bigchaindb/bigchaindb/core.py

import random
import math
import collections
from time import time

from itertools import compress
from bigchaindb_common import crypto, exceptions
from bigchaindb_common.util import gen_timestamp, serialize
from bigchaindb_common.transaction import TransactionLink, Metadata

import rethinkdb as r

import bigchaindb

from bigchaindb.db.utils import Connection
from bigchaindb import config_utils, util
from bigchaindb.consensus import BaseConsensusRules
from bigchaindb.models import Block, Transaction


class Bigchain(object):
    """Bigchain API

    Create, read, sign, write transactions to the database
    """

    # return if a block has been voted invalid
    BLOCK_INVALID = 'invalid'
    # return if a block is valid, or tx is in valid block
    BLOCK_VALID = TX_VALID = 'valid'
    # return if block is undecided, or tx is in undecided block
    BLOCK_UNDECIDED = TX_UNDECIDED = 'undecided'
    # return if transaction is in backlog
    TX_IN_BACKLOG = 'backlog'

    def __init__(self, host=None, port=None, dbname=None,
                 public_key=None, private_key=None, keyring=[],
                 backlog_reassign_delay=None):
        """Initialize the Bigchain instance

        A Bigchain instance has several configuration parameters (e.g. host).
        If a parameter value is passed as an argument to the Bigchain
        __init__ method, then that is the value it will have.
        Otherwise, the parameter value will come from an environment variable.
        If that environment variable isn't set, then the value
        will come from the local configuration file. And if that variable
        isn't in the local configuration file, then the parameter will have
        its default value (defined in bigchaindb.__init__).

        Args:
            host (str): hostname where RethinkDB is running.
            port (int): port in which RethinkDB is running (usually 28015).
            dbname (str): the name of the database to connect to (usually bigchain).
            public_key (str): the base58 encoded public key for the ED25519 curve.
            private_key (str): the base58 encoded private key for the ED25519 curve.
            keyring (list[str]): list of base58 encoded public keys of the federation nodes.
        """

        config_utils.autoconfigure()
        self.host = host or bigchaindb.config['database']['host']
        self.port = port or bigchaindb.config['database']['port']
        self.dbname = dbname or bigchaindb.config['database']['name']
        self.me = public_key or bigchaindb.config['keypair']['public']
        self.me_private = private_key or bigchaindb.config['keypair']['private']
        self.nodes_except_me = keyring or bigchaindb.config['keyring']
        self.backlog_reassign_delay = backlog_reassign_delay or bigchaindb.config['backlog_reassign_delay']
        self.consensus = BaseConsensusRules
        # change RethinkDB read mode to majority.  This ensures consistency in query results
        self.read_mode = 'majority'

        if not self.me or not self.me_private:
            raise exceptions.KeypairNotFoundException()

        self._conn = None
        self.connection = Connection(host=self.host, port=self.port, db=self.dbname)

    @property
    def conn(self):
        if not self._conn:
            self._conn = self.reconnect()
        return self._conn

    def reconnect(self):
        return r.connect(host=self.host, port=self.port, db=self.dbname)

    def write_transaction(self, signed_transaction, durability='soft'):
        """Write the transaction to bigchain.

        When first writing a transaction to the bigchain the transaction will be kept in a backlog until
        it has been validated by the nodes of the federation.

        Args:
            signed_transaction (Transaction): transaction with the `signature` included.

        Returns:
            dict: database response
        """
        signed_transaction = signed_transaction.to_dict()

        # we will assign this transaction to `one` node. This way we make sure that there are no duplicate
        # transactions on the bigchain
        if self.nodes_except_me:
            assignee = random.choice(self.nodes_except_me)
        else:
            # I am the only node
            assignee = self.me

        signed_transaction.update({'assignee': assignee})
        signed_transaction.update({'assignment_timestamp': time()})

        # write to the backlog
        response = self.connection.run(
                r.table('backlog')
                .insert(signed_transaction, durability=durability))
        return response

    def reassign_transaction(self, transaction, durability='hard'):
        """Assign a transaction to a new node

