Add aggregate function for multi-kernel transaction (#966)

* Add aggregate function

* Rustfmt

* Add multikernel tx test

* Rustfmt

* Add test bad multi-kernel transaction

* Add sorting

* Modified multikernel test

core/src/core/mod.rs

@@ -353,26 +353,7 @@ mod test {
 		assert!(tx2.validate().is_ok());
 
 		// now build a "cut_through" tx from tx1 and tx2
-		let mut tx3 = tx1.clone();
-		tx3.inputs.extend(tx2.inputs.iter().cloned());
-		tx3.outputs.extend(tx2.outputs.iter().cloned());
-		tx3.kernels.extend(tx2.kernels.iter().cloned());
-
-		// make sure everything is sorted
-		tx3.inputs.sort();
-		tx3.outputs.sort();
-		tx3.kernels.sort();
-
-		// finally sum the offsets up
-		// TODO - hide this in a convenience function somewhere
-		tx3.offset = {
-			let secp = static_secp_instance();
-			let secp = secp.lock().unwrap();
-			let skey1 = tx1.offset.secret_key(&secp).unwrap();
-			let skey2 = tx2.offset.secret_key(&secp).unwrap();
-			let skey3 = secp.blind_sum(vec![skey1, skey2], vec![]).unwrap();
-			BlindingFactor::from_secret_key(skey3)
-		};
+		let tx3 = aggregate(vec![tx1, tx2]).unwrap();
 
 		assert!(tx3.validate().is_ok());
 	}

core/src/core/transaction.rs

@@ -16,6 +16,7 @@
 use util::secp::{self, Message, Signature};
 use util::{kernel_sig_msg, static_secp_instance};
 use util::secp::pedersen::{Commitment, ProofMessage, RangeProof};
+use std::collections::HashSet;
 use std::cmp::max;
 use std::cmp::Ordering;
 use std::{error, fmt};
@@ -488,6 +489,80 @@ impl Transaction {
 	}
 }
 
+/// Aggregate a vec of transactions into a multi-kernel transaction
+pub fn aggregate(transactions: Vec<Transaction>) -> Result<Transaction, Error> {
+	let mut inputs: Vec<Input> = vec![];
+	let mut outputs: Vec<Output> = vec![];
+	let mut kernels: Vec<TxKernel> = vec![];
+
+	// we will sum these together at the end to give us the overall offset for the
+	// transaction
+	let mut kernel_offsets = vec![];
+
+	for mut transaction in transactions {
+		// we will summ these later to give a single aggregate offset
+		kernel_offsets.push(transaction.offset);
+
+		inputs.append(&mut transaction.inputs);
+		outputs.append(&mut transaction.outputs);
+		kernels.append(&mut transaction.kernels);
+	}
+
+	// now sum the kernel_offsets up to give us an aggregate offset for the
+	// transaction
+	let total_kernel_offset = {
+		let secp = static_secp_instance();
+		let secp = secp.lock().unwrap();
+		let mut keys = kernel_offsets
+			.iter()
+			.cloned()
+			.filter(|x| *x != BlindingFactor::zero())
+			.filter_map(|x| x.secret_key(&secp).ok())
+			.collect::<Vec<_>>();
+
+		if keys.is_empty() {
+			BlindingFactor::zero()
+		} else {
+			let sum = secp.blind_sum(keys, vec![])?;
+			BlindingFactor::from_secret_key(sum)
+		}
+	};
+
+	let in_set = inputs
+		.iter()
+		.map(|inp| inp.commitment())
+		.collect::<HashSet<_>>();
+
+	let out_set = outputs
+		.iter()
+		.filter(|out| !out.features.contains(OutputFeatures::COINBASE_OUTPUT))
+		.map(|out| out.commitment())
+		.collect::<HashSet<_>>();
+
+	let to_cut_through = in_set.intersection(&out_set).collect::<HashSet<_>>();
+
+	let mut new_inputs = inputs
+		.iter()
+		.filter(|inp| !to_cut_through.contains(&inp.commitment()))
+		.cloned()
+		.collect::<Vec<_>>();
+
+	let mut new_outputs = outputs
+		.iter()
+		.filter(|out| !to_cut_through.contains(&out.commitment()))
+		.cloned()
+		.collect::<Vec<_>>();
+
+	// sort them lexicographically
+	new_inputs.sort();
+	new_outputs.sort();
+	kernels.sort();
+
+	let tx = Transaction::new(new_inputs, new_outputs, kernels);
+
+	Ok(tx.with_offset(total_kernel_offset))
+}
+
 /// A transaction input.
 ///
 /// Primarily a reference to an output being spent by the transaction.

