pageserver: tweak logs for index_part loading (#6005)

## Problem

On pageservers upgraded to enable generations, these INFO level logs
were rather frequent. If a tenant timeline hasn't written new layers
since the upgrade, it will emit the "No index_part.json*" log every time
it starts.

## Summary of changes

- Downgrade two log lines from info to debug
- Add a tiny unit test that I wrote for sanity-checking that there
wasn't something wrong with our Generation-comparing logic when loading
index parts.

libs/remote_storage/src/s3_bucket.rs

@@ -378,7 +378,7 @@ impl RemoteStorage for S3Bucket {
             let empty = Vec::new();
             let prefixes = response.common_prefixes.as_ref().unwrap_or(&empty);
 
-            tracing::info!("list: {} prefixes, {} keys", prefixes.len(), keys.len());
+            tracing::debug!("list: {} prefixes, {} keys", prefixes.len(), keys.len());
 
             for object in keys {
                 let object_path = object.key().expect("response does not contain a key");

libs/utils/src/generation.rs

@@ -152,3 +152,16 @@ impl Debug for Generation {
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn generation_gt() {
+        // Important that a None generation compares less than a valid one, during upgrades from
+        // pre-generation systems.
+        assert!(Generation::none() < Generation::new(0));
+        assert!(Generation::none() < Generation::new(1));
+    }
+}

pageserver/src/tenant/remote_timeline_client/download.rs

@@ -363,7 +363,7 @@ pub(super) async fn download_index_part(
         None => {
             // Migration from legacy pre-generation state: we have a generation but no prior
             // attached pageservers did.  Try to load from a no-generation path.
-            tracing::info!("No index_part.json* found");
+            tracing::debug!("No index_part.json* found");
             do_download_index_part(
                 storage,
                 tenant_shard_id,