        Args:
            transaction (dict): assigned transaction

        Returns:
            dict: database response or None if no reassignment is possible
        """

        if self.nodes_except_me:
            try:
                federation_nodes = self.nodes_except_me + [self.me]
                index_current_assignee = federation_nodes.index(transaction['assignee'])
                new_assignee = random.choice(federation_nodes[:index_current_assignee] +
                                             federation_nodes[index_current_assignee + 1:])
            except ValueError:
                # current assignee not in federation
                new_assignee = random.choice(self.nodes_except_me)

        else:
            # There is no other node to assign to
            new_assignee = self.me

        response = r.table('backlog')\
            .get(transaction['id'])\
            .update({'assignee': new_assignee,
                     'assignment_timestamp': time()},
                    durability=durability).run(self.conn)
        return response

    def get_stale_transactions(self):
        """Get a RethinkDB cursor of stale transactions

        Transactions are considered stale if they have been assigned a node, but are still in the
        backlog after some amount of time specified in the configuration
        """

        return r.table('backlog')\
            .filter(lambda tx: time() - tx['assignment_timestamp'] >
                    self.backlog_reassign_delay).run(self.conn)

    def validate_transaction(self, transaction):
        """Validate a transaction.

        Args:
            transaction (Transaction): transaction to validate.

        Returns:
            The transaction if the transaction is valid else it raises an
            exception describing the reason why the transaction is invalid.
        """

        return self.consensus.validate_transaction(self, transaction)

    def is_valid_transaction(self, transaction):
        """Check whether a transaction is valid or invalid.

        Similar to :meth:`~bigchaindb.Bigchain.validate_transaction`
        but never raises an exception. It returns :obj:`False` if
        the transaction is invalid.

        Args:
            transaction (:Class:`~bigchaindb.models.Transaction`): transaction
                to check.

        Returns:
            The :class:`~bigchaindb.models.Transaction` instance if valid,
            otherwise :obj:`False`.
        """

        try:
            return self.validate_transaction(transaction)
        except (ValueError, exceptions.OperationError, exceptions.TransactionDoesNotExist,
                exceptions.TransactionOwnerError, exceptions.DoubleSpend,
                exceptions.InvalidHash, exceptions.InvalidSignature):
            return False

    def get_transaction(self, txid, include_status=False):
        """Retrieve a transaction with `txid` from bigchain.

        Queries the bigchain for a transaction, if it's in a valid or invalid
        block.

        Args:
            txid (str): transaction id of the transaction to query
            include_status (bool): also return the status of the transaction
                                   the return value is then a tuple: (tx, status)

        Returns:
            A dict with the transaction details if the transaction was found.
            Will add the transaction status to payload ('valid', 'undecided',
            or 'backlog'). If no transaction with that `txid` was found it
            returns `None`
        """

        response, tx_status = None, None

        validity = self.get_blocks_status_containing_tx(txid)

        if validity:
            # Disregard invalid blocks, and return if there are no valid or undecided blocks
            validity = {_id: status for _id, status in validity.items()
                                    if status != Bigchain.BLOCK_INVALID}
            if validity:

                tx_status = self.TX_UNDECIDED
                # If the transaction is in a valid or any undecided block, return it. Does not check
                # if transactions in undecided blocks are consistent, but selects the valid block before
                # undecided ones
                for target_block_id in validity:
                    if validity[target_block_id] == Bigchain.BLOCK_VALID:
                        tx_status = self.TX_VALID
                        break

                # Query the transaction in the target block and return
                response = self.connection.run(
                        r.table('bigchain', read_mode=self.read_mode)
                        .get(target_block_id)
                        .get_field('block')
                        .get_field('transactions')
                        .filter(lambda tx: tx['id'] == txid))[0]

        else:
            # Otherwise, check the backlog
            response = self.connection.run(r.table('backlog')
                                           .get(txid)
                                           .without('assignee', 'assignment_timestamp')
                                           .default(None))
            if response:
                tx_status = self.TX_IN_BACKLOG

        if response:
            response = Transaction.from_dict(response)

        if include_status:
            return response, tx_status
        else:
            return response

    def get_status(self, txid):
        """Retrieve the status of a transaction with `txid` from bigchain.

