Optional upstream?

This might only make sense for the timeout toxic used with timeout=0, but wouldn't it be nice if it could work without a responding upstream?

I tried with a "dummy" upstream, just picking a port where nothing was listening

[
  {
    "name": "timeout_dummy",
    "listen": "127.0.0.1:8080",
    "upstream": "127.0.0.1:9999"
  }
]

$ toxiproxy-cli toxic add timeout_dummy -t timeout -a timeout=0
Added downstream timeout toxic 'timeout_downstream' on proxy 'timeout_dummy'

But toxiproxy didn't like that

$ curl -o /dev/null -s -v -m 3 http://127.0.0.1:8080
* Rebuilt URL to: http://127.0.0.1:8080/
*   Trying 127.0.0.1...
* Connected to 127.0.0.1 (127.0.0.1) port 8080 (#0)
> GET / HTTP/1.1
> Host: 127.0.0.1:8080
> User-Agent: curl/7.43.0
> Accept: */*
>
* Empty reply from server
* Connection #0 to host 127.0.0.1 left intact

$ toxiproxy-server -config tmp/toxiproy.json
INFO[0000] Started proxy                                 name=timeout_dummy proxy=127.0.0.1:8080 upstream=127.0.0.1:9999
INFO[0000] Populated proxies from file                   config=tmp/toxiproy.json proxies=1
INFO[0000] API HTTP server starting                      host=localhost port=8474 version=2.1.0
INFO[0004] Accepted client                               client=127.0.0.1:62301 name=timeout_dummy proxy=127.0.0.1:8080 upstream=127.0.0.1:9999
ERRO[0004] Unable to open connection to upstream         client=127.0.0.1:62301 name=timeout_dummy proxy=127.0.0.1:8080 upstream=127.0.0.1:9999

I was actually able to workaround this by setting the upstream to toxiproxy itself ("upstream": "127.0.0.1:8474"):

$ toxiproxy-server -config tmp/toxiproy.json
INFO[0000] Started proxy                                 name=timeout_dummy proxy=127.0.0.1:8080 upstream=127.0.0.1:8474
INFO[0000] Populated proxies from file                   config=tmp/toxiproy.json proxies=1
INFO[0000] API HTTP server starting                      host=localhost port=8474 version=2.1.0
INFO[0017] Accepted client                               client=127.0.0.1:62254 name=timeout_dummy proxy=127.0.0.1:8080 upstream=127.0.0.1:8474
WARN[0020] Source terminated                             bytes=176 err=read tcp 127.0.0.1:62255->127.0.0.1:8474: use of closed network connection name=timeout_dummy

$ curl -o /dev/null -s -v -m 3 http://127.0.0.1:8080
* Rebuilt URL to: http://127.0.0.1:8080/
*   Trying 127.0.0.1...
* Connected to 127.0.0.1 (127.0.0.1) port 8080 (#0)
> GET / HTTP/1.1
> Host: 127.0.0.1:8080
> User-Agent: curl/7.43.0
> Accept: */*
>
* Operation timed out after 3002 milliseconds with 0 bytes received
* Closing connection 0

WDYT about the workaround, might be enough that that works?

Start with the timeout toxic documentation and reproduce the reported timeout=0 behavior using the dummy upstream and the toxiproxy API port workaround. Trace how the proxy handles an unreachable upstream, then define and test the desired behavior for a timeout toxic that does not require a responding upstream.