Only calculate tree route during finalization when there are multiple leaves (#14067) (#14069)

* Only calculate tree route when there are multiple leaves

* Update client/service/src/client/client.rs



---------

Co-authored-by: Sebastian Kunert <skunert49@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>

Author: 3 people

client/service/src/client/client.rs

@@ -936,19 +936,24 @@ where
 			return Err(sp_blockchain::Error::NotInFinalizedChain)
 		}
 
-		let route_from_best =
-			sp_blockchain::tree_route(self.backend.blockchain(), best_block, block)?;
-
-		// if the block is not a direct ancestor of the current best chain,
-		// then some other block is the common ancestor.
-		if route_from_best.common_block().hash != block {
-			// NOTE: we're setting the finalized block as best block, this might
-			// be slightly inaccurate since we might have a "better" block
-			// further along this chain, but since best chain selection logic is
-			// plugable we cannot make a better choice here. usages that need
-			// an accurate "best" block need to go through `SelectChain`
-			// instead.
-			operation.op.mark_head(block)?;
+		// If there is only one leaf, best block is guaranteed to be
+		// a descendant of the new finalized block. If not,
+		// we need to check.
+		if self.backend.blockchain().leaves()?.len() > 1 {
+			let route_from_best =
+				sp_blockchain::tree_route(self.backend.blockchain(), best_block, block)?;
+
+			// if the block is not a direct ancestor of the current best chain,
+			// then some other block is the common ancestor.
+			if route_from_best.common_block().hash != block {
+				// NOTE: we're setting the finalized block as best block, this might
+				// be slightly inaccurate since we might have a "better" block
+				// further along this chain, but since best chain selection logic is
+				// plugable we cannot make a better choice here. usages that need
+				// an accurate "best" block need to go through `SelectChain`
+				// instead.
+				operation.op.mark_head(block)?;
+			}
 		}
 
 		let enacted = route_from_finalized.enacted();