Make total_difficulty the sum of network difficulty [testnet2] (#523)

* Make total_difficulty the sum of network difficulty, not whatever the miner happened to mine.
  - Only for Testnet2 / Mainnet (hardforks Testnet1)
  - update chain::pipe validate_header to validate according to Testnet2 rules for cumulative difference

Fixes #280

* tests that should ignore (network) difficulty

* fn new_block is explained as "utility to create a block without worrying about the key or previous header" so it gets network difficulty := 0 too

* update tx pool tests (going with "minimum" for network difficulty for now)

* add ERR outputs about bannable offences
(#406 should know about these)

* whitespace fix

* mine_simple_chain: Probably DON'T overwrite difficulty (?)

* core/mod tests "reward_empty_block" and "reward_with_tx_block" tests set to use lowest network difficulty possible

core/src/core/block.rs

@@ -292,10 +292,11 @@ impl Block {
 		txs: Vec<&Transaction>,
 		keychain: &keychain::Keychain,
 		key_id: &keychain::Identifier,
+		difficulty: Difficulty,
 	) -> Result<Block, Error> {
 		let fees = txs.iter().map(|tx| tx.fee).sum();
 		let (reward_out, reward_proof) = Block::reward_output(keychain, key_id, fees)?;
-		let block = Block::with_reward(prev, txs, reward_out, reward_proof)?;
+		let block = Block::with_reward(prev, txs, reward_out, reward_proof, difficulty)?;
 		Ok(block)
 	}
 
@@ -307,6 +308,7 @@ impl Block {
 		txs: Vec<&Transaction>,
 		reward_out: Output,
 		reward_kern: TxKernel,
+		difficulty: Difficulty,
 	) -> Result<Block, Error> {
 		let mut kernels = vec![];
 		let mut inputs = vec![];
@@ -350,7 +352,7 @@ impl Block {
 						..time::now_utc()
 					},
 					previous: prev.hash(),
-					total_difficulty: prev.pow.clone().to_difficulty() +
+					total_difficulty: difficulty +
 						prev.total_difficulty.clone(),
 					..Default::default()
 				},
@@ -615,7 +617,13 @@ mod test {
 	// header
 	fn new_block(txs: Vec<&Transaction>, keychain: &Keychain) -> Block {
 		let key_id = keychain.derive_key_id(1).unwrap();
-		Block::new(&BlockHeader::default(), txs, keychain, &key_id).unwrap()
+		Block::new(
+			&BlockHeader::default(),
+			txs,
+			keychain,
+			&key_id,
+			Difficulty::minimum()
+		).unwrap()
 	}
 
 	// utility producing a transaction that spends an output with the provided

core/src/core/mod.rs

@@ -28,6 +28,7 @@ use std::cmp::Ordering;
 use std::num::ParseFloatError;
 use consensus::GRIN_BASE;
 
+use core::target::Difficulty;
 use util::{secp, static_secp_instance};
 use util::secp::pedersen::*;
 
@@ -388,7 +389,13 @@ mod test {
 		let keychain = keychain::Keychain::from_random_seed().unwrap();
 		let key_id = keychain.derive_key_id(1).unwrap();
 
-		let b = Block::new(&BlockHeader::default(), vec![], &keychain, &key_id).unwrap();
+		let b = Block::new(
+			&BlockHeader::default(),
+			vec![],
+			&keychain,
+			&key_id,
+			Difficulty::minimum(),
+		).unwrap();
 		b.compact().validate().unwrap();
 	}
 
@@ -400,7 +407,13 @@ mod test {
 		let mut tx1 = tx2i1o();
 		tx1.verify_sig().unwrap();
 
-		let b = Block::new(&BlockHeader::default(), vec![&mut tx1], &keychain, &key_id).unwrap();
+		let b = Block::new(
+			&BlockHeader::default(),
+			vec![&mut tx1],
+			&keychain,
+			&key_id,
+			Difficulty::minimum(),
+		).unwrap();
 		b.compact().validate().unwrap();
 	}
 
@@ -417,6 +430,7 @@ mod test {
 			vec![&mut tx1, &mut tx2],
 			&keychain,
 			&key_id,
+			Difficulty::minimum(),
 		).unwrap();
 		b.validate().unwrap();
 	}
@@ -447,6 +461,7 @@ mod test {
 			vec![&tx1],
 			&keychain,
 			&key_id3.clone(),
+			Difficulty::minimum(),
 		).unwrap();
 		b.validate().unwrap();
 
@@ -467,6 +482,7 @@ mod test {
 			vec![&tx1],
 			&keychain,
 			&key_id3.clone(),
+			Difficulty::minimum(),
 		).unwrap();
 		match b.validate() {
 			Err(KernelLockHeight { lock_height: height }) => {