Added the possibility to use Redis for quota locks

src/pretix/base/models.py

@@ -1408,22 +1408,8 @@ class Quota(Versionable):
         :raises Quota.LockTimeoutException: if the quota is locked every time we try
                                             to obtain the lock
         """
-        retries = 5
-        for i in range(retries):
-            dt = now()
-            updated = Quota.objects.current.filter(
-                Q(identity=self.identity)
-                & Q(Q(locked__lt=dt - timedelta(seconds=120)) | Q(locked__isnull=True))
-                & Q(version_end_date__isnull=True)
-            ).update(
-                locked=dt
-            )
-            if updated:
-                self.locked_here = dt
-                self.locked = dt
-                return True
-            time.sleep(2 ** i / 100)
-        raise Quota.LockTimeoutException()
+        from .services import locking
+        return locking.lock_quota(self)
 
     def release(self, force=False):
         """
@@ -1431,17 +1417,8 @@ class Quota(Versionable):
         the lock will only be released if it was issued in _this_ python
         representation of the database object.
         """
-        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
-        )
-        self.locked_here = None
-        self.locked = None
-        return updated
+        from .services import locking
+        return locking.release_quota(self, force)
 
 
 class Order(Versionable):

src/pretix/base/services/locking.py

@@ -0,0 +1,111 @@
+from datetime import timedelta
+import logging
+import time
+from django.db.models import Q
+from django.utils.timezone import now
+from pretix import settings
+
+from pretix.base.models import Quota
+from redis import RedisError
+
+logger = logging.getLogger('pretix.base.locking')
+
+
+def lock_quota(quota):
+    """
+    Issue a lock on this quota so nobody can take tickets from this quota until
+    you release the lock. Will retry 5 times on failure.
+
+    :raises Quota.LockTimeoutException: if the quota is locked every time we try
+                                        to obtain the lock
+    """
+    if settings.HAS_REDIS:
+        return lock_quota_redis(quota)
+    else:
+        return lock_quota_db(quota)
+
+
+def lock_quota_db(quota):
+    retries = 5
+    for i in range(retries):
+        dt = now()
+
+        updated = Quota.objects.current.filter(
+            Q(identity=quota.identity)
+            & Q(Q(locked__lt=dt - timedelta(seconds=120)) | Q(locked__isnull=True))
+            & Q(version_end_date__isnull=True)
+        ).update(
+            locked=dt
+        )
+        if updated:
+            quota.locked_here = dt
+            quota.locked = dt
+            return True
+        time.sleep(2 ** i / 100)
+    raise Quota.LockTimeoutException()
+
+
+def release_quota(quota, force=False):
+    """
+    Release a lock placed by :py:meth:`lock()`. If the parameter force is not set to ``True``,
+    the lock will only be released if it was issued in _this_ python
+    representation of the database object.
+    """
+    if not quota.locked_here and not force:
+        return False
+    if settings.HAS_REDIS:
+        return release_quota_redis(quota)
+    else:
+        return release_quota_db(quota)
+
+
+def release_quota_db(quota):
+    updated = Quota.objects.current.filter(
+        identity=quota.identity,
+        version_end_date__isnull=True
+    ).update(
+        locked=None
+    )
+    quota.locked_here = None
+    quota.locked = None
+    return updated
+
+
+def redis_lock_from_quota(quota):
+    from django_redis import get_redis_connection
+    from redis.lock import Lock
+
+    if not hasattr(quota, '_redis_lock'):
+        rc = get_redis_connection("redis")
+        quota._redis_lock = Lock(redis=rc, name='pretix_quota_%s' % quota.identity, timeout=120)
+    return quota._redis_lock
+
+
+def lock_quota_redis(quota):
+    from redis.exceptions import RedisError
+    lock = redis_lock_from_quota(quota)
+    retries = 5
+    for i in range(retries):
+        dt = now()
+        try:
+            if lock.acquire(False):
+                quota.locked_here = dt
+                quota.locked = dt
+                return True
+        except RedisError:
+            logger.exception('Error locking a quota')
+            raise Quota.LockTimeoutException()
+        time.sleep(2 ** i / 100)
+    raise Quota.LockTimeoutException()
+
+
+def release_quota_redis(quota):
+    lock = redis_lock_from_quota(quota)
+    try:
+        lock.release()
+    except RedisError:
+        logger.exception('Error releasing a quota lock')
+        raise Quota.LockTimeoutException()
+    quota.locked_here = None
+    quota.locked = None
+    return True

src/pretix/settings.py

@@ -76,13 +76,34 @@ SESSION_COOKIE_SECURE = SESSION_COOKIE_HTTPONLY = config.getboolean(
 LANGUAGE_COOKIE_DOMAIN = SESSION_COOKIE_DOMAIN = CSRF_COOKIE_DOMAIN = config.get(
     'pretix', 'cookiedomain', fallback=None)
 
-if config.has_option('memcached', 'location'):
-    CACHES = {
-        'default': {
-            'BACKEND': 'django.core.cache.backends.memcached.PyLibMCCache',
-            'LOCATION': config.get('memcached', 'location'),
+CACHES = {
+    'default': {
+        'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
+        'LOCATION': 'unique-snowflake',
+    }
+}
+
+HAS_MEMCACHED = config.has_option('memcached', 'location')
+if HAS_MEMCACHED:
+    CACHES['default'] = {
+        'BACKEND': 'django.core.cache.backends.memcached.PyLibMCCache',
+        'LOCATION': config.get('memcached', 'location'),
+    }
+
+HAS_REDIS = config.has_option('redis', 'location')
+if HAS_REDIS:
+    CACHES['redis'] = {
+        "BACKEND": "django_redis.cache.RedisCache",
+        "LOCATION": config.get('redis', 'location'),
+        "OPTIONS": {
+            "CLIENT_CLASS": "django_redis.client.DefaultClient",
         }
     }
+    if not HAS_MEMCACHED:
+        CACHES['default'] = CACHES['redis']
+    if config.getboolean('redis', 'sessions', fallback=False):
+        SESSION_ENGINE = "django.contrib.sessions.backends.cache"
+        SESSION_CACHE_ALIAS = "redis"
 
 # Internal settings