Locking optimizations

src/pretix/base/models/orders.py

@@ -32,6 +32,7 @@ from pretix.base.decimal import round_decimal
 from pretix.base.i18n import language
 from pretix.base.models import User
 from pretix.base.reldate import RelativeDateWrapper
+from pretix.base.services.locking import NoLockManager
 from pretix.base.settings import PERSON_NAME_SCHEMES
 
 from .base import LockModel, LoggedModel
@@ -1222,13 +1223,13 @@ class OrderPayment(models.Model):
         if (self.order.status == Order.STATUS_PENDING and self.order.expires > now() + timedelta(hours=12)) or not lock:
             # Performance optimization. In this case, there's really no reason to lock everything and an atomic
             # database transaction is more than enough.
-            with transaction.atomic():
-                self._mark_paid(force, count_waitinglist, user, auth, overpaid=payment_sum - refund_sum > self.order.total,
-                                ignore_date=ignore_date)
+            lockfn = NoLockManager
         else:
-            with self.order.event.lock():
-                self._mark_paid(force, count_waitinglist, user, auth, overpaid=payment_sum - refund_sum > self.order.total,
-                                ignore_date=ignore_date)
+            lockfn = self.order.event.lock
+
+        with lockfn():
+            self._mark_paid(force, count_waitinglist, user, auth, overpaid=payment_sum - refund_sum > self.order.total,
+                            ignore_date=ignore_date)
 
         invoice = None
         if invoice_qualified(self.order):

src/pretix/base/services/cart.py

@@ -5,7 +5,7 @@ from typing import List, Optional
 
 from celery.exceptions import MaxRetriesExceededError
 from django.core.exceptions import ValidationError
-from django.db import transaction
+from django.db import DatabaseError, transaction
 from django.db.models import Q
 from django.dispatch import receiver
 from django.utils.timezone import make_aware, now
@@ -21,7 +21,7 @@ from pretix.base.models.orders import OrderFee
 from pretix.base.models.tax import TAXED_ZERO, TaxedPrice, TaxRule
 from pretix.base.reldate import RelativeDateWrapper
 from pretix.base.services.checkin import _save_answers
-from pretix.base.services.locking import LockTimeoutException
+from pretix.base.services.locking import LockTimeoutException, NoLockManager
 from pretix.base.services.pricing import get_price
 from pretix.base.services.tasks import ProfiledTask
 from pretix.base.settings import PERSON_NAME_SCHEMES
@@ -791,7 +791,11 @@ class CartManager:
                     if available_count == 1:
                         op.position.expires = self._expiry
                         op.position.price = op.price.gross
-                        op.position.save()
+                        try:
+                            op.position.save(force_update=True)
+                        except DatabaseError:
+                            # Best effort... The position might have been deleted in the meantime!
+                            pass
                     elif available_count == 0:
                         op.position.addons.all().delete()
                         op.position.delete()
@@ -806,17 +810,33 @@ class CartManager:
         CartPosition.objects.bulk_create([p for p in new_cart_positions if not getattr(p, '_answers', None) and not p.pk])
         return err
 
+    def _require_locking(self):
+        if self._voucher_use_diff:
+            # If any vouchers are used, we lock to make sure we don't redeem them to often
+            return True
+
+        if self._quota_diff and any(q.size is not None for q in self._quota_diff):
+            # If any quotas are affected that are not unlimited, we lock
+            return True
+
+        return False
+
     def commit(self):
         self._check_presale_dates()
         self._check_max_cart_size()
         self._calculate_expiry()
 
-        with self.event.lock() as now_dt:
+        err = self._delete_out_of_timeframe()
+        err = self.extend_expired_positions() or err
+
+        lockfn = NoLockManager
+        if self._require_locking():
+            lockfn = self.event.lock
+
+        with lockfn() as now_dt:
             with transaction.atomic():
                 self.now_dt = now_dt
                 self._extend_expiry_of_valid_existing_positions()
-                err = self._delete_out_of_timeframe()
-                err = self.extend_expired_positions() or err
                 err = self._perform_operations() or err
             if err:
                 raise CartError(err)

src/pretix/base/services/locking.py

@@ -13,6 +13,18 @@ logger = logging.getLogger('pretix.base.locking')
 LOCK_TIMEOUT = 120
 
 
+class NoLockManager:
+    def __init__(self):
+        pass
+
+    def __enter__(self):
+        return now()
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if exc_type is not None:
+            return False
+
+
 class LockManager:
     def __init__(self, event):
         self.event = event

src/pretix/base/services/orders.py

@@ -39,7 +39,7 @@ from pretix.base.services import tickets
 from pretix.base.services.invoices import (
     generate_cancellation, generate_invoice, invoice_qualified,
 )
-from pretix.base.services.locking import LockTimeoutException
+from pretix.base.services.locking import LockTimeoutException, NoLockManager
 from pretix.base.services.mail import SendMailException
 from pretix.base.services.pricing import get_price
 from pretix.base.services.tasks import ProfiledTask
@@ -665,9 +665,18 @@ def _perform_order(event: str, payment_provider: str, position_ids: List[str],
         except InvoiceAddress.DoesNotExist:
             pass
 
-    with event.lock() as now_dt:
-        positions = list(CartPosition.objects.filter(
-            id__in=position_ids).select_related('item', 'variation', 'subevent', 'addon_to').prefetch_related('addons'))
+    positions = CartPosition.objects.filter(id__in=position_ids, event=event)
+
+    lockfn = NoLockManager
+    locked = False
+    if positions.filter(Q(voucher__isnull=False) | Q(expires__lt=now() + timedelta(minutes=2))).exists():
+        # Performance optimization: If no voucher is used and no cart position is dangerously close to its expiry date,
+        # creating this order shouldn't be prone to any race conditions and we don't need to lock the event.
+        locked = True
+        lockfn = event.lock
+
+    with lockfn() as now_dt:
+        positions = list(positions.select_related('item', 'variation', 'subevent', 'addon_to').prefetch_related('addons'))
         if len(positions) == 0:
             raise OrderError(error_messages['empty'])
         if len(position_ids) != len(positions):
@@ -679,7 +688,7 @@ def _perform_order(event: str, payment_provider: str, position_ids: List[str],
         free_order_flow = payment and payment_provider == 'free' and order.total == Decimal('0.00') and not order.require_approval
         if free_order_flow:
             try:
-                payment.confirm(send_mail=False, lock=False)
+                payment.confirm(send_mail=False, lock=not locked)
             except Quota.QuotaExceededException:
                 pass