fix(scrubber): log even if no refs are found (#11160)

## Problem

Investigate #11159

## Summary of changes

This doesn't fix the issue, but at least we can narrow down the cause
next time it happens by logging ancestor referenced layer cnt even if
it's 0.

Signed-off-by: Alex Chi Z <chi@neon.tech>

Author: skyzh

pageserver/src/tenant/timeline/compaction.rs

@@ -1091,7 +1091,7 @@ impl Timeline {
         let latest_gc_cutoff = self.get_applied_gc_cutoff_lsn();
 
         tracing::info!(
-            "latest_gc_cutoff: {}, pitr cutoff {}",
+            "starting shard ancestor compaction, latest_gc_cutoff: {}, pitr cutoff {}",
             *latest_gc_cutoff,
             self.gc_info.read().unwrap().cutoffs.time
         );
@@ -1120,6 +1120,7 @@ impl Timeline {
                     // Expensive, exhaustive check of keys in this layer: this guards against ShardedRange's calculations being
                     // wrong.  If ShardedRange claims the local page count is zero, then no keys in this layer
                     // should be !is_key_disposable()
+                    // TODO: exclude sparse keyspace from this check, otherwise it will infinitely loop.
                     let range = layer_desc.get_key_range();
                     let mut key = range.start;
                     while key < range.end {

storage_scrubber/src/pageserver_physical_gc.rs

@@ -152,10 +152,8 @@ impl TenantRefAccumulator {
             }
         }
 
-        if !ancestor_refs.is_empty() {
-            tracing::info!(%ttid, "Found {} ancestor refs", ancestor_refs.len());
-            self.ancestor_ref_shards.update(ttid, ancestor_refs);
-        }
+        tracing::info!(%ttid, "Found {} ancestor refs", ancestor_refs.len());
+        self.ancestor_ref_shards.update(ttid, ancestor_refs);
     }
 
     /// Consume Self and return a vector of ancestor tenant shards that should be GC'd, and map of referenced ancestor layers to preserve
@@ -779,7 +777,7 @@ pub async fn pageserver_physical_gc(
 
     let mut summary = GcSummary::default();
     {
-        let timelines = std::pin::pin!(timelines.try_buffered(CONCURRENCY));
+        let timelines = timelines.try_buffered(CONCURRENCY);
         let timelines = timelines.try_flatten();
 
         let timelines = timelines.map_ok(|(ttid, tenant_manifest_arc)| {
@@ -793,8 +791,8 @@ pub async fn pageserver_physical_gc(
                 tenant_manifest_arc,
             )
         });
-        let mut timelines = std::pin::pin!(timelines.try_buffered(CONCURRENCY));
-
+        let timelines = timelines.try_buffered(CONCURRENCY);
+        let mut timelines = std::pin::pin!(timelines);
         // Drain futures for per-shard GC, populating accumulator as a side effect
         while let Some(i) = timelines.next().await {
             summary.merge(i?);