return 503 to /jobs/update if an update is pending

inventory_provider/routes/jobs.py

-from flask import Blueprint, current_app, jsonify
+from flask import Blueprint, current_app, jsonify, Response
 from inventory_provider.tasks import worker
 from inventory_provider.routes import common
+from inventory_provider.tasks.common import get_current_redis, get_latch
 
 routes = Blueprint("inventory-data-job-routes", __name__)
 
@@ -13,6 +14,15 @@ def after_request(resp):
 @routes.route("/update", methods=['GET', 'POST'])
 @common.require_accepts_json
 def update():
+
+    config = current_app.config['INVENTORY_PROVIDER_CONFIG']
+    latch = get_latch(get_current_redis(config))
+    if latch and latch['pending']:
+        return Response(
+            response='an update is already in progress',
+            status=503,
+            mimetype="text/html")
+
     job_ids = worker.launch_refresh_cache_all(
         current_app.config["INVENTORY_PROVIDER_CONFIG"])
     return jsonify(job_ids)