add flush to ExtendedQueryHandler (#220)

`Flush` is part of https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY

src/api/query.rs

@@ -17,7 +17,7 @@ use crate::api::PgWireConnectionState;
 use crate::error::{PgWireError, PgWireResult};
 use crate::messages::data::{NoData, ParameterDescription};
 use crate::messages::extendedquery::{
-    Bind, BindComplete, Close, CloseComplete, Describe, Execute, Parse, ParseComplete,
+    Bind, BindComplete, Close, CloseComplete, Describe, Execute, Flush, Parse, ParseComplete,
     Sync as PgSync, TARGET_TYPE_BYTE_PORTAL, TARGET_TYPE_BYTE_STATEMENT,
 };
 use crate::messages::response::{EmptyQueryResponse, ReadyForQuery, TransactionStatus};
@@ -300,6 +300,19 @@ pub trait ExtendedQueryHandler: Send + Sync {
         Ok(())
     }
 
+    /// Called when client sends `flush` command.
+    ///
+    /// The default implementation flushes client buffer
+    async fn on_flush<C>(&self, client: &mut C, _message: Flush) -> PgWireResult<()>
+    where
+        C: ClientInfo + Sink<PgWireBackendMessage> + Unpin + Send + Sync,
+        C::Error: Debug,
+        PgWireError: From<<C as Sink<PgWireBackendMessage>>::Error>,
+    {
+        client.flush().await?;
+        Ok(())
+    }
+
     /// Called when client sends `sync` command.
     ///
     /// The default implementation flushes client buffer and sends

src/tokio/server.rs

@@ -216,6 +216,9 @@ where
                 PgWireFrontendMessage::Describe(describe) => {
                     extended_query_handler.on_describe(socket, describe).await?;
                 }
+                PgWireFrontendMessage::Flush(flush) => {
+                    extended_query_handler.on_flush(socket, flush).await?;
+                }
                 PgWireFrontendMessage::Sync(sync) => {
                     extended_query_handler.on_sync(socket, sync).await?;
                 }