panic expecting "response receiver is open"

[jameysharp]

I'm sometimes seeing this panic kill a thread that's handling an incoming HTTP request:

thread 'tokio-runtime-worker' panicked at lib/src/session/downstream.rs:52:54:
response receiver is open: ()

Viceroy continues running afterward and responding to further requests.

The panic in question is from expect in this function:

Viceroy/lib/src/session/downstream.rs

Lines 49 to 57 in 8feaf78

	// Mark this `DownstreamResponse` as having been sent, and match on the previous value.
	match mem::replace(self, Sent) {
	Closed => panic!("downstream response channel was closed"),
	Pending(sender) => sender
	.send(response)
	.map_err(|_| ())
	.expect("response receiver is open"),
	Sent => return Err(Error::DownstreamRespSending),
	}

I think this means that the guest service called send multiple times, but it's not clear to me how my service could have done that. If it is due to a bug in my guest code, it would be nice to get a better error message. I tried upgrading my service from fastly crate version 0.9.4 to 0.9.11 in case that mattered, but I saw this panic at both versions.

[jameysharp]

Oh, it looks like this panic is only happening when my browser requests /favicon.ico from my service. My service doesn't look at the path at all and always returns a text/plain response. I'm wondering if the browser sees that, closes the connection early, and then Viceroy gets confused.