f use bits 6 and 7 as joint descriptions of additional data length

Author: arik-so

lightning-rapid-gossip-sync/src/processing.rs

@@ -183,13 +183,25 @@ impl<NG: Deref<Target=NetworkGraph<L>>, L: Deref> RapidGossipSync<NG, L> where L
 					node_modifications.insert(current_pubkey, synthetic_node_announcement);
 				}
 
-				let has_additional_data = additional_data_marker > 0;
 				if additional_data_marker > 0 {
-					// 1 => u8, 2 => u16, 3 => u32
-					let _length_prefix_bytes = 1u8 << (additional_data_marker - 1);
-					// let _length_prefix = match length_prefix_bytes {
-					//
-					// };
+					let additional_byte_count: u64 = match additional_data_marker {
+						1 => {
+							u8::read(read_cursor)? as u64
+						}
+						2 => {
+							u16::read(read_cursor)? as u64
+						}
+						3 => {
+							u32::read(read_cursor)? as u64
+						}
+						_ => {
+							return Err(DecodeError::BadLengthDescriptor.into());
+						}
+					};
+
+					let mut additional_data_reader = FixedLengthReader::new(&mut read_cursor, additional_byte_count);
+					let mut additional_data = Vec::new();
+					additional_data_reader.read_to_end(&mut additional_data)?;
 				}
 
 				if has_additional_data {