🐛(scalingo) isolate the metrics endpoint in its own gunicorn pool

bin/scalingo_run_web

@@ -3,8 +3,26 @@
 # Trigger collabora configuration
 python manage.py trigger_wopi_configuration
 
+# Keep the total worker count at WEB_CONCURRENCY: the metrics pool below
+# takes one worker, the main pool gets the rest (at least one).
+MAIN_WORKERS=$((${WEB_CONCURRENCY:-2} - 1))
+if [ "$MAIN_WORKERS" -lt 1 ]; then
+    MAIN_WORKERS=1
+fi
+echo "WEB_CONCURRENCY=${WEB_CONCURRENCY:-unset} MAIN_WORKERS=$MAIN_WORKERS"
+
 # Start the Django backend
-gunicorn -b :8000 drive.wsgi:application --log-file - &
+gunicorn -b :8000 -w "$MAIN_WORKERS" drive.wsgi:application --log-file - &
+
+# Start a dedicated Django backend pool for the usage metrics endpoint.
+# DeployCenter calls it synchronously while Drive itself is blocked waiting
+# on a DeployCenter entitlements response: served by the main pool, this
+# callback would queue behind the very workers waiting for it (deadlock
+# until timeout). An isolated pool guarantees it always gets a worker.
+# Its access log is enabled with a distinctive prefix so the platform
+# logs show which requests were served by this pool.
+gunicorn -b :8001 -w 1 drive.wsgi:application --log-file - \
+    --access-logfile - --access-logformat 'metrics-pool %(h)s "%(r)s" %(s)s' &
 
 # Start the Nginx server
 bin/run &

src/nginx/servers.conf.erb

@@ -5,6 +5,13 @@ upstream backend_server {
 	server localhost:8000 fail_timeout=0;
 }
 
+# Dedicated gunicorn pool for the usage metrics endpoint, so DeployCenter's
+# synchronous callback never competes with user requests blocked on
+# DeployCenter itself (see bin/scalingo_run_web).
+upstream metrics_server {
+	server localhost:8001 fail_timeout=0;
+}
+
 server {
 
 	listen <%= ENV["PORT"] %>;
@@ -14,6 +21,18 @@ server {
 
     error_page 404 /404.html;
 
+    # Usage metrics, isolated from the main backend pool. Must stay above
+    # the generic api/external_api block: nginx picks the first matching
+    # regex location.
+    location ~ ^/external_api/v[0-9.]+/metrics/ {
+        proxy_set_header X-Forwarded-Proto https;
+        proxy_set_header Host $http_host;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+
+        proxy_redirect off;
+        proxy_pass http://metrics_server;
+    }
+
     # Django rest framework and external API
     location ~ ^/(api|external_api)/ {
         proxy_set_header X-Forwarded-Proto https;