[proxy/tokio-postgres] garbage collection for codec buffers (#12701)

## Problem

A large insert or a large row will cause the codec to allocate a large
buffer. The codec never shrinks the buffer however. LKB-2496

## Summary of changes

1. Introduce a naive GC system for codec buffers
2. Try and reduce copies as much as possible

libs/proxy/tokio-postgres2/src/codec.rs

@@ -1,13 +1,13 @@
 use std::io;
 
-use bytes::{Bytes, BytesMut};
+use bytes::BytesMut;
 use fallible_iterator::FallibleIterator;
 use postgres_protocol2::message::backend;
 use tokio::sync::mpsc::UnboundedSender;
 use tokio_util::codec::{Decoder, Encoder};
 
 pub enum FrontendMessage {
-    Raw(Bytes),
+    Raw(BytesMut),
     RecordNotices(RecordNotices),
 }
 
@@ -17,7 +17,10 @@ pub struct RecordNotices {
 }
 
 pub enum BackendMessage {
-    Normal { messages: BackendMessages },
+    Normal {
+        messages: BackendMessages,
+        ready: bool,
+    },
     Async(backend::Message),
 }
 
@@ -40,11 +43,18 @@ impl FallibleIterator for BackendMessages {
 
 pub struct PostgresCodec;
 
-impl Encoder<Bytes> for PostgresCodec {
+impl Encoder<BytesMut> for PostgresCodec {
     type Error = io::Error;
 
-    fn encode(&mut self, item: Bytes, dst: &mut BytesMut) -> io::Result<()> {
-        dst.extend_from_slice(&item);
+    fn encode(&mut self, item: BytesMut, dst: &mut BytesMut) -> io::Result<()> {
+        // When it comes to request/response workflows, we usually flush the entire write
+        // buffer in order to wait for the response before we send a new request.
+        // Therefore we can avoid the copy and just replace the buffer.
+        if dst.is_empty() {
+            *dst = item;
+        } else {
+            dst.extend_from_slice(&item);
+        }
         Ok(())
     }
 }
@@ -56,6 +66,7 @@ impl Decoder for PostgresCodec {
     fn decode(&mut self, src: &mut BytesMut) -> Result<Option<BackendMessage>, io::Error> {
         let mut idx = 0;
 
+        let mut ready = false;
         while let Some(header) = backend::Header::parse(&src[idx..])? {
             let len = header.len() as usize + 1;
             if src[idx..].len() < len {
@@ -79,6 +90,7 @@ impl Decoder for PostgresCodec {
             idx += len;
 
             if header.tag() == backend::READY_FOR_QUERY_TAG {
+                ready = true;
                 break;
             }
         }
@@ -88,6 +100,7 @@ impl Decoder for PostgresCodec {
         } else {
             Ok(Some(BackendMessage::Normal {
                 messages: BackendMessages(src.split_to(idx)),
+                ready,
             }))
         }
     }