Set page LSN when reconstructing VM in page server (#7935)

## Problem

Page LSN is not set while VM update.
May be reason of test_vm_bits flukyness.
Buit more serious issues can be also caused by wrong LSN.

Related: https://github.com/neondatabase/neon/pull/7935

## Summary of changes

- In `apply_in_neon`, set the LSN bytes when applying records of type
`ClearVisibilityMapFlags`

pageserver/src/tenant.rs

@@ -3992,8 +3992,8 @@ pub(crate) mod harness {
                 let base_img = base_img.expect("Neon WAL redo requires base image").1;
                 let mut page = BytesMut::new();
                 page.extend_from_slice(&base_img);
-                for (_record_lsn, record) in records {
-                    apply_neon::apply_in_neon(&record, key, &mut page)?;
+                for (record_lsn, record) in records {
+                    apply_neon::apply_in_neon(&record, record_lsn, key, &mut page)?;
                 }
                 Ok(page.freeze())
             } else {

pageserver/src/walredo.rs

@@ -361,10 +361,10 @@ impl PostgresRedoManager {
         &self,
         key: Key,
         page: &mut BytesMut,
-        _record_lsn: Lsn,
+        record_lsn: Lsn,
         record: &NeonWalRecord,
     ) -> anyhow::Result<()> {
-        apply_neon::apply_in_neon(record, key, page)?;
+        apply_neon::apply_in_neon(record, record_lsn, key, page)?;
 
         Ok(())
     }

pageserver/src/walredo/apply_neon.rs

@@ -14,6 +14,7 @@ use postgres_ffi::v14::nonrelfile_utils::{
 use postgres_ffi::BLCKSZ;
 use tracing::*;
 use utils::bin_ser::BeSer;
+use utils::lsn::Lsn;
 
 /// Can this request be served by neon redo functions
 /// or we need to pass it to wal-redo postgres process?
@@ -32,6 +33,7 @@ pub(crate) fn can_apply_in_neon(rec: &NeonWalRecord) -> bool {
 
 pub(crate) fn apply_in_neon(
     record: &NeonWalRecord,
+    lsn: Lsn,
     key: Key,
     page: &mut BytesMut,
 ) -> Result<(), anyhow::Error> {
@@ -67,6 +69,7 @@ pub(crate) fn apply_in_neon(
                 let map = &mut page[pg_constants::MAXALIGN_SIZE_OF_PAGE_HEADER_DATA..];
 
                 map[map_byte as usize] &= !(flags << map_offset);
+                postgres_ffi::page_set_lsn(page, lsn);
             }
 
             // Repeat for 'old_heap_blkno', if any
@@ -80,6 +83,7 @@ pub(crate) fn apply_in_neon(
                 let map = &mut page[pg_constants::MAXALIGN_SIZE_OF_PAGE_HEADER_DATA..];
 
                 map[map_byte as usize] &= !(flags << map_offset);
+                postgres_ffi::page_set_lsn(page, lsn);
             }
         }
         // Non-relational WAL records are handled here, with custom code that has the
@@ -285,7 +289,7 @@ mod test {
         let mut page = BytesMut::from_iter(base_image);
 
         for record in deltas {
-            apply_in_neon(&record, file_path, &mut page)?;
+            apply_in_neon(&record, Lsn(8), file_path, &mut page)?;
         }
 
         let reconstructed = AuxFilesDirectory::des(&page)?;