Add assumevalid testcase

Adds a qa testcase testing the new "-assumevalid" option. The testcase builds
a chain that includes and invalid signature for one of the transactions and
sends that chain to three nodes:

 - node0 has no -assumevalid parameter and rejects the invalid chain.
 - node1 has -assumevalid set and accepts the invalid chain.
 - node2 has -assumevalid set but the invalid block is not buried deep
   enough to assume invalid, and so rejects the invalid chain.

qa/rpc-tests/assumevalid.py

@@ -0,0 +1,191 @@
+#!/usr/bin/env python3
+# Copyright (c) 2014-2016 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+'''
+assumevalid.py
+
+Test logic for skipping signature validation on blocks which we've assumed
+valid (https://github.com/bitcoin/bitcoin/pull/9484)
+
+We build a chain that includes and invalid signature for one of the
+transactions:
+
+    0:        genesis block
+    1:        block 1 with coinbase transaction output.
+    2-101:    bury that block with 100 blocks so the coinbase transaction
+              output can be spent
+    102:      a block containing a transaction spending the coinbase
+              transaction output. The transaction has an invalid signature. 
+    103-2202: bury the bad block with just over two weeks' worth of blocks
+              (2100 blocks)
+
+Start three nodes:
+
+    - node0 has no -assumevalid parameter. Try to sync to block 2202. It will
+      reject block 102 and only sync as far as block 101
+    - node1 has -assumevalid set to the hash of block 102. Try to sync to
+      block 2202. node1 will sync all the way to block 2202.
+    - node2 has -assumevalid set to the hash of block 102. Try to sync to
+      block 200. node2 will reject block 102 since it's assumed valid, but it
+      isn't buried by at least two weeks' work.
+'''
+
+from test_framework.mininode import *
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import *
+from test_framework.blocktools import create_block, create_coinbase
+from test_framework.key import CECKey
+from test_framework.script import *
+
+class BaseNode(SingleNodeConnCB):
+    def __init__(self):
+        SingleNodeConnCB.__init__(self)
+        self.last_inv = None
+        self.last_headers = None
+        self.last_block = None
+        self.last_getdata = None
+        self.block_announced = False
+        self.last_getheaders = None
+        self.disconnected = False
+        self.last_blockhash_announced = None
+
+    def on_close(self, conn):
+        self.disconnected = True
+
+    def wait_for_disconnect(self, timeout=60):
+        test_function = lambda: self.disconnected
+        assert(wait_until(test_function, timeout=timeout))
+        return
+
+    def send_header_for_blocks(self, new_blocks):
+        headers_message = msg_headers()
+        headers_message.headers = [ CBlockHeader(b) for b in new_blocks ]
+        self.send_message(headers_message)
+
+class SendHeadersTest(BitcoinTestFramework):
+    def __init__(self):
+        super().__init__()
+        self.setup_clean_chain = True
+        self.num_nodes = 3
+
+    def setup_network(self):
+        # Start node0. We don't start the other nodes yet since
+        # we need to pre-mine a block with an invalid transaction
+        # signature so we can pass in the block hash as assumevalid.
+        self.nodes = []
+        self.nodes.append(start_node(0, self.options.tmpdir, ["-debug"]))
+
+    def run_test(self):
+
+        # Connect to node0
+        node0 = BaseNode()
+        connections = []
+        connections.append(NodeConn('127.0.0.1', p2p_port(0), self.nodes[0], node0))
+        node0.add_connection(connections[0])
+
+        NetworkThread().start() # Start up network handling in another thread
+        node0.wait_for_verack()
+
+        # Build the blockchain
+        self.tip = int(self.nodes[0].getbestblockhash(), 16)
+        self.block_time = self.nodes[0].getblock(self.nodes[0].getbestblockhash())['time'] + 1
+
+        self.blocks = []
+
+        # Get a pubkey for the coinbase TXO
+        coinbase_key = CECKey()
+        coinbase_key.set_secretbytes(b"horsebattery")
+        coinbase_pubkey = coinbase_key.get_pubkey()
