fix(pageserver): ensure initial image layers have correct key ranges (#10374)

## Problem

Discovered during the relation dir refactor work.

If we do not create images as in this patch, we would get two set of
image layers:

```
0000...METADATA_KEYS
0000...REL_KEYS
```

They overlap at the same LSN and would cause data loss for relation
keys. This doesn't happen in prod because initial image layer generation
is never called, but better to be fixed to avoid future issues with the
reldir refactors.

## Summary of changes

* Consolidate create_image_layers call into a single one.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/tenant/timeline.rs

@@ -3781,36 +3781,35 @@ impl Timeline {
                 return Err(FlushLayerError::Cancelled);
             }
 
-            let mut layers_to_upload = Vec::new();
-            layers_to_upload.extend(
-                self.create_image_layers(
-                    &rel_partition,
-                    self.initdb_lsn,
-                    ImageLayerCreationMode::Initial,
-                    ctx,
-                )
-                .await?,
-            );
+            // Ensure that we have a single call to `create_image_layers` with a combined dense keyspace.
+            // So that the key ranges don't overlap.
+            let mut partitions = KeyPartitioning::default();
+            partitions.parts.extend(rel_partition.parts);
             if !metadata_partition.parts.is_empty() {
                 assert_eq!(
                     metadata_partition.parts.len(),
                     1,
                     "currently sparse keyspace should only contain a single metadata keyspace"
                 );
-                layers_to_upload.extend(
-                    self.create_image_layers(
-                        // Safety: create_image_layers treat sparse keyspaces differently that it does not scan
-                        // every single key within the keyspace, and therefore, it's safe to force converting it
-                        // into a dense keyspace before calling this function.
-                        &metadata_partition.into_dense(),
-                        self.initdb_lsn,
-                        ImageLayerCreationMode::Initial,
-                        ctx,
-                    )
-                    .await?,
-                );
+                // Safety: create_image_layers treat sparse keyspaces differently that it does not scan
+                // every single key within the keyspace, and therefore, it's safe to force converting it
+                // into a dense keyspace before calling this function.
+                partitions
+                    .parts
+                    .extend(metadata_partition.into_dense().parts);
             }
 
+            let mut layers_to_upload = Vec::new();
+            layers_to_upload.extend(
+                self.create_image_layers(
+                    &partitions,
+                    self.initdb_lsn,
+                    ImageLayerCreationMode::Initial,
+                    ctx,
+                )
+                .await?,
+            );
+
             (layers_to_upload, None)
         } else {
             // Normal case, write out a L0 delta layer file.