Add workaround for Django 3 memory leak under gevent

We've been suffering from a problematic memory leak since upgrading to Django 3 which is believed to be caused by django/asgiref#144.

The memory leak is causing Gunicorn worker processes to exhaust available memory and crash and restart.

This adds a workaround which is inactive by default. This is so it can be we can verify that it fixes the problem in e.g. the dev environment.

README.md

@@ -302,6 +302,7 @@ Data Hub API can run on any Heroku-style platform. Configuration is performed vi
 | `GUNICORN_ACCESS_LOG_FORMAT`  | No |  |
 | `GUNICORN_ENABLE_ASYNC_PSYCOPG2` | No | Whether to enable asynchronous psycopg2 when the worker class is 'gevent' (default=True). |
 | `GUNICORN_ENABLE_STATSD` | No | Whether to enable Gunicorn StatD instrumentation (default=False). |
+| `GUNICORN_PATCH_ASGIREF` | No | Whether to enable enable a workaround for https://github.com/django/asgiref/issues/144 when the worker class is 'gevent' (default=False). |
 | `GUNICORN_WORKER_CLASS`  | No | [Type of Gunicorn worker.](http://docs.gunicorn.org/en/stable/settings.html#worker-class) Uses async workers via gevent by default. |
 | `GUNICORN_WORKER_CONNECTIONS`  | No | Maximum no. of connections for async workers (default=10). |
 | `INTERACTION_ADMIN_CSV_IMPORT_MAX_SIZE` | No | Maximum file size in bytes for interaction admin CSV uploads (default=2MB). |

changelog/django-3-gevent-workaround.internal.md

@@ -0,0 +1 @@
+A workaround was added for a memory leak when using Django 3 with gevent. This is inactive by default.

config/gunicorn.py

@@ -41,6 +41,9 @@
 _enable_async_psycopg2 = (
     os.environ.get('GUNICORN_ENABLE_ASYNC_PSYCOPG2', 'true').lower() in ('true', '1')
 )
+_patch_asgiref = (
+    os.environ.get('GUNICORN_PATCH_ASGIREF', 'false').lower() in ('true', '1')
+)
 
 
 def post_fork(server, worker):
@@ -49,6 +52,17 @@ def post_fork(server, worker):
 
     Enables async processing in Psycopg2 if GUNICORN_ENABLE_ASYNC_PSYCOPG2 is set.
     """
-    if worker_class == 'gevent' and _enable_async_psycopg2:
-        patch_psycopg()
-        worker.log.info('Enabled async Psycopg2')
+    if worker_class == 'gevent':
+        if _enable_async_psycopg2:
+            patch_psycopg()
+            worker.log.info('Enabled async Psycopg2')
+
+        # Temporary workaround for https://github.com/django/asgiref/issues/144.
+        # Essentially reverts part of
+        # https://github.com/django/django/commit/a415ce70bef6d91036b00dd2c8544aed7aeeaaed.
+        if _patch_asgiref:
+            import asgiref.local
+            import threading
+
+            asgiref.local.Local = lambda **kwargs: threading.local()
+            worker.log.info('Patched asgiref.local.Local')