router

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Describe the bug It is currently not possible to set the Vary header when using Rhai scripts, as it's overwritten by the router.

To Reproduce Using the following script and configuration, you are returned the following headers. Notably missing is the potato entry for the Vary header.

fn router_service(service) {
    const response_callback = Fn("process_response");
    service.map_response(response_callback);
}

fn process_response(response) {
    response.headers["vary"] = "potato";
    response.headers["vary2"] = "potato";
}

plugins:
  experimental.rhai:
    filename: "./main.rhai"

Expected behavior Headers set via Rhai are respected over default values.

Desktop (please complete the following information):

• OS: OSX
• Version: v0.9.5

Additional context Alternatively, document headers that are unable to be overridden within documentation.

This may not be a Rhai issue, but a plugin issue: i.e. you would have the same problem if you did this from a rust plugin. I'll confirm that as part of investigating this.

I've verified that this is a "plugin" problem by trying to do the same thing from a rust plugin and getting the same result. I think the solution will be related to the fix for #1284, so we should make sure this use case is also addressed.

After a little digging, I've confirmed that the root cause of the problem is the CORSLayer which is operating in the router.

By default, the layer sets the vary headers as follows: https://docs.rs/tower-http/latest/tower_http/cors/struct.CorsLayer.html#method.vary

Since we are not doing anything to change this behaviour, any vary header set in a response will be clobbered by this.

More digging shows that setting an empty .vary([]) list when creating the CorsLayer means no default value is set and any value set by users is preserved. I'll proceed with a fix based on this.