Implement backpressure for compute node to avoid WAL overflow

Co-authored-by: Arseny Sher <sher-ars@yandex.ru>
Co-authored-by: Alexey Kondratov <kondratov.aleksey@gmail.com>

zenith_utils/src/pq_proto.rs

@@ -358,6 +358,7 @@ pub enum BeMessage<'a> {
     RowDescription(&'a [RowDescriptor<'a>]),
     XLogData(XLogDataBody<'a>),
     NoticeResponse(String),
+    KeepAlive(WalSndKeepAlive),
 }
 
 // One row desciption in RowDescription packet.
@@ -409,6 +410,13 @@ pub struct XLogDataBody<'a> {
     pub data: &'a [u8],
 }
 
+#[derive(Debug)]
+pub struct WalSndKeepAlive {
+    pub sent_ptr: u64,
+    pub timestamp: i64,
+    pub request_reply: bool,
+}
+
 pub static HELLO_WORLD_ROW: BeMessage = BeMessage::DataRow(&[Some(b"hello world")]);
 
 // single text column
@@ -721,6 +729,18 @@ impl<'a> BeMessage<'a> {
                 })
                 .unwrap();
             }
+
+            BeMessage::KeepAlive(req) => {
+                buf.put_u8(b'd');
+                write_body(buf, |buf| {
+                    buf.put_u8(b'k');
+                    buf.put_u64(req.sent_ptr);
+                    buf.put_i64(req.timestamp);
+                    buf.put_u8(if req.request_reply { 1u8 } else { 0u8 });
+                    Ok::<_, io::Error>(())
+                })
+                .unwrap();
+            }
         }
         Ok(())
     }