fix partial send of outbound data

[bentsku]

We got a report from a customer that running a migration with diesel would hang.

After investigating, it seems running an insert query of approximately 25MiB would just hang.

I've tracked down the reason for it: the flow of our proxy is flawed. Basically, we wait on the "receiving" socket, in this migration case, the client, so that it sends data. We then construct our packet, and once this packet is complete, we intercept it and populate the "outbound" packet to be sent to the server (the packet sent by the client and intercepted).
However, we only call Socket.send once there, and do the right thing by slicing the outbound data with what we really sent. However, to get to send again, we need the client to send more data. But the client is waiting for the server to respond, and the server hasn't received anything (or just a partial send, not a full query).

Until we refactor the proxy deeper, the fix consist of iterating over the outbound data until there's none left. I've tested it locally and it solved the issue.

As an alternative solution, using Socket.sendall could also work, and we should then remove the whole logic around slicing the outbound data with the sent length.

[steffyP]

Thanks so much for investigating, @bentsku! 🚀
That's a great catch!