+
+        # Create the first block with a coinbase output to our key
+        height = 1
+        block = create_block(self.tip, create_coinbase(height, coinbase_pubkey), self.block_time)
+        self.blocks.append(block)
+        self.block_time += 1
+        block.solve()
+        # Save the coinbase for later
+        self.block1 = block
+        self.tip = block.sha256
+        height += 1
+
+        # Bury the block 100 deep so the coinbase output is spendable
+        for i in range(100):
+            block = create_block(self.tip, create_coinbase(height), self.block_time)
+            block.solve()
+            self.blocks.append(block)
+            self.tip = block.sha256
+            self.block_time += 1
+            height += 1
+
+        # Create a transaction spending the coinbase output with an invalid (null) signature
+        tx = CTransaction()
+        tx.vin.append(CTxIn(COutPoint(self.block1.vtx[0].sha256, 0), scriptSig=b""))
+        tx.vout.append(CTxOut(49*100000000, CScript([OP_TRUE])))
+        tx.calc_sha256()
+
+        block102 = create_block(self.tip, create_coinbase(height), self.block_time)
+        self.block_time += 1
+        block102.vtx.extend([tx])
+        block102.hashMerkleRoot = block102.calc_merkle_root()
+        block102.rehash()
+        block102.solve()
+        self.blocks.append(block102)
+        self.tip = block102.sha256
+        self.block_time += 1
+        height += 1
+
+        # Bury the assumed valid block 2100 deep
+        for i in range(2100):
+            block = create_block(self.tip, create_coinbase(height), self.block_time)
+            block.nVersion = 4
+            block.solve()
+            self.blocks.append(block)
+            self.tip = block.sha256
+            self.block_time += 1
+            height += 1
+
+        # Start node1 and node2 with assumevalid so they accept a block with a bad signature.
+        self.nodes.append(start_node(1, self.options.tmpdir,
+                                     ["-debug", "-assumevalid=" + hex(block102.sha256)]))
+        node1 = BaseNode()  # connects to node1
+        connections.append(NodeConn('127.0.0.1', p2p_port(1), self.nodes[1], node1))
+        node1.add_connection(connections[1])
+        node1.wait_for_verack()
+
+        self.nodes.append(start_node(2, self.options.tmpdir,
+                                     ["-debug", "-assumevalid=" + hex(block102.sha256)]))
+        node2 = BaseNode()  # connects to node2
+        connections.append(NodeConn('127.0.0.1', p2p_port(2), self.nodes[2], node2))
+        node2.add_connection(connections[2])
+        node2.wait_for_verack()
+
+        # send header lists to all three nodes
+        node0.send_header_for_blocks(self.blocks[0:2000])
+        node0.send_header_for_blocks(self.blocks[2000:])
+        node1.send_header_for_blocks(self.blocks[0:2000])
+        node1.send_header_for_blocks(self.blocks[2000:])
+        node2.send_header_for_blocks(self.blocks[0:200])
+
+        # Send 102 blocks to node0. Block 102 will be rejected.
+        for i in range(101):
+            node0.send_message(msg_block(self.blocks[i]))
+        node0.sync_with_ping() # make sure the most recent block is synced
+        node0.send_message(msg_block(self.blocks[101]))
+        assert_equal(self.nodes[0].getblock(self.nodes[0].getbestblockhash())['height'], 101)
+
+        # Send 3102 blocks to node1. All blocks will be accepted.
+        for i in range(2202):
+            node1.send_message(msg_block(self.blocks[i]))
+        node1.sync_with_ping() # make sure the most recent block is synced
+        assert_equal(self.nodes[1].getblock(self.nodes[1].getbestblockhash())['height'], 2202)
+
+        # Send 102 blocks to node2. Block 102 will be rejected.
+        for i in range(101):
+            node2.send_message(msg_block(self.blocks[i]))
+        node2.sync_with_ping() # make sure the most recent block is synced
+        node2.send_message(msg_block(self.blocks[101]))
+        assert_equal(self.nodes[2].getblock(self.nodes[2].getbestblockhash())['height'], 101)
+
+if __name__ == '__main__':
+    SendHeadersTest().main()