[qa] Test non-atomic chainstate writes

Adds new functional test, dbcrash.py, which uses -dbcrashratio to exercise the
logic for recovering from a crash during chainstate flush.

dbcrash.py is added to the extended tests, as it may take ~10 minutes to run

Use _Exit() instead of exit() for crash simulation

This eliminates stderr output such as:
    terminate called without an active exception
or
    Assertion failed: (!pthread_mutex_destroy(&m)), function ~recursive_mutex, file /usr/local/include/boost/thread/pthread/recursive_mutex.hpp, line 104.

Eliminating the stderr output on crash simulation allows testing with
test_runner.py, which reports a test as failed if stderr is produced.

src/txdb.cpp

@@ -124,7 +124,7 @@ bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) {
                 static FastRandomContext rng;
                 if (rng.randrange(crash_simulate) == 0) {
                     LogPrintf("Simulating a crash. Goodbye.\n");
-                    exit(0);
+                    _Exit(0);
                 }
             }
         }

test/functional/dbcrash.py

@@ -0,0 +1,268 @@
+#!/usr/bin/env python3
+# Copyright (c) 2017 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test recovery from a crash during chainstate writing."""
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import *
+from test_framework.script import *
+from test_framework.mininode import *
+import random
+try:
+    import http.client as httplib
+except ImportError:
+    import httplib
+import errno
+
+'''
+Test structure:
+
+- 4 nodes
+  * node0, node1, and node2 will have different dbcrash ratios, and different
+    dbcache sizes
+  * node3 will be a regular node, with no crashing.
+  * The nodes will not connect to each other.
+
+- use default test framework starting chain. initialize starting_tip_height to
+  tip height.
+
+- Main loop:
+  * generate lots of transactions on node3, enough to fill up a block.
+  * uniformly randomly pick a tip height from starting_tip_height to
+    tip_height; with probability 1/(height_difference+4), invalidate this block.
+  * mine enough blocks to overtake tip_height at start of loop.
+  * for each node in [node0,node1,node2]:
+     - for each mined block:
+       * submit block to node
+       * if node crashed on/after submitting:
+         - restart until recovery succeeds
+         - check that utxo matches node3 using gettxoutsetinfo
+'''
+
+class ChainstateWriteCrashTest(BitcoinTestFramework):
+
+    def __init__(self):
+        super().__init__()
+        self.num_nodes = 4
+        self.setup_clean_chain = False
+
+        # Set -maxmempool=0 to turn off mempool memory sharing with dbcache
+        # Set -rpcservertimeout=900 to reduce socket disconnects in this
+        # long-running test
+        self.base_args = ["-limitdescendantsize=0", "-maxmempool=0", "-rpcservertimeout=900"]
+
+        # Set different crash ratios and cache sizes.  Note that not all of
+        # -dbcache goes to pcoinsTip.
+        self.node0_args = ["-dbcrashratio=8", "-dbcache=4", "-dbbatchsize=200000"] + self.base_args
+        self.node1_args = ["-dbcrashratio=16", "-dbcache=8", "-dbbatchsize=200000"] + self.base_args
+        self.node2_args = ["-dbcrashratio=24", "-dbcache=16", "-dbbatchsize=200000"] + self.base_args
+
+        # Node3 is a normal node with default args, except will mine full blocks
+        self.node3_args = ["-blockmaxweight=4000000"]
+        self.extra_args = [self.node0_args, self.node1_args, self.node2_args, self.node3_args]
+
+        # We'll track some test coverage statistics
+        self.restart_counts = [0, 0, 0] # Track the restarts for nodes 0-2
+        self.crashed_on_restart = 0     # Track count of crashes during recovery
+
+    def setup_network(self):
+        self.setup_nodes()
+        # Leave them unconnected, we'll use submitblock directly in this test
+
+    # Starts up a given node id, waits for the tip to reach the given block
+    # hash, and calculates the utxo hash.  Exceptions on startup should
+    # indicate node crash (due to -dbcrashratio), in which case we try again.
+    # Give up after 60 seconds.
+    # Returns the utxo hash of the given node.
+    def restart_node(self, node_index, expected_tip):
+        time_start = time.time()
+        while time.time() - time_start < 60:
+            try:
+                # Any of these RPC calls could throw due to node crash
+                self.nodes[node_index] = self.start_node(node_index, self.options.tmpdir, self.extra_args[node_index])
+                self.nodes[node_index].waitforblock(expected_tip)
+                utxo_hash = self.nodes[node_index].gettxoutsetinfo()['hash_serialized_2']
+                return utxo_hash
+            except:
+                # An exception here should mean the node is about to crash.
+                # If bitcoind exits, then try again.  wait_for_node_exit()
