uwsgi: Force Django load before returning the uwsgi worker function.

Django lazy-loads much of its modules, including the application's.
This defers the time to during the first request it serves.  When
doing rolling-restarts, this means that the worker is marked "ready"
despite having multiple seconds more work to do.  With small numbers
of workers, this causes a significant capacity drop, since effectively
more than one worker can be still reloading at a time.  It also
results in poor user experience for the requests which are unlucky
enough to be served first.

Use the technique detailed in the uwsgi documentation[^1] to fake a
request during initial application load, which forces the application
to be loaded.

[^1]: https://uwsgi-docs.readthedocs.io/en/latest/articles/TheArtOfGracefulReloading.html#dealing-with-ultra-lazy-apps-like-django

zproject/wsgi.py

@@ -25,6 +25,9 @@ setup_path()
 
 os.environ.setdefault("DJANGO_SETTINGS_MODULE", "zproject.settings")
 
+from collections.abc import Callable
+from typing import Any
+
 from django.core.wsgi import get_wsgi_application
 
 try:
@@ -33,6 +36,37 @@ try:
     # setting points here.
 
     application = get_wsgi_application()
+
+    # We force loading of the main parts of the application now, by
+    # handing it a fake request, rather than have to pay that price
+    # during the first request served by this process.  Hitting the
+    # /health endpoint will not only load Django and all of the views
+    # (by loading the URL dispatcher) but will also force open any
+    # lazy-loaded service connections.
+    #
+    # The return value (and thus response status) of this healthcheck
+    # request is ignored, so we do return the application handler even
+    # if connections are not fully available yet.  This at least
+    # allows application logging to handle any such errors, instead of
+    # arcane errors from uwsgi not being able to load its handler
+    # function.
+    def ignored_start_response(
+        status: str, headers: list[tuple[str, str]], exc_info: Any = None, /
+    ) -> Callable[[bytes], object]:
+        return lambda x: None
+
+    application(
+        {
+            "REQUEST_METHOD": "GET",
+            "SERVER_NAME": "127.0.0.1",
+            "SERVER_PORT": "443",
+            "PATH_INFO": "/health",
+            "REMOTE_ADDR": "127.0.0.1",
+            "wsgi.input": sys.stdin,
+            "wsgi.url_scheme": "https",
+        },
+        ignored_start_response,
+    )
 except Exception:
     # If /etc/zulip/settings.py contains invalid syntax, Django
     # initialization will fail in django.setup().  In this case, our