Do not include :code in the storage proof if not required (#480)

This is basically a hack that prevents the inclusion of `:code` in the
storage proof. A proper fix requires some changes to the trie crate to
not include nodes that aren't read.

client/collator/src/lib.rs

@@ -333,6 +333,8 @@ mod tests {
 	use polkadot_overseer::{AllSubsystems, HeadSupportsParachains, Overseer};
 	use sp_consensus::BlockOrigin;
 	use sp_core::{testing::TaskExecutor, Pair};
+	use sp_runtime::traits::BlakeTwo256;
+	use sp_state_machine::Backend;
 
 	struct AlwaysSupportsParachains;
 	impl HeadSupportsParachains for AlwaysSupportsParachains {
@@ -376,7 +378,7 @@ mod tests {
 	}
 
 	#[test]
-	fn collates_produces_a_block() {
+	fn collates_produces_a_block_and_storage_proof_does_not_contains_code() {
 		let _ = env_logger::try_init();
 
 		let spawner = TaskExecutor::new();
@@ -432,8 +434,21 @@ mod tests {
 
 		let block_data = collation.proof_of_validity.block_data;
 
-		let block = Block::decode(&mut &block_data.0[..]).expect("Is a valid block");
+		let block =
+			ParachainBlockData::<Block>::decode(&mut &block_data.0[..]).expect("Is a valid block");
 
 		assert_eq!(1, *block.header().number());
+
+		// Ensure that we did not include `:code` in the proof.
+		let db = block.storage_proof().clone().into_memory_db();
+
+		let backend =
+			sp_state_machine::new_in_mem::<BlakeTwo256>().update_backend(*header.state_root(), db);
+
+		// Should return an error, as it was not included while building the proof.
+		assert!(backend
+			.storage(sp_core::storage::well_known_keys::CODE)
+			.unwrap_err()
+			.contains("Trie lookup error: Database missing expected key"));
 	}
 }