The experimental mirroring feature used a lot of memory and CPU when put under production traffic. This change attempts to reduce memory and CPU usage.
Memory footprint is reduced by making the channel smaller. CPU usage is reduced by avoiding allocations if the channel is full or is closed.
We might lose more messages this way if the mirror falls behind but that is more acceptable than crashing the entire process when it goes out-of-memory (OOM)
This is an implementation of Query mirroring in PgCat (outlined here #302)
In configs, we match mirror hosts with the servers handling the traffic. A mirror host will receive the same protocol messages as the main server it was matched with.
This is done by creating an async task for each mirror server, it communicates with the main server through two channels, one for the protocol messages and one for the exit signal. The mirror server sends the protocol packets to the underlying PostgreSQL server. We receive from the underlying PostgreSQL server as soon as the data is available and we immediately discard it. We use bb8 to manage the life cycle of the connection, not for pooling since each mirror server handler is more or less single-threaded.
We don't have any connection pooling in the mirrors. Matching each mirror connection to an actual server connection guarantees that we will not have more connections to any of the mirrors than the parent pool would allow.
