Avoid sending Z packet in the middle of extended protocol packet sequence if we fail to get connection from pool (#137)

* Failing test

* maybe

* try fail

* try

* add message

* pool size

* correct user

* more

* debug

* try fix

* see stdout

* stick?

* fix configs

* modify

* types

* m

* maybe

* make tests idempotent

* hopefully fails

* Add client fix

* revert pgcat.toml change

* Fix tests

src/client.rs

@@ -643,9 +643,20 @@ where
                     conn
                 }
                 Err(err) => {
-                    error!("Could not get connection from pool: {:?}", err);
-                    error_response(&mut self.write, "could not get connection from the pool")
-                        .await?;
+                    // Clients do not expect to get SystemError followed by ReadyForQuery in the middle
+                    // of extended protocol submission. So we will hold off on sending the actual error
+                    // message to the client until we get 'S' message
+                    match message[0] as char {
+                        'P' | 'B' | 'E' | 'D' => (),
+                        _ => {
+                            error!("Could not get connection from pool: {:?}", err);
+                            error_response(
+                                &mut self.write,
+                                "could not get connection from the pool",
+                            )
+                            .await?;
+                        }
+                    }
                     continue;
                 }
             };