pageserver: fix read path max lsn bug (#7007)

## Summary of changes
The problem it fixes is when `request_lsn` is `u64::MAX-1` the
`cont_lsn` becomes `u64::MAX` which is the same as `prev_lsn` which
stops the loop.

Closes https://github.com/neondatabase/neon/issues/6812

pageserver/src/tenant.rs

@@ -4625,10 +4625,7 @@ mod tests {
         drop(guard);
 
         // Pick a big LSN such that we query over all the changes.
-        // Technically, u64::MAX - 1 is the largest LSN supported by the read path,
-        // but there seems to be a bug on the non-vectored search path which surfaces
-        // in that case.
-        let reads_lsn = Lsn(u64::MAX - 1000);
+        let reads_lsn = Lsn(u64::MAX - 1);
 
         for read in reads {
             info!("Doing vectored read on {:?}", read);
@@ -5145,4 +5142,23 @@ mod tests {
 
         Ok(())
     }
+
+    #[tokio::test]
+    async fn test_read_at_max_lsn() -> anyhow::Result<()> {
+        let harness = TenantHarness::create("test_read_at_max_lsn")?;
+        let (tenant, ctx) = harness.load().await;
+        let tline = tenant
+            .create_test_timeline(TIMELINE_ID, Lsn(0x08), DEFAULT_PG_VERSION, &ctx)
+            .await?;
+
+        let lsn = Lsn(0x10);
+        bulk_insert_compact_gc(tline.clone(), &ctx, lsn, 50, 10000).await?;
+
+        let test_key = Key::from_hex("010000000033333333444444445500000000").unwrap();
+        let read_lsn = Lsn(u64::MAX - 1);
+
+        assert!(tline.get(test_key, read_lsn, &ctx).await.is_ok());
+
+        Ok(())
+    }
 }

pageserver/src/tenant/timeline.rs

@@ -2478,7 +2478,7 @@ impl Timeline {
         // 'prev_lsn' tracks the last LSN that we were at in our search. It's used
         // to check that each iteration make some progress, to break infinite
         // looping if something goes wrong.
-        let mut prev_lsn = Lsn(u64::MAX);
+        let mut prev_lsn = None;
 
         let mut result = ValueReconstructResult::Continue;
         let mut cont_lsn = Lsn(request_lsn.0 + 1);
@@ -2498,18 +2498,20 @@ impl Timeline {
                         MATERIALIZED_PAGE_CACHE_HIT.inc_by(1);
                         return Ok(traversal_path);
                     }
-                    if prev_lsn <= cont_lsn {
-                        // Didn't make any progress in last iteration. Error out to avoid
-                        // getting stuck in the loop.
-                        return Err(layer_traversal_error(format!(
-                            "could not find layer with more data for key {} at LSN {}, request LSN {}, ancestor {}",
-                            key,
-                            Lsn(cont_lsn.0 - 1),
-                            request_lsn,
-                            timeline.ancestor_lsn
-                        ), traversal_path));
+                    if let Some(prev) = prev_lsn {
+                        if prev <= cont_lsn {
+                            // Didn't make any progress in last iteration. Error out to avoid
+                            // getting stuck in the loop.
+                            return Err(layer_traversal_error(format!(
+                                "could not find layer with more data for key {} at LSN {}, request LSN {}, ancestor {}",
+                                key,
+                                Lsn(cont_lsn.0 - 1),
+                                request_lsn,
+                                timeline.ancestor_lsn
+                            ), traversal_path));
+                        }
                     }
-                    prev_lsn = cont_lsn;
+                    prev_lsn = Some(cont_lsn);
                 }
                 ValueReconstructResult::Missing => {
                     return Err(layer_traversal_error(
@@ -2539,7 +2541,7 @@ impl Timeline {
 
                 timeline_owned = timeline.get_ready_ancestor_timeline(ctx).await?;
                 timeline = &*timeline_owned;
-                prev_lsn = Lsn(u64::MAX);
+                prev_lsn = None;
                 continue 'outer;
             }