Events never show to the browser when using a `ReadableStream` with the `text/event-stream` content type

[hugo]

What version of Remix are you using?

1.5.0

Steps to Reproduce

Create an app using @remix-run/serve that returns a ReadableStream body in a loader, with the content-type text/event-stream. Send events.

On the client side connect to this endpoint using an EventSource.

See this reproduction repository for a full example.

The issue appears to be compression in the Express server. If you cURL the endpoint you'll see events streaming as expected:

curl -N http://localhost:3000/api/events
data: asdfghjkl

data: asdfghjkl

This pointed me to the underlying problem. cURL doesn't do compression by default. If you enable it, with curl --compressed you'll also have a connection that hangs. I'm not sure why but the compression middleware in Express borks streaming responses somehow.

If you edit the "@remix-run/serve" server to disable compression for "text/event-stream" content types, things work as expected:

app.use(compression({filter: (req) => req.headers['content-type']?.startsWith('text/event-stream')}))

(n.b. there may be a preferable way to do this, I'm not sure.)

---

While I'm here I should just add that this new streaming functionality is delightful.

Expected Behavior

Events would start to appear via the EventSource connection.

Actual Behavior

No events appear.

[jangerhofer]

The express compression library documentation explicitly calls out the need to flush entries in an SSE stream. I'm not sure how to square that against the ReadableStream controller's interface, but my first thought is that a middleware in @remix/serve may be able to call flush on application code's behalf.

[jacob-ebey]

The express adapter now calls flush appropriately: https://github.com/remix-run/remix/blob/main/packages/remix-node/stream.ts#L23

If this is still an issue, please re-open the issue or file a new one.