refactor: introduce entry struct

pageserver/src/tenant/storage_layer/inmemory_layer.rs

@@ -82,7 +82,7 @@ pub struct InMemoryLayerInner {
     /// All versions of all pages in the layer are kept here. Indexed
     /// by block number and LSN. The value is an offset into the
     /// ephemeral file where the page version is stored.
-    index: BTreeMap<Key, VecMap<Lsn, u64>>,
+    index: BTreeMap<Key, VecMap<Lsn, InMemoryLayerIndexValue>>,
 
     /// The values are stored in a serialized format in this file.
     /// Each serialized Value is preceded by a 'u32' length field.
@@ -91,6 +91,11 @@ pub struct InMemoryLayerInner {
 
     resource_units: GlobalResourceUnits,
 }
+struct InMemoryLayerIndexValue {
+    /// The offset returned by [`EphemeralFile::write_blob`]
+    /// when we wrote the [`Value`] in [`InMemoryLayerInner::put_value`].
+    pos: u64,
+}
 
 impl std::fmt::Debug for InMemoryLayerInner {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
@@ -274,7 +279,8 @@ impl InMemoryLayer {
         let cursor = inner.file.block_cursor();
         let mut buf = Vec::new();
         for (key, vec_map) in inner.index.iter() {
-            for (lsn, pos) in vec_map.as_slice() {
+            for (lsn, value) in vec_map.as_slice() {
+                let InMemoryLayerIndexValue { pos } = value;
                 let mut desc = String::new();
                 cursor.read_blob_into_buf(*pos, &mut buf, ctx).await?;
                 let val = Value::des(&buf);
@@ -325,7 +331,8 @@ impl InMemoryLayer {
         // Scan the page versions backwards, starting from `lsn`.
         if let Some(vec_map) = inner.index.get(&key) {
             let slice = vec_map.slice_range(lsn_range);
-            for (entry_lsn, pos) in slice.iter().rev() {
+            for (entry_lsn, value) in slice.iter().rev() {
+                let InMemoryLayerIndexValue { pos } = value;
                 let buf = reader.read_blob(*pos, &ctx).await?;
                 let value = Value::des(&buf)?;
                 match value {
@@ -384,7 +391,8 @@ impl InMemoryLayer {
 
                 let slice = vec_map.slice_range(lsn_range);
 
-                for (entry_lsn, pos) in slice.iter().rev() {
+                for (entry_lsn, value) in slice.iter().rev() {
+                    let InMemoryLayerIndexValue { pos } = value;
                     // TODO: this uses the page cache => https://github.com/neondatabase/neon/issues/8183
                     let buf = reader.read_blob(*pos, &ctx).await;
                     if let Err(e) = buf {
@@ -516,9 +524,10 @@ impl InMemoryLayer {
                 )
                 .await?
         };
+        let entry = InMemoryLayerIndexValue { pos: off };
 
         let vec_map = locked_inner.index.entry(key).or_default();
-        let old = vec_map.append_or_update_last(lsn, off).unwrap().0;
+        let old = vec_map.append_or_update_last(lsn, entry).unwrap().0;
         if old.is_some() {
             // We already had an entry for this LSN. That's odd..
             warn!("Key {} at {} already exists", key, lsn);
@@ -640,7 +649,8 @@ impl InMemoryLayer {
 
                 for (key, vec_map) in inner.index.iter() {
                     // Write all page versions
-                    for (lsn, pos) in vec_map.as_slice() {
+                    for (lsn, entry) in vec_map.as_slice() {
+                        let InMemoryLayerIndexValue { pos } = entry;
                         cursor.read_blob_into_buf(*pos, &mut buf, &ctx).await?;
                         let will_init = Value::des(&buf)?.will_init();
                         let res;
@@ -673,7 +683,8 @@ impl InMemoryLayer {
 
                 for (key, vec_map) in inner.index.iter() {
                     // Write all page versions
-                    for (lsn, pos) in vec_map.as_slice() {
+                    for (lsn, entry) in vec_map.as_slice() {
+                        let InMemoryLayerIndexValue { pos } = entry;
                         // TODO: once we have blob lengths in the in-memory index, we can
                         // 1. get rid of the blob_io / BlockReaderRef::Slice business and
                         // 2. load the file contents into a Bytes and