From 5f4f7d976252477470f9b9a8d1a99ec67eb9753b Mon Sep 17 00:00:00 2001
From: Patrick Insinger <patrickinsinger@gmail.com>
Date: Tue, 12 Oct 2021 21:44:21 -0700
Subject: [PATCH] pageserver - PageVersions switch to iterator

---
 .../src/layered_repository/inmemory_layer.rs  |  3 +--
 .../src/layered_repository/page_versions.rs   | 20 +++++++++++--------
 2 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/pageserver/src/layered_repository/inmemory_layer.rs b/pageserver/src/layered_repository/inmemory_layer.rs
index bd1860fd47..7b486b2652 100644
--- a/pageserver/src/layered_repository/inmemory_layer.rs
+++ b/pageserver/src/layered_repository/inmemory_layer.rs
@@ -157,8 +157,7 @@ impl Layer for InMemoryLayer {
             // Scan the page versions backwards, starting from `lsn`.
             let iter = inner
                 .page_versions
-                .get_block_lsn_range(blknum, ..=lsn)
-                .iter()
+                .iter_block_lsn_range(blknum, ..=lsn)
                 .rev();
             for (_entry_lsn, entry) in iter {
                 if let Some(img) = &entry.page_image {
diff --git a/pageserver/src/layered_repository/page_versions.rs b/pageserver/src/layered_repository/page_versions.rs
index 90321f96cd..2d5ce13a56 100644
--- a/pageserver/src/layered_repository/page_versions.rs
+++ b/pageserver/src/layered_repository/page_versions.rs
@@ -21,23 +21,25 @@ impl PageVersions {
     }
 
     /// Get all [`PageVersion`]s in a block
-    pub fn get_block_slice(&self, blknum: u32) -> &[(Lsn, PageVersion)] {
+    pub fn iter_block(&self, blknum: u32) -> BlockVersionIter<'_> {
         self.0
             .get(&blknum)
             .map(VecMap::as_slice)
             .unwrap_or(EMPTY_SLICE)
+            .iter()
     }
 
     /// Get a range of [`PageVersions`] in a block
-    pub fn get_block_lsn_range<R: RangeBounds<Lsn>>(
+    pub fn iter_block_lsn_range<R: RangeBounds<Lsn>>(
         &self,
         blknum: u32,
         range: R,
-    ) -> &[(Lsn, PageVersion)] {
+    ) -> BlockVersionIter<'_> {
         self.0
             .get(&blknum)
             .map(|vec_map| vec_map.slice_range(range))
             .unwrap_or(EMPTY_SLICE)
+            .iter()
     }
 
     /// Iterate through [`PageVersion`]s in (block, lsn) order.
@@ -46,21 +48,23 @@ impl PageVersions {
         let mut ordered_blocks: Vec<u32> = self.0.keys().cloned().collect();
         ordered_blocks.sort_unstable();
 
-        let slice = ordered_blocks
+        let iter = ordered_blocks
             .first()
-            .map(|&blknum| self.get_block_slice(blknum))
-            .unwrap_or(EMPTY_SLICE);
+            .map(|&blknum| self.iter_block(blknum))
+            .unwrap_or_else(|| EMPTY_SLICE.iter());
 
         OrderedPageVersionIter {
             page_versions: self,
             ordered_blocks,
             cur_block_idx: 0,
             cutoff_lsn,
-            cur_slice_iter: slice.iter(),
+            cur_slice_iter: iter,
         }
     }
 }
 
+pub type BlockVersionIter<'a> = std::slice::Iter<'a, (Lsn, PageVersion)>;
+
 pub struct OrderedPageVersionIter<'a> {
     page_versions: &'a PageVersions,
 
@@ -97,7 +101,7 @@ impl<'a> Iterator for OrderedPageVersionIter<'a> {
             let next_block_idx = self.cur_block_idx + 1;
             let blknum: u32 = *self.ordered_blocks.get(next_block_idx)?;
             self.cur_block_idx = next_block_idx;
-            self.cur_slice_iter = self.page_versions.get_block_slice(blknum).iter();
+            self.cur_slice_iter = self.page_versions.iter_block(blknum);
         }
     }
 }