Merge pull request #1196 from alphagov/ris-gunicorn-context-recycling-eventlet-worker

eventlet: add `ContextRecyclingEventletWorker`

Author: risicle

CHANGELOG.md

@@ -1,5 +1,10 @@
 # CHANGELOG
 
+## 96.0.0
+
+* BREAKING CHANGE: the `gunicorn_defaults` module has been moved to `gunicorn.defaults` to make space for gunicorn-related utils that don't have the restricted-import constraints of the gunicorn defaults. Imports of `notifications_utils.gunicorn_defaults` should be changed to `notifications_utils.gunicorn.defaults`.
+* Added `ContextRecyclingEventletWorker` custom gunicorn worker class.
+
 ## 95.2.0
 
 * Implement `InsensitiveSet.__contains__`

notifications_utils/gunicorn/__init__.py

@@ -0,0 +1,3 @@
+# Avoid additions to this file as it will implicitly be imported before all
+# notifications_utils.gunicorn.* imports and we need to be able to use the
+# defaults module from an early stage of app bringup

notifications_utils/gunicorn_defaults.py renamed to notifications_utils/gunicorn/defaults.py

@@ -0,0 +1,34 @@
+import contextvars
+from collections import deque
+
+import greenlet
+from gunicorn.workers import geventlet
+
+
+class ContextRecyclingEventletWorker(geventlet.EventletWorker):
+    """
+    Because eventlet's GreenPool discards GreenThreads after they have performed a task,
+    to reduce wasteful continual re-creation of thread-local resources this class will
+    maintain a pool of thread contexts suitable for reuse with new GreenThreads. In
+    theory at least this means we will never have more thread contexts than the maximum
+    number of concurrent GreenThreads handling connections we've ever had.
+    """
+
+    def __init__(self, *args, **kwargs):
+        self.context_pool = deque()  # a stack of unused thread contexts
+        super().__init__(*args, **kwargs)
+
+    def handle(self, *args, **kwargs):
+        g = greenlet.getcurrent()
+        if self.context_pool:
+            # reuse an existing thread context from the pool
+            g.gr_context = self.context_pool.pop()
+
+        ret = super().handle(*args, **kwargs)
+
+        # stash potentially-populated thread context in context_pool
+        self.context_pool.append(g.gr_context)
+        # replace reference to now-stashed context with an empty one
+        g.gr_context = contextvars.Context()
+
+        return ret

notifications_utils/gunicorn/eventlet.py

@@ -5,4 +5,4 @@
 # - `make version-minor` for new features
 # - `make version-patch` for bug fixes
 
-__version__ = "95.2.0"  # e423953f9dec74010897c62aa4e035de
+__version__ = "96.0.0"  # 6fdd72884bdeadbeef