feat: support for cancel requests in handshake

proto/src/handshake/errors.rs

@@ -4,6 +4,7 @@ use crate::reader::errors::{ProtoConsumeError, ProtoPeekError, ProtoReadError};
 use crate::writer::errors::ProtoWriteError;
 use thiserror::Error;
 use tokio::io;
+use crate::message::special::CancelRequestData;
 
 #[derive(Debug, Error)]
 pub enum ClientHandshakeError {
@@ -23,6 +24,8 @@ pub enum ClientHandshakeError {
 pub enum ServerHandshakeError {
     #[error("startup message not found")]
     MissingStartupMessage,
+    #[error("cancel request found instead of startup message")]
+    IsCancelRequest(CancelRequestData),
     #[error("socket communication failed")]
     Io(#[from] io::Error),
     #[error("deserialization of inner data failed")]

proto/src/handshake/server.rs

@@ -8,6 +8,7 @@ use crate::writer::backend::BackendProtoWriter;
 use crate::writer::protowriter::ProtoFlush;
 
 /// Performs server-side handshake with the client until ending it with `ReadyForQuery` message.
+/// Client can send `CancelRequest` message instead of `StartupMessage` to cancel the request.
 /// For more info visit the [`55.2.1. Start-up`](https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-FLOW-START-UP)
 pub async fn do_server_handshake(
     writer: &mut (impl BackendProtoWriter + ProtoFlush),
@@ -27,12 +28,16 @@ pub async fn do_server_handshake(
         }
     }
 
-    //  Wait for mandatory StartupMessage
+    //  Wait for mandatory StartupMessage or CancelRequest
     let startup_message = match &reader.peek_special_message().await? {
         Some(msg @ SpecialMessage::StartupMessage(data)) => {
             reader.consume_special_message(msg).await?;
             data.clone()
         }
+        Some(msg @ SpecialMessage::CancelRequest(data)) => {
+            reader.consume_special_message(msg).await?;
+            return Err(ServerHandshakeError::IsCancelRequest(data.clone()));
+        }
         _ => {
             return Err(ServerHandshakeError::MissingStartupMessage);
         }