Add block validation consensus rule for block coinbase output (#647)

and kernel counts

core/src/consensus.rs

@@ -44,6 +44,16 @@ pub fn reward(fee: u64) -> u64 {
 /// Number of blocks before a coinbase matures and can be spent
 pub const COINBASE_MATURITY: u64 = 1_000;
 
+/// Max number of coinbase outputs in a valid block.
+/// This is to prevent a miner generating an excessively large "compact block".
+/// But we do techincally support blocks with multiple coinbase outputs/kernels.
+pub const MAX_BLOCK_COINBASE_OUTPUTS: u64 = 1;
+
+/// Max number of coinbase kernels in a valid block.
+/// This is to prevent a miner generating an excessively large "compact block".
+/// But we do techincally support blocks with multiple coinbase outputs/kernels.
+pub const MAX_BLOCK_COINBASE_KERNELS: u64 = 1;
+
 /// Block interval, in seconds, the network will tune its next_target for. Note
 /// that we may reduce this value in the future as we get more data on mining
 /// with Cuckoo Cycle, networks improve and block propagation is optimized

core/src/core/block.rs

@@ -73,6 +73,10 @@ pub enum Error {
 		/// The lock_height needed to be reached for the coinbase output to mature
 		lock_height: u64,
 	},
+	/// Limit on number of coinbase outputs in a valid block.
+	CoinbaseOutputCountExceeded,
+	/// Limit on number of coinbase kernels in a valid block.
+	CoinbaseKernelCountExceeded,
 	/// Other unspecified error condition
 	Other(String)
 }
@@ -674,11 +678,12 @@ impl Block {
 		Ok(())
 	}
 
-	// Validate the coinbase outputs generated by miners. Entails 2 main checks:
+	/// Validate the coinbase outputs generated by miners. Entails 3 main checks:
-	//
+	///
-	// * That the sum of all coinbase-marked outputs equal the supply.
+	/// * That the block does not exceed the number of permitted coinbase outputs or kernels.
-	// * That the sum of blinding factors for all coinbase-marked outputs match
+	/// * That the sum of all coinbase-marked outputs equal the supply.
-	//   the coinbase-marked kernels.
+	/// * That the sum of blinding factors for all coinbase-marked outputs match
+	///   the coinbase-marked kernels.
 	fn verify_coinbase(&self) -> Result<(), Error> {
 		let cb_outs = self.outputs
 			.iter()
@@ -692,6 +697,16 @@ impl Block {
 			.cloned()
 			.collect::<Vec<TxKernel>>();
 
+		// First check that we do not have too many coinbase outputs in the block.
+		if cb_outs.len() as u64 > consensus::MAX_BLOCK_COINBASE_OUTPUTS {
+			return Err(Error::CoinbaseOutputCountExceeded);
+		}
+
+		// And that we do not have too many coinbase kernels in the block.
+		if cb_kerns.len() as u64 > consensus::MAX_BLOCK_COINBASE_KERNELS {
+			return Err(Error::CoinbaseKernelCountExceeded);
+		}
+
 		let over_commit;
 		let out_adjust_sum;
 		let kerns_sum;