Use the query start block for ReplyChannelRange response messages

C-Lightning versions prior to 0.10 (incorrectly) enforce that the
reply_channel_range first_blocknum field is set to at least the
value they sent in their query_channel_range message. Sending a 0
results in them responding with an Error message, closing open
channels spuriously.

This code is extracted from a previous version of the original
query_channel_range PR in commit
44ba52c. The original commit is by
`bmancini55 <[email protected]>`.

Author: TheBlueMatt

lightning/src/routing/network_graph.rs

@@ -375,17 +375,20 @@ impl<C: Deref , L: Deref > RoutingMessageHandler for NetGraphMsgHandler<C, L> wh
 		let batch_count = batches.len();
 		for (batch_index, batch) in batches.into_iter().enumerate() {
 			// Per spec, the initial first_blocknum needs to be <= the query's first_blocknum and subsequent
-			// must be >= the prior reply. We'll simplify this by using zero since its still spec compliant and
-			// sequence completion is now explicitly.
-			let first_blocknum = 0;
+			// must be >= the prior reply. We'll simplify this by using the query's first_blocknum since its
+			// still spec compliant and sequence completion is now explicit.
+			let first_blocknum = msg.first_blocknum;
 
-			// Per spec, the final end_blocknum needs to be >= the query's end_blocknum, so we'll use the
-			// query's value. Prior batches must use the number of blocks that fit into the message. We'll
-			// base this off the last SCID in the batch since we've somewhat abusing first_blocknum.
+			// Per spec, the last end_block needs to be >= the query's end_blocknum. Last
+			// reply calculates difference between the query's end_blocknum and the start of the reply.
+			// Overflow safe since end_blocknum must be >= msg.first_block_num.
 			let number_of_blocks = if batch_index == batch_count-1 {
-				msg.end_blocknum()
-			} else {
-				block_from_scid(batch.last().unwrap()) + 1
+				msg.end_blocknum() - first_blocknum
+			}
+			// Prior replies should use the number of blocks that fit into the reply. Overflow
+			// safe since first_blocknum is always <= last SCID's block.
+			else {
+				block_from_scid(batch.last().unwrap()) - first_blocknum + 1
 			};
 
 			// Only true for the last message in a sequence
@@ -2235,8 +2238,8 @@ mod tests {
 			vec![
 				ReplyChannelRange {
 					chain_hash: chain_hash.clone(),
-					first_blocknum: 0,
-					number_of_blocks: 0x01000000,
+					first_blocknum: 0xffffff,
+					number_of_blocks: 1,
 					sync_complete: true,
 					short_channel_ids: vec![]
 				},
@@ -2256,8 +2259,8 @@ mod tests {
 			vec![
 				ReplyChannelRange {
 					chain_hash: chain_hash.clone(),
-					first_blocknum: 0,
-					number_of_blocks: 2000,
+					first_blocknum: 1000,
+					number_of_blocks: 1000,
 					sync_complete: true,
 					short_channel_ids: vec![],
 				}
@@ -2277,8 +2280,8 @@ mod tests {
 			vec![
 				ReplyChannelRange {
 					chain_hash: chain_hash.clone(),
-					first_blocknum: 0,
-					number_of_blocks: 0xffffffff,
+					first_blocknum: 0xfe0000,
+					number_of_blocks: 0xffffffff - 0xfe0000,
 					sync_complete: true,
 					short_channel_ids: vec![
 						0xfffffe_ffffff_ffff, // max
@@ -2300,8 +2303,8 @@ mod tests {
 			vec![
 				ReplyChannelRange {
 					chain_hash: chain_hash.clone(),
-					first_blocknum: 0,
-					number_of_blocks: 108000,
+					first_blocknum: 100000,
+					number_of_blocks: 8000,
 					sync_complete: true,
 					short_channel_ids: (100000..=107999)
 						.map(|block| scid_from_parts(block, 0, 0).unwrap())
@@ -2323,17 +2326,17 @@ mod tests {
 			vec![
 				ReplyChannelRange {
 					chain_hash: chain_hash.clone(),
-					first_blocknum: 0,
-					number_of_blocks: 108000,
+					first_blocknum: 100000,
+					number_of_blocks: 8000,
 					sync_complete: false,
 					short_channel_ids: (100000..=107999)
 						.map(|block| scid_from_parts(block, 0, 0).unwrap())
 						.collect(),
 				},
 				ReplyChannelRange {
 					chain_hash: chain_hash.clone(),
-					first_blocknum: 0,
-					number_of_blocks: 108001,
+					first_blocknum: 100000,
+					number_of_blocks: 8001,
 					sync_complete: true,
 					short_channel_ids: vec![
 						scid_from_parts(108000, 0, 0).unwrap(),
@@ -2355,17 +2358,17 @@ mod tests {
 			vec![
 				ReplyChannelRange {
 					chain_hash: chain_hash.clone(),
-					first_blocknum: 0,
-					number_of_blocks: 108002,
+					first_blocknum: 100002,
+					number_of_blocks: 8000,
 					sync_complete: false,
 					short_channel_ids: (100002..=108001)
 						.map(|block| scid_from_parts(block, 0, 0).unwrap())
 						.collect(),
 				},
 				ReplyChannelRange {
 					chain_hash: chain_hash.clone(),
-					first_blocknum: 0,
-					number_of_blocks: 108002,
+					first_blocknum: 100002,
+					number_of_blocks: 8000,
 					sync_complete: true,
 					short_channel_ids: vec![
 						scid_from_parts(108001, 1, 0).unwrap(),