+                locked=dt
+            )
+            if updated:
+                self.locked_here = dt
+                return True
+            time.sleep(2 ** i / 100)
+        raise Quota.LockTimeoutException()
 
     def release(self, force=False):
         """
@@ -1118,7 +1142,15 @@ class Quota(Versionable):
         the lock will only be released if it was issued in _this_ python
         representation of the database object.
         """
-        pass
+        if not self.locked_here and not force:
+            return False
+        updated = Quota.objects.current.filter(
+            identity=self.identity,
+            version_end_date__isnull=True
+        ).update(
+            locked=None
+        )
+        return updated
 
 
 class Order(Versionable):

src/pretixpresale/views/cart.py

@@ -94,13 +94,22 @@ class CartAdd(EventViewMixin, CartActionMixin, View):
             price = item.execute_restrictions() if variation is None else variation.execute_restrictions()
 
             if price is False:
-                msg_some_unavailable = True
-                messages.error(self.request,
-                               _('Some of the items you selected were no longer available. '
-                                 'Please see below for details.'))
+                if not msg_some_unavailable:
+                    msg_some_unavailable = True
+                    messages.error(self.request,
+                                   _('Some of the items you selected were no longer available. '
+                                     'Please see below for details.'))
                 continue
 
             quotas = list(item.quotas.all()) if variation is None else list(variation.quotas.all())
+            if len(quotas) == 0:
+                if not msg_some_unavailable:
+                    msg_some_unavailable = True
+                    messages.error(self.request,
+                                   _('Some of the items you selected were no longer available. '
+                                     'Please see below for details.'))
+                continue
+
             quota_ok = i[2]
             try:
                 for quota in quotas:
@@ -132,6 +141,12 @@ class CartAdd(EventViewMixin, CartActionMixin, View):
                         price=price,
                         expires=now() + timedelta(minutes=30)
                     )
+            except Quota.LockTimeoutException:
+                if not msg_some_unavailable:
+                    msg_some_unavailable = True
+                    messages.error(self.request,
+                                   _('We were not able to process your request completely as the '
+                                     'server was too busy. Please try again.'))
             finally:
                 for quota in quotas:
                     quota.release()