feat: add CORS to operations API

functions-python/operations_api/function_config.json

@@ -10,6 +10,9 @@
   "environment_variables": [
     {
       "key": "GOOGLE_CLIENT_ID"
+    },
+    {
+      "key": "CORS_ALLOWED_ORIGINS"
     }
   ],
   "secret_environment_variables": [

functions-python/operations_api/src/main.py

@@ -14,8 +14,11 @@
 #  limitations under the License.
 #
 
+import os
+
 from flask import Request, Response
 from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
 from feeds_gen.apis.operations_api import router as FeedsApiRouter
 from feeds_gen.apis.licenses_api import router as LicenseApiRouter
 from feeds_gen.apis.users_api import router as UsersApiRouter
@@ -35,6 +38,21 @@
 
 # Add here middlewares that should be applied to all routes.
 app.add_middleware(RequestContextMiddleware)
+# Added last so it becomes the outermost ASGI layer (Starlette middlewares wrap
+# in reverse order of registration) and can answer CORS preflight OPTIONS
+# requests before they ever reach RequestContextMiddleware's auth check, which
+# would otherwise 401 every preflight since browsers never send Authorization
+# on an OPTIONS request.
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=[
+        origin.strip()
+        for origin in os.getenv("CORS_ALLOWED_ORIGINS", "*").split(",")
+        if origin.strip()
+    ],
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
 app.include_router(FeedsApiRouter)
 app.include_router(LicenseApiRouter)
 app.include_router(UsersApiRouter)