pool/src/pool.rs

@@ -860,7 +860,7 @@ mod tests {
 	use core::core::hash::{Hash, Hashed};
 	use core::core::pmmr::MerkleProof;
 	use core::core::target::Difficulty;
-	use core::core::transaction::ProofMessageElements;
+	use core::core::transaction::{self, ProofMessageElements};
 	use types::PoolError::InvalidTx;
 
 	macro_rules! expect_output_parent {
@@ -942,6 +942,106 @@ mod tests {
 		}
 	}
 
+	#[test]
+	/// Attempt to add a multi kernel transaction to the mempool
+	fn test_multikernel_pool_add() {
+		let mut dummy_chain = DummyChainImpl::new();
+		let head_header = block::BlockHeader {
+			height: 1,
+			..block::BlockHeader::default()
+		};
+		dummy_chain.store_head_header(&head_header);
+
+		let parent_transaction = test_transaction(vec![5, 6, 7], vec![11, 3]);
+		// We want this transaction to be rooted in the blockchain.
+		let new_output = DummyOutputSet::empty()
+			.with_output(test_output(5))
+			.with_output(test_output(6))
+			.with_output(test_output(7))
+			.with_output(test_output(8));
+
+		// Prepare a second transaction, connected to the first.
+		let child_transaction = test_transaction(vec![11, 3], vec![12]);
+
+		let txs = vec![parent_transaction, child_transaction];
+		let multi_kernel_transaction = transaction::aggregate(txs).unwrap();
+
+		dummy_chain.update_output_set(new_output);
+
+		// To mirror how this construction is intended to be used, the pool
+		// is placed inside a RwLock.
+		let pool = RwLock::new(test_setup(&Arc::new(dummy_chain)));
+
+		// Take the write lock and add a pool entry
+		{
+			let mut write_pool = pool.write().unwrap();
+			assert_eq!(write_pool.total_size(), 0);
+
+			// First, add the transaction rooted in the blockchain
+			let result =
+				write_pool.add_to_memory_pool(test_source(), multi_kernel_transaction, false);
+			if result.is_err() {
+				panic!(
+					"got an error adding multi-kernel tx: {:?}",
+					result.err().unwrap()
+				);
+			}
+		}
+
+		// Now take the read lock and use a few exposed methods to check consistency
+		{
+			let read_pool = pool.read().unwrap();
+			assert_eq!(read_pool.total_size(), 1);
+			expect_output_parent!(read_pool, Parent::PoolTransaction{tx_ref: _}, 12);
+			expect_output_parent!(read_pool, Parent::AlreadySpent{other_tx: _}, 5);
+			expect_output_parent!(read_pool, Parent::BlockTransaction, 8);
+			expect_output_parent!(read_pool, Parent::Unknown, 11, 3, 20);
+		}
+	}
+
+	#[test]
+	/// Attempt to add a bad multi kernel transaction to the mempool should get rejected
+	fn test_bad_multikernel_pool_add() {
+		let mut dummy_chain = DummyChainImpl::new();
+		let head_header = block::BlockHeader {
+			height: 1,
+			..block::BlockHeader::default()
+		};
+		dummy_chain.store_head_header(&head_header);
+
+		let parent_transaction = test_transaction(vec![5, 6, 7], vec![11, 3]);
+		// We want this transaction to be rooted in the blockchain.
+		let new_output = DummyOutputSet::empty()
+			.with_output(test_output(5))
+			.with_output(test_output(6))
+			.with_output(test_output(7))
+			.with_output(test_output(8));
+
+		// Prepare a second transaction, connected to the first.
+		let child_transaction1 = test_transaction(vec![11, 3], vec![12]);
+		let child_transaction2 = test_transaction(vec![11, 3], vec![10]);
+
+		let txs = vec![parent_transaction, child_transaction1, child_transaction2];
+		let bad_multi_kernel_transaction = transaction::aggregate(txs).unwrap();
+
+		dummy_chain.update_output_set(new_output);
+
+		// To mirror how this construction is intended to be used, the pool
+		// is placed inside a RwLock.
+		let pool = RwLock::new(test_setup(&Arc::new(dummy_chain)));
+
+		// Take the write lock and add a pool entry
+		{
+			let mut write_pool = pool.write().unwrap();
+			assert_eq!(write_pool.total_size(), 0);
+
+			// First, add the transaction rooted in the blockchain
+			let result =
+				write_pool.add_to_memory_pool(test_source(), bad_multi_kernel_transaction, false);
+			assert!(result.is_err());
+		}
+	}
+
 	#[test]
 	/// A basic test; add a transaction to the pool and add the child to the
 	/// stempool