fix: vectored get returns incorrect result on inexact materialized pa…

…ge cache hit (#8050)

# Problem

Suppose our vectored get starts with an inexact materialized page cache
hit ("cached lsn") that is shadowed by a newer image layer image layer.
Like so:


```
    <inmemory layers>

    +-+ < delta layer
    | |
   -|-|----- < image layer
    | |
    | |
   -|-|----- < cached lsn for requested key
    +_+
```

The correct visitation order is
1. inmemory layers
2. delta layer records in LSN range `[image_layer.lsn,
oldest_inmemory_layer.lsn_range.start)`
3. image layer

However, the vectored get code, when it visits the delta layer, it
(incorrectly!) returns with state `Complete`.

The reason why it returns is that it calls `on_lsn_advanced` with
`self.lsn_range.start`, i.e., the layer's LSN range.

Instead, it should use `lsn_range.start`, i.e., the LSN range from the
correct visitation order listed above.

# Solution

Use `lsn_range.start` instead of `self.lsn_range.start`.

# Refs

discovered by & fixes #6967

Co-authored-by: Vlad Lazar <vlad@neon.tech>

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -219,7 +219,6 @@ pub struct DeltaLayerInner {
     // values copied from summary
     index_start_blk: u32,
     index_root_blk: u32,
-    lsn_range: Range<Lsn>,
 
     file: VirtualFile,
     file_id: FileId,
@@ -785,7 +784,6 @@ impl DeltaLayerInner {
             file_id,
             index_start_blk: actual_summary.index_start_blk,
             index_root_blk: actual_summary.index_root_blk,
-            lsn_range: actual_summary.lsn_range,
             max_vectored_read_bytes,
         }))
     }
@@ -911,7 +909,7 @@ impl DeltaLayerInner {
 
         let reads = Self::plan_reads(
             &keyspace,
-            lsn_range,
+            lsn_range.clone(),
             data_end_offset,
             index_reader,
             planner,
@@ -924,7 +922,7 @@ impl DeltaLayerInner {
         self.do_reads_and_update_state(reads, reconstruct_state, ctx)
             .await;
 
-        reconstruct_state.on_lsn_advanced(&keyspace, self.lsn_range.start);
+        reconstruct_state.on_lsn_advanced(&keyspace, lsn_range.start);
 
         Ok(())
     }