Rename module async to tasks (#994)

Fixes #993

src/pretix/base/services/tasks.py

@@ -0,0 +1,77 @@
+"""
+This code has been taken from
+https://blog.hypertrack.io/2016/10/08/dealing-with-database-transactions-in-django-celery/
+
+Usage:
+    from pretix.base.services.async import TransactionAwareTask
+    @task(base=TransactionAwareTask)
+    def task_…():
+"""
+import cProfile
+import os
+import random
+import time
+
+from django.conf import settings
+from django.db import transaction
+
+from pretix.base.metrics import (
+    pretix_task_duration_seconds, pretix_task_runs_total,
+)
+from pretix.celery_app import app
+
+
+class ProfiledTask(app.Task):
+    def __call__(self, *args, **kwargs):
+
+        if settings.PROFILING_RATE > 0 and random.random() < settings.PROFILING_RATE / 100:
+            profiler = cProfile.Profile()
+            profiler.enable()
+            t0 = time.perf_counter()
+            ret = super().__call__(*args, **kwargs)
+            tottime = time.perf_counter() - t0
+            profiler.disable()
+            profiler.dump_stats(os.path.join(settings.PROFILE_DIR, '{time:.0f}_{tottime:.3f}_celery_{t}.pstat'.format(
+                t=self.name, tottime=tottime, time=time.time()
+            )))
+        else:
+            t0 = time.perf_counter()
+            ret = super().__call__(*args, **kwargs)
+            tottime = time.perf_counter() - t0
+
+        if settings.METRICS_ENABLED:
+            pretix_task_duration_seconds.observe(tottime, task_name=self.name)
+        return ret
+
+    def on_failure(self, exc, task_id, args, kwargs, einfo):
+        if settings.METRICS_ENABLED:
+            expected = False
+            for t in self.throws:
+                if isinstance(exc, t):
+                    expected = True
+                    break
+            pretix_task_runs_total.inc(1, task_name=self.name, status="expected-error" if expected else "error")
+
+        return super().on_failure(exc, task_id, args, kwargs, einfo)
+
+    def on_success(self, retval, task_id, args, kwargs):
+        if settings.METRICS_ENABLED:
+            pretix_task_runs_total.inc(1, task_name=self.name, status="success")
+
+        return super().on_success(retval, task_id, args, kwargs)
+
+
+class TransactionAwareTask(ProfiledTask):
+    """
+    Task class which is aware of django db transactions and only executes tasks
+    after transaction has been committed
+    """
+
+    def apply_async(self, *args, **kwargs):
+        """
+        Unlike the default task in celery, this task does not return an async
+        result
+        """
+        transaction.on_commit(
+            lambda: super(TransactionAwareTask, self).apply_async(*args, **kwargs)
+        )