Toxic with toxicity: 0 still affects proxy

Contrary to my expectations, it seems that when the toxicity of a reset_peer toxic is set to 0, the toxic still has a residual impact on the proxy.

To pinpoint the issue, I have created a single toxic with toxicity: 0:

{
  "name": "connection_reset",
  "type": "reset_peer",
  "stream": "downstream",
  "toxicity": 0
}

Despite the toxicity set to 0, we can still see some exceptions thrown during the test (~1 every 10 seconds). This only happens when using toxiproxy.

The README states

toxicity: probability of the toxic being applied to a link

A toxicity value of 0 then means that the toxic has no probability of being applied, suggesting that it is the same as not having the toxic defined at all. However, this was not what I observed.

Evidence

To make sure the exceptions were not caused by my server, I conducted a test where I created a single toxic with toxicity: 0, followed by the deletion of the toxic. I repeated this process many times, with 5-minute waits between each create and delete operation.

1. Create the toxic

curl -X POST http://localhost:63264/proxies/my-proxy/toxics \
  -H "Content-Type: application/json" \
  -d '{
        "name": "connection_reset",
        "type": "reset_peer",
        "stream": "downstream",
        "toxicity": 0
      }'

2. Delete the toxic

curl -X DELETE http://localhost:63264/proxies/my-proxy/toxics/connection_reset

When the toxic is deleted, the proxy operates without issue. However, when it is introduced with toxicity set to 0, it causes some requests to fail with "Connection reset" errors. The chart below highlights this difference.

Conclusion

Based on the documentation, I expected that a toxicity of 0 would mean the toxic would have no effect, but the observed behavior suggests that toxicity: 0 does not completely disable the toxic.

Start by reproducing the issue with the provided curl commands: create a downstream reset_peer toxic with toxicity 0, observe proxy requests, then delete it for comparison. Trace how reset_peer toxicity is handled and add or update coverage for zero toxicity. Done means toxicity 0 produces no more connection resets than having no toxic.