define no-op request/response type inside pagestream & impl server side

libs/pageserver_api/src/models.rs

@@ -572,15 +572,18 @@ pub enum PagestreamFeMessage {
     Nblocks(PagestreamNblocksRequest),
     GetPage(PagestreamGetPageRequest),
     DbSize(PagestreamDbSizeRequest),
+    NoOp,
 }
 
 // Wrapped in libpq CopyData
+#[derive(Debug)]
 pub enum PagestreamBeMessage {
     Exists(PagestreamExistsResponse),
     Nblocks(PagestreamNblocksResponse),
     GetPage(PagestreamGetPageResponse),
     Error(PagestreamErrorResponse),
     DbSize(PagestreamDbSizeResponse),
+    NoOp,
 }
 
 #[derive(Debug, PartialEq, Eq)]
@@ -679,6 +682,10 @@ impl PagestreamFeMessage {
                 bytes.put_u64(req.lsn.0);
                 bytes.put_u32(req.dbnode);
             }
+
+            Self::NoOp => {
+                bytes.put_u8(4);
+            }
         }
 
         bytes.into()
@@ -729,6 +736,7 @@ impl PagestreamFeMessage {
                 lsn: Lsn::from(body.read_u64::<BigEndian>()?),
                 dbnode: body.read_u32::<BigEndian>()?,
             })),
+            4 => Ok(PagestreamFeMessage::NoOp),
             _ => bail!("unknown smgr message tag: {:?}", msg_tag),
         }
     }
@@ -763,6 +771,9 @@ impl PagestreamBeMessage {
                 bytes.put_u8(104); /* tag from pagestore_client.h */
                 bytes.put_i64(resp.db_size);
             }
+            Self::NoOp => {
+                bytes.put_u8(105);
+            }
         }
 
         bytes.into()
@@ -794,6 +805,9 @@ impl PagestreamBeMessage {
                 }))
             }
             104 => todo!(),
+            105 => {
+                Ok(PagestreamBeMessage::NoOp)
+            },
             _ => bail!("unknown tag: {:?}", msg_tag),
         }
     }

pageserver/src/metrics.rs

@@ -760,6 +760,7 @@ pub enum SmgrQueryType {
     GetRelSize,
     GetPageAtLsn,
     GetDbSize,
+    NoOp,
 }
 
 #[derive(Debug)]

pageserver/src/page_service.rs

@@ -491,6 +491,11 @@ impl PageServerHandler {
                         span,
                     )
                 }
+                PagestreamFeMessage::NoOp => {
+                    let _timer = metrics.start_timer(metrics::SmgrQueryType::NoOp);
+                    let span = tracing::info_span!("no_op");
+                    (Ok(PagestreamBeMessage::NoOp), span)
+                }
             };
 
             if let Err(e) = &response {

pgxn/neon/pagestore_client.h

@@ -33,6 +33,7 @@ typedef enum
 	T_NeonNblocksRequest,
 	T_NeonGetPageRequest,
 	T_NeonDbSizeRequest,
+	T_NeonPagestoreNoopRequest,
 
 	/* pagestore -> pagestore_client */
 	T_NeonExistsResponse = 100,
@@ -40,6 +41,7 @@ typedef enum
 	T_NeonGetPageResponse,
 	T_NeonErrorResponse,
 	T_NeonDbSizeResponse,
+	T_NeonPagestoreNoopResponse,
 }			NeonMessageTag;
 
 /* base struct for c-style inheritance */

trace/src/main.rs

@@ -74,6 +74,7 @@ fn analyze_trace<R: std::io::Read>(mut reader: R) {
                 prev = Some(req);
             }
             PagestreamFeMessage::DbSize(_) => {}
+            PagestreamFeMessage::NoOp => {},
         };
     }