New validation check: warn when ERL instance is created inside of a request handler

[nfriedly]

I'd like to add a validation check that looks for things like this https://stackoverflow.com/questions/68503106/is-it-possible-to-use-app-use-inside-the-if-else-statement-in-express and https://stackoverflow.com/questions/77945846/how-to-add-dynamically-windowms-and-max-value-in-ratelimit-using-express-rate-li/77957023 and then logs a warning that it should be created at the top-level, not in response to a request.

I'm not entirely sure how to do this, though.

• One idea I had was to create (but don't throw) and error and inspect it's stack trace checking to see if we're in an express route handler. But this seems brittle.
  • Draft PR implementing this at feat: check for instance creation while handling a request #438
• Another idea is to just create and store the stack trace, and then if a second instance is created, see if it has the same stack trace as the first. This seems less brittle, but still not great.
  • Also, we normally shut down validation after the first request, and this would require two requests to detect the problem.
• I'm wondering if there's any global state in express or node.js that keeps a list of open requests? If so, we could check it and warn if it's not empty when the instance is created.
• Maybe the value of this could be helpful here?

[nfriedly]

Here's stack traces for top-level instance creation, normal middleware call, and during a request handling:

Error: instance creation
    at rateLimit (file:///config/workspace/express-rate-limit/dist/index.mjs:589:16)
    at file:///config/workspace/erl-test-server/server.js:9:5
    at ModuleJob.run (node:internal/modules/esm/module_job:217:25)
    at async ModuleLoader.import (node:internal/modules/esm/loader:316:24)
    at async loadESM (node:internal/process/esm_loader:34:7)
    at async handleMainPromise (node:internal/modules/run_main:66:12) { this: undefined }
ERL test server listening at on http://localhost:3000/
Error: call middleware
    at file:///config/workspace/express-rate-limit/dist/index.mjs:596:20
    at file:///config/workspace/express-rate-limit/dist/index.mjs:583:27
    at Layer.handle [as handle_request] (/config/workspace/erl-test-server/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/config/workspace/erl-test-server/node_modules/express/lib/router/index.js:328:13)
    at /config/workspace/erl-test-server/node_modules/express/lib/router/index.js:286:9
    at Function.process_params (/config/workspace/erl-test-server/node_modules/express/lib/router/index.js:346:12)
    at next (/config/workspace/erl-test-server/node_modules/express/lib/router/index.js:280:10)
    at expressInit (/config/workspace/erl-test-server/node_modules/express/lib/middleware/init.js:40:5)
    at Layer.handle [as handle_request] (/config/workspace/erl-test-server/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/config/workspace/erl-test-server/node_modules/express/lib/router/index.js:328:13) { this: undefined }
Error: request handler
    at file:///config/workspace/erl-test-server/server.js:14:18
    at Layer.handle [as handle_request] (/config/workspace/erl-test-server/node_modules/express/lib/router/layer.js:95:5)
    at next (/config/workspace/erl-test-server/node_modules/express/lib/router/route.js:144:13)
    at Route.dispatch (/config/workspace/erl-test-server/node_modules/express/lib/router/route.js:114:3)
    at Layer.handle [as handle_request] (/config/workspace/erl-test-server/node_modules/express/lib/router/layer.js:95:5)
    at /config/workspace/erl-test-server/node_modules/express/lib/router/index.js:284:15
    at Function.process_params (/config/workspace/erl-test-server/node_modules/express/lib/router/index.js:346:12)
    at next (/config/workspace/erl-test-server/node_modules/express/lib/router/index.js:280:10)
    at file:///config/workspace/express-rate-limit/dist/index.mjs:669:7
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) { this: undefined }

Looking for Layer.handle in the stack trace on instance creation might be a good indication that we're in a request handler.

I also logged this, but it's undefined in each of them, so that's not helpful.

[gamemaker1]

Layer.handle sounds good if we can't use the router stack, nice find!

[gamemaker1]

I'm wondering if there's any global state in express or node.js that keeps a list of open requests? If so, we could check it and warn if it's not empty when the instance is created.

Maybe we could use app._router.stack?

[nfriedly]

I don't think we have access to app at instance creation time :(

[gamemaker1]

Oh right sorry!