+                # should raise an exception if bitcoind doesn't exit.
+                wait_for_node_exit(node_index, timeout=10)
+            self.crashed_on_restart += 1
+            time.sleep(1)
+
+        # If we got here, bitcoind isn't coming back up on restart.  Could be a
+        # bug in bitcoind, or we've gotten unlucky with our dbcrash ratio --
+        # perhaps we generated a test case that blew up our cache?
+        # TODO: If this happens a lot, we should try to restart without -dbcrashratio
+        # and make sure that recovery happens.
+        raise AssertionError("Unable to successfully restart node %d in allotted time", node_index)
+
+    # Try submitting a block to the given node.
+    # Catch any exceptions that indicate the node has crashed.
+    # Returns true if the block was submitted successfully; false otherwise.
+    def submit_block_catch_error(self, node_index, block):
+        try:
+            self.nodes[node_index].submitblock(block)
+            return True
+        except (httplib.CannotSendRequest, httplib.RemoteDisconnected) as e:
+            self.log.debug("node %d submitblock raised exception: %s", node_index, e)
+            return False
+        except OSError as e:
+            self.log.debug("node %d submitblock raised OSError exception: errno=%s", node_index, e.errno)
+            if e.errno in [errno.EPIPE, errno.ECONNREFUSED, errno.ECONNRESET]:
+                # The node has likely crashed
+                return False
+            else:
+                # Unexpected exception, raise
+                raise
+
+    # Use submitblock to sync node3's chain with the other nodes
+    # If submitblock fails, restart the node and get the new utxo hash.
+    def sync_node3blocks(self, block_hashes):
+        # If any nodes crash while updating, we'll compare utxo hashes to
+        # ensure recovery was successful.
+        node3_utxo_hash = self.nodes[3].gettxoutsetinfo()['hash_serialized_2']
+
+        # Retrieve all the blocks from node3
+        blocks = []
+        for block_hash in block_hashes:
+            blocks.append([block_hash, self.nodes[3].getblock(block_hash, 0)])
+
+        # Deliver each block to each other node
+        for i in range(3):
+            nodei_utxo_hash = None
+            self.log.debug("Syncing blocks to node %d", i)
+            for (block_hash, block) in blocks:
+                # Get the block from node3, and submit to node_i
+                self.log.debug("submitting block %s", block_hash)
+                if not self.submit_block_catch_error(i, block):
+                    # TODO: more carefully check that the crash is due to -dbcrashratio
+                    # (change the exit code perhaps, and check that here?)
+                    wait_for_node_exit(i, timeout=30)
+                    self.log.debug("Restarting node %d after block hash %s", i, block_hash)
+                    nodei_utxo_hash = self.restart_node(i, block_hash)
+                    assert nodei_utxo_hash is not None
+                    self.restart_counts[i] += 1
+                else:
+                    # Clear it out after successful submitblock calls -- the cached
+                    # utxo hash will no longer be correct
+                    nodei_utxo_hash = None
+
+            # Check that the utxo hash matches node3's utxo set
+            # NOTE: we only check the utxo set if we had to restart the node
+            # after the last block submitted:
+            # - checking the utxo hash causes a cache flush, which we don't
+            # want to do every time; so
+            # - we only update the utxo cache after a node restart, since flushing
+            # the cache is a no-op at that point
+            if nodei_utxo_hash is not None:
+                self.log.debug("Checking txoutsetinfo matches for node %d", i)
+                assert_equal(nodei_utxo_hash, node3_utxo_hash)
+
+    # Verify that the utxo hash of each node matches node3.
+    # Restart any nodes that crash while querying.
+    def verify_utxo_hash(self):
+        node3_utxo_hash = self.nodes[3].gettxoutsetinfo()['hash_serialized_2']
+        self.log.info("Verifying utxo hash matches for all nodes")
+
+        for i in range(3):
+            try:
+                nodei_utxo_hash = self.nodes[i].gettxoutsetinfo()['hash_serialized_2']
+            except OSError:
+                # probably a crash on db flushing
+                nodei_utxo_hash = self.restart_node(i, self.nodes[3].getbestblockhash())
+            assert_equal(nodei_utxo_hash, node3_utxo_hash)
+
+
+    def generate_small_transactions(self, node, count, utxo_list):
+        FEE = 1000 # TODO: replace this with node relay fee based calculation
+        num_transactions = 0
+        random.shuffle(utxo_list)
+        while len(utxo_list) >= 2 and num_transactions < count:
+            tx = CTransaction()
