Downward Message Processing implementation (#1859)

* DMP: data structures and plumbing

* DMP: Implement DMP logic in the router module

DMP: Integrate DMP parts into the inclusion module

* DMP: Introduce the max size limit for the size of a downward message

* DMP: Runtime API for accessing inbound messages

* OCD

Small clean ups

* DMP: fix the naming of the error

* DMP: add caution about a non-existent recipient

polkadot/node/core/backing/src/lib.rs

@@ -682,6 +682,7 @@ impl CandidateBackingJob {
 			erasure_root,
 			new_validation_code: outputs.new_validation_code,
 			head_data: outputs.head_data,
+			processed_downward_messages: outputs.processed_downward_messages,
 		};
 
 		let res = match with_commitments(commitments) {
@@ -977,12 +978,14 @@ mod tests {
 					parent_head: HeadData(vec![7, 8, 9]),
 					block_number: Default::default(),
 					hrmp_mqc_heads: Vec::new(),
+					dmq_mqc_head: Default::default(),
 				},
 				transient: TransientValidationData {
 					max_code_size: 1000,
 					max_head_data_size: 1000,
 					balance: Default::default(),
 					code_upgrade_allowed: None,
+					dmq_length: 0,
 				},
 			};
 
@@ -1159,6 +1162,7 @@ mod tests {
 							upward_messages: Vec::new(),
 							fees: Default::default(),
 							new_validation_code: None,
+							processed_downward_messages: 0,
 						}, test_state.validation_data.persisted),
 					)).unwrap();
 				}
@@ -1278,6 +1282,7 @@ mod tests {
 							upward_messages: Vec::new(),
 							fees: Default::default(),
 							new_validation_code: None,
+							processed_downward_messages: 0,
 						}, test_state.validation_data.persisted),
 					)).unwrap();
 				}
@@ -1416,6 +1421,7 @@ mod tests {
 							upward_messages: Vec::new(),
 							fees: Default::default(),
 							new_validation_code: None,
+							processed_downward_messages: 0,
 						}, test_state.validation_data.persisted),
 					)).unwrap();
 				}
@@ -1571,6 +1577,7 @@ mod tests {
 							upward_messages: Vec::new(),
 							fees: Default::default(),
 							new_validation_code: None,
+							processed_downward_messages: 0,
 						}, test_state.validation_data.persisted),
 					)).unwrap();
 				}