Fix infinite loop in looking up predecessor layer

Commit 960c7d69a8 changed the LSN returned in the Continue case in
InMemoryLayer::get_page_reconstruct_data(), but neglected to make the
same change in DeltaLayer.

Also add an escape hatch to the loop in materialize_page() to avoid
getting stuck in an infinite loop, if a bug like this reoccurs.

pageserver/src/layered_repository.rs

@@ -1534,6 +1534,15 @@ impl LayeredTimeline {
                 PageReconstructResult::Continue(cont_lsn) => {
                     // Fetch base image / more WAL from the returned predecessor layer
                     if let Some((cont_layer, cont_lsn)) = self.get_layer_for_read(seg, cont_lsn)? {
+                        if cont_lsn == curr_lsn {
+                            // We landed on the same layer again. Shouldn't happen, but if it does,
+                            // don't get stuck in an infinite loop.
+                            bail!(
+                                "could not find predecessor layer of segment {} at {}",
+                                seg.rel,
+                                cont_lsn
+                            );
+                        }
                         layer_arc = cont_layer;
                         layer_ref = &*layer_arc;
                         curr_lsn = cont_lsn;

pageserver/src/layered_repository/delta_layer.rs

@@ -230,7 +230,7 @@ impl Layer for DeltaLayer {
         // If an older page image is needed to reconstruct the page, let the
         // caller know.
         if need_image {
-            Ok(PageReconstructResult::Continue(self.start_lsn))
+            Ok(PageReconstructResult::Continue(Lsn(self.start_lsn.0 - 1)))
         } else {
             Ok(PageReconstructResult::Complete)
         }