+            input_amount = 0
+            for i in range(2):
+                utxo = utxo_list.pop()
+                tx.vin.append(CTxIn(COutPoint(int(utxo['txid'], 16), utxo['vout'])))
+                input_amount += int(utxo['amount']*COIN)
+            output_amount = (input_amount - FEE)//3
+
+            if output_amount <= 0:
+                # Sanity check -- if we chose inputs that are too small, skip
+                continue
+
+            for i in range(3):
+                tx.vout.append(CTxOut(output_amount, hex_str_to_bytes(utxo['scriptPubKey'])))
+
+            # Sign and send the transaction to get into the mempool
+            tx_signed_hex = node.signrawtransaction(ToHex(tx))['hex']
+            node.sendrawtransaction(tx_signed_hex)
+            num_transactions += 1
+
+    def run_test(self):
+
+        # Start by creating a lot of utxos on node3
+        initial_height = self.nodes[3].getblockcount()
+        utxo_list = create_confirmed_utxos(self.nodes[3].getnetworkinfo()['relayfee'], self.nodes[3], 5000)
+        self.log.info("Prepped %d utxo entries", len(utxo_list))
+
+        # Sync these blocks with the other nodes
+        block_hashes_to_sync = []
+        for height in range(initial_height+1, self.nodes[3].getblockcount()+1):
+            block_hashes_to_sync.append(self.nodes[3].getblockhash(height))
+
+        self.log.debug("Syncing %d blocks with other nodes", len(block_hashes_to_sync))
+        # Syncing the blocks could cause nodes to crash, so the test begins here.
+        self.sync_node3blocks(block_hashes_to_sync)
+
+        starting_tip_height = self.nodes[3].getblockcount()
+
+        # Main test loop:
+        # each time through the loop, generate a bunch of transactions,
+        # and then either mine a single new block on the tip, or some-sized reorg.
+        for i in range(40):
+            self.log.info("Iteration %d, generating 2500 transactions %s", i, self.restart_counts)
+            # Generate a bunch of small-ish transactions
+            self.generate_small_transactions(self.nodes[3], 2500, utxo_list)
+            # Pick a random block between current tip, and starting tip
+            current_height = self.nodes[3].getblockcount()
+            random_height = random.randint(starting_tip_height, current_height)
+            self.log.debug("At height %d, considering height %d", current_height, random_height)
+            if random_height > starting_tip_height:
+                # Randomly reorg from this point with some probability (1/4 for
+                # tip, 1/5 for tip-1, ...)
+                if random.random() < 1.0/(current_height + 4 - random_height):
+                    self.log.debug("Invalidating block at height %d", random_height)
+                    self.nodes[3].invalidateblock(self.nodes[3].getblockhash(random_height))
+
+            # Now generate new blocks until we pass the old tip height
+            self.log.debug("Mining longer tip")
+            block_hashes = self.nodes[3].generate(current_height+1-self.nodes[3].getblockcount())
+            self.log.debug("Syncing %d new blocks...", len(block_hashes))
+            self.sync_node3blocks(block_hashes)
+            utxo_list = self.nodes[3].listunspent()
+            self.log.debug("Node3 utxo count: %d", len(utxo_list))
+
+        # Check that the utxo hashes agree with node3
+        # Useful side effect: each utxo cache gets flushed here, so that we
+        # won't get crashes on shutdown at the end of the test.
+        self.verify_utxo_hash()
+
+        # Check the test coverage
+        self.log.info("Restarted nodes: %s; crashes on restart: %d", self.restart_counts, self.crashed_on_restart)
+
+        # If no nodes were restarted, we didn't test anything.
+        assert self.restart_counts != [0, 0, 0]
+
+        # Make sure we tested the case of crash-during-recovery.
+        assert self.crashed_on_restart > 0
+
+        # Warn if any of the nodes escaped restart.
+        for i in range(3):
+            if self.restart_counts[i] == 0:
+                self.log.warn("Node %d never crashed during utxo flush!", i)
+
+if __name__ == "__main__":
+    ChainstateWriteCrashTest().main()

test/functional/test_framework/util.py

@@ -249,6 +249,8 @@ def wait_for_bitcoind_start(process, datadir, i, rpchost=None):
                 raise
         time.sleep(0.25)
 
+def wait_for_node_exit(node_index, timeout):
+    bitcoind_processes[node_index].wait(timeout)
 
 def _start_node(i, dirname, extra_args=None, rpchost=None, timewait=None, binary=None, stderr=None):
     """Start a bitcoind and return RPC connection to it

test/functional/test_runner.py

@@ -124,6 +124,7 @@ EXTENDED_SCRIPTS = [
     # vv Tests less than 5m vv
     'maxuploadtarget.py',
     'mempool_packages.py',
+    'dbcrash.py',
     # vv Tests less than 2m vv
     'bip68-sequence.py',
     'getblocktemplate_longpoll.py',