feat: implement /healthcheck endpoint

implements the healthcheck as a middleware before the HOST_HEADER verification (in django.middleware.common.CommonMiddleware), so healthcheck requests always pass

based on https://stackoverflow.com/a/64623669

pems/core/middleware.py

@@ -0,0 +1,19 @@
+"""
+The core application: middleware definitions for request/response cycle.
+"""
+
+from django.http import HttpResponse
+
+HEALTHCHECK_PATH = "/healthcheck"
+
+
+class Healthcheck:
+    """Middleware intercepts and accepts /healthcheck requests."""
+
+    def __init__(self, get_response):
+        self.get_response = get_response
+
+    def __call__(self, request):
+        if request.path == HEALTHCHECK_PATH:
+            return HttpResponse("Healthy", content_type="text/plain")
+        return self.get_response(request)

pems/settings.py

@@ -36,6 +36,7 @@ def _filter_empty(ls):
 MIDDLEWARE = [
     "django.middleware.security.SecurityMiddleware",
     "django.contrib.sessions.middleware.SessionMiddleware",
+    "pems.core.middleware.Healthcheck",
     "django.middleware.common.CommonMiddleware",
     "django.middleware.csrf.CsrfViewMiddleware",
     "django.contrib.auth.middleware.AuthenticationMiddleware",

tests/pytest/core/test_middleware_healthcheck.py

@@ -0,0 +1,6 @@
+from pems.core.middleware import HEALTHCHECK_PATH
+
+
+def test_healthcheck(client):
+    response = client.get(HEALTHCHECK_PATH)
+    assert response.status_code == 200