Allow to reduce the interval of some cronjobs (#1753)

src/pretix/helpers/periodic.py

@@ -0,0 +1,61 @@
+import logging
+import uuid
+from functools import wraps
+
+from django.core.cache import cache
+
+logger = logging.getLogger(__name__)
+
+
+def minimum_interval(minutes_after_success, minutes_after_error=0, minutes_running_timeout=30):
+    """
+    This is intended to be used as a decorator on receivers of the ``periodic_task`` signal.
+    It stores the result in the task in the cache (usually redis) to ensure the receiver function
+    isn't executed less than ``minutes_after_success`` after the last successful run and no less
+    than ``minutes_after_error`` after the last failed run. There's also a simple locking mechanism
+    implemented making sure the function is not called a second time while it is running, unless
+    ``minutes_running_timeout`` have passed. This locking mechanism is naive and not safe of
+    race-conditions, it should not be relied upon.
+    """
+    def deco(f):
+        @wraps(f)
+        def wrapper(*args, **kwargs):
+            key_running = f'pretix_periodic_{f.__module__}.{f.__name__}_running'
+            key_result = f'pretix_periodic_{f.__module__}.{f.__name__}_result'
+
+            running_val = cache.get(key_running)
+            if running_val:
+                # Currently running
+                return
+
+            result_val = cache.get(key_result)
+            if result_val:
+                # Has run recently
+                return
+
+            uniqid = str(uuid.uuid4())
+            cache.set(key_running, uniqid, timeout=minutes_running_timeout * 60)
+            try:
+                retval = f(*args, **kwargs)
+            except Exception as e:
+                try:
+                    cache.set(key_result, 'error', timeout=minutes_after_error * 60)
+                except:
+                    logger.exception('Could not store result')
+                raise e
+            else:
+                try:
+                    cache.set(key_result, 'success', timeout=minutes_after_success * 60)
+                except:
+                    logger.exception('Could not store result')
+                return retval
+            finally:
+                try:
+                    if cache.get(key_running) == uniqid:
+                        cache.delete(key_running)
+                except:
+                    logger.exception('Could not release lock')
+
+        return wrapper
+
+    return deco