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: https://github.com/fastly/Viceroy/blob/8feaf7888250a4429933a7a170759d5b7e91ee53/lib/src/session/downstream.rs#L49-L57

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.