pageserver: only zero truncated FSM page on owning shard (#10032)

## Problem

FSM pages are managed like regular relation pages, and owned by a single
shard. However, when truncating the FSM relation the last FSM page was
zeroed out on all shards. This is unnecessary and potentially confusing.

The superfluous keys will be removed during compactions, as they do not
belong on these shards.

Resolves #10027.

## Summary of changes

Only zero out the truncated FSM page on the owning shard.

pageserver/src/walingest.rs

@@ -582,18 +582,21 @@ impl WalIngest {
                 forknum: FSM_FORKNUM,
             };
 
+            // Zero out the last remaining FSM page, if this shard owns it. We are not precise here,
+            // and instead of digging in the FSM bitmap format we just clear the whole page.
             let fsm_logical_page_no = blkno / pg_constants::SLOTS_PER_FSM_PAGE;
             let mut fsm_physical_page_no = fsm_logical_to_physical(fsm_logical_page_no);
-            if blkno % pg_constants::SLOTS_PER_FSM_PAGE != 0 {
-                // Tail of last remaining FSM page has to be zeroed.
-                // We are not precise here and instead of digging in FSM bitmap format just clear the whole page.
+            if blkno % pg_constants::SLOTS_PER_FSM_PAGE != 0
+                && self
+                    .shard
+                    .is_key_local(&rel_block_to_key(rel, fsm_physical_page_no))
+            {
                 modification.put_rel_page_image_zero(rel, fsm_physical_page_no)?;
                 fsm_physical_page_no += 1;
             }
-            // TODO: re-examine the None case here wrt. sharding; should we error?
+            // Truncate this shard's view of the FSM relation size, if it even has one.
             let nblocks = get_relsize(modification, rel, ctx).await?.unwrap_or(0);
             if nblocks > fsm_physical_page_no {
-                // check if something to do: FSM is larger than truncate position
                 self.put_rel_truncation(modification, rel, fsm_physical_page_no, ctx)
                     .await?;
             }
@@ -617,7 +620,7 @@ impl WalIngest {
             // tail bits in the last remaining map page, representing truncated heap
             // blocks, need to be cleared. This is not only tidy, but also necessary
             // because we don't get a chance to clear the bits if the heap is extended
-            // again.
+            // again. Only do this on the shard that owns the page.
             if (trunc_byte != 0 || trunc_offs != 0)
                 && self.shard.is_key_local(&rel_block_to_key(rel, vm_page_no))
             {
@@ -631,10 +634,9 @@ impl WalIngest {
                 )?;
                 vm_page_no += 1;
             }
-            // TODO: re-examine the None case here wrt. sharding; should we error?
+            // Truncate this shard's view of the VM relation size, if it even has one.
             let nblocks = get_relsize(modification, rel, ctx).await?.unwrap_or(0);
             if nblocks > vm_page_no {
-                // check if something to do: VM is larger than truncate position
                 self.put_rel_truncation(modification, rel, vm_page_no, ctx)
                     .await?;
             }