Limit stagnant checks to a certain amount of entries (#5742)

* Limit number of elements loaded from the stagnant key

This will likely be required if we enable stagnant prunning as currently database has way
too many entries to be prunned in a single iteration

* Fmt run

* Slightly improve logging

* Some more debug nits

* Fmt pass

polkadot/node/core/chain-selection/src/tree.rs

@@ -534,12 +534,28 @@ pub(super) fn approve_block(
 pub(super) fn detect_stagnant<'a, B: 'a + Backend>(
 	backend: &'a B,
 	up_to: Timestamp,
+	max_elements: usize,
 ) -> Result<OverlayedBackend<'a, B>, Error> {
-	let stagnant_up_to = backend.load_stagnant_at_up_to(up_to)?;
+	let stagnant_up_to = backend.load_stagnant_at_up_to(up_to, max_elements)?;
 	let mut backend = OverlayedBackend::new(backend);
 
+	let (min_ts, max_ts) = match stagnant_up_to.len() {
+		0 => (0 as Timestamp, 0 as Timestamp),
+		1 => (stagnant_up_to[0].0, stagnant_up_to[0].0),
+		n => (stagnant_up_to[0].0, stagnant_up_to[n - 1].0),
+	};
+
 	// As this is in ascending order, only the earliest stagnant
 	// blocks will involve heavy viability propagations.
+	gum::debug!(
+		target: LOG_TARGET,
+		?up_to,
+		?min_ts,
+		?max_ts,
+		"Prepared {} stagnant entries for pruning",
+		stagnant_up_to.len()
+	);
+
 	for (timestamp, maybe_stagnant) in stagnant_up_to {
 		backend.delete_stagnant_at(timestamp);
 
@@ -550,12 +566,27 @@ pub(super) fn detect_stagnant<'a, B: 'a + Backend>(
 					entry.viability.approval = Approval::Stagnant;
 				}
 				let is_viable = entry.viability.is_viable();
+				gum::trace!(
+					target: LOG_TARGET,
+					?block_hash,
+					?timestamp,
+					?was_viable,
+					?is_viable,
+					"Found existing stagnant entry"
+				);
 
 				if was_viable && !is_viable {
 					propagate_viability_update(&mut backend, entry)?;
 				} else {
 					backend.write_block_entry(entry);
 				}
+			} else {
+				gum::trace!(
+					target: LOG_TARGET,
+					?block_hash,
+					?timestamp,
+					"Found non-existing stagnant entry"
+				);
 			}
 		}
 	}