Better handling extended protocol messages in the event of busy pool (#155)

* Better handling for checkout errors during extended protocol messages

* Fix specs

* comment

Author: drdrsh

src/client.rs

@@ -600,19 +600,32 @@ where
                 message_result = read_message(&mut self.read) => message_result?
             };
 
-            // Avoid taking a server if the client just wants to disconnect.
-            if message[0] as char == 'X' {
-                debug!("Client disconnecting");
-                return Ok(());
-            }
-
             // Handle admin database queries.
             if self.admin {
                 debug!("Handling admin command");
                 handle_admin(&mut self.write, message, self.client_server_map.clone()).await?;
                 continue;
             }
 
+            match message[0] as char {
+                // Buffer extended protocol messages even if we do not have
+                // a server connection yet. Hopefully, when we get the S message
+                // we'll be able to allocate a connection. Also, clients do not expect
+                // the server to respond to these messages so even if we were not able to
+                // allocate a connection, we wouldn't be able to send back an error message
+                // to the client so we buffer them and defer the decision to error out or not
+                // to when we get the S message
+                'P' | 'B' | 'D' | 'E' => {
+                    self.buffer.put(&message[..]);
+                    continue;
+                }
+                'X' => {
+                    debug!("Client disconnecting");
+                    return Ok(());
+                }
+                _ => (),
+            }
+
             // Get a pool instance referenced by the most up-to-date
             // pointer. This ensures we always read the latest config
             // when starting a query.
@@ -714,22 +727,17 @@ where
                     conn
                 }
                 Err(err) => {
-                    // 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_response(
-                                &mut self.write,
-                                "could not get connection from the pool",
-                            )
-                            .await?;
-                        }
-                    };
-
+                    // Client is attempting to get results from the server,
+                    // but we were unable to grab a connection from the pool
+                    // We'll send back an error message and clean the extended
+                    // protocol buffer
+                    if message[0] as char == 'S' {
+                        error!("Got Sync message but failed to get a connection from the pool");
+                        self.buffer.clear();
+                    }
+                    error_response(&mut self.write, "could not get connection from the pool")
+                        .await?;
                     error!("Could not get connection from pool: {:?}", err);
-
                     continue;
                 }
             };