        Args:
            txid (str): transaction id of the transaction to query

        Returns:
            (string): transaction status ('valid', 'undecided',
            or 'backlog'). If no transaction with that `txid` was found it
            returns `None`
        """
        _, status = self.get_transaction(txid, include_status=True)
        return status

    def search_block_election_on_index(self, value, index):
        """Retrieve block election information given a secondary index and value

        Args:
            value: a value to search (e.g. transaction id string, payload hash string)
            index (str): name of a secondary index, e.g. 'transaction_id'

        Returns:
            :obj:`list` of :obj:`dict`: A list of blocks with with only election information
        """
        # First, get information on all blocks which contain this transaction
        response = self.connection.run(
                r.table('bigchain', read_mode=self.read_mode)
                .get_all(value, index=index)
                .pluck('votes', 'id', {'block': ['voters']}))

        return list(response)

    def get_blocks_status_containing_tx(self, txid):
        """Retrieve block ids and statuses related to a transaction

        Transactions may occur in multiple blocks, but no more than one valid block.

        Args:
            txid (str): transaction id of the transaction to query

        Returns:
            A dict of blocks containing the transaction,
            e.g. {block_id_1: 'valid', block_id_2: 'invalid' ...}, or None
        """

        # First, get information on all blocks which contain this transaction
        blocks = self.search_block_election_on_index(txid, 'transaction_id')
        if blocks:
            # Determine the election status of each block
            validity = {
                block['id']: self.block_election_status(
                    block['id'],
                    block['block']['voters']
                ) for block in blocks
            }

            # NOTE: If there are multiple valid blocks with this transaction,
            #       something has gone wrong
            if list(validity.values()).count(Bigchain.BLOCK_VALID) > 1:
                block_ids = str([block for block in validity
                                 if validity[block] == Bigchain.BLOCK_VALID])
                raise exceptions.DoubleSpend('Transaction {tx} is present in '
                                             'multiple valid blocks: '
                                             '{block_ids}'
                                             .format(tx=txid,
                                                     block_ids=block_ids))

            return validity

        else:
            return None

    def get_tx_by_metadata_id(self, metadata_id):
        """Retrieves transactions related to a metadata.

        When creating a transaction one of the optional arguments is the `metadata`. The metadata is a generic
        dict that contains extra information that can be appended to the transaction.

        To make it easy to query the bigchain for that particular metadata we create a UUID for the metadata and
        store it with the transaction.

        Args:
            metadata_id (str): the id for this particular metadata.

        Returns:
            A list of transactions containing that metadata. If no transaction exists with that metadata it
            returns an empty list `[]`
        """
        cursor = r.table('bigchain', read_mode=self.read_mode) \
            .get_all(metadata_id, index='metadata_id') \
            .concat_map(lambda block: block['block']['transactions']) \
            .filter(lambda transaction: transaction['transaction']['metadata']['id'] == metadata_id) \
            .run(self.conn)

        transactions = list(cursor)
        return [Transaction.from_dict(tx) for tx in transactions]

    def get_txs_by_asset_id(self, asset_id):
        """Retrieves transactions related to a particular asset.

        A digital asset in bigchaindb is identified by an uuid. This allows us to query all the transactions
        related to a particular digital asset, knowing the id.

        Args:
            asset_id (str): the id for this particular metadata.

        Returns:
            A list of transactions containing related to the asset. If no transaction exists for that asset it
            returns an empty list `[]`
        """
        cursor = self.connection.run(
            r.table('bigchain', read_mode=self.read_mode)
             .get_all(asset_id, index='asset_id')
             .concat_map(lambda block: block['block']['transactions'])
             .filter(lambda transaction: transaction['transaction']['asset']['id'] == asset_id))

        return [Transaction.from_dict(tx) for tx in cursor]

    def get_spent(self, txid, cid):
        """Check if a `txid` was already used as an input.

        A transaction can be used as an input for another transaction. Bigchain needs to make sure that a
        given `txid` is only used once.

        Args:
            txid (str): The id of the transaction
            cid (num): the index of the condition in the respective transaction

        Returns:
            The transaction (Transaction) that used the `txid` as an input else
            `None`
        """
        # checks if an input was already spent
        # checks if the bigchain has any transaction with input {'txid': ..., 'cid': ...}
        response = self.connection.run(
                r.table('bigchain', read_mode=self.read_mode)
                .concat_map(lambda doc: doc['block']['transactions'])
                .filter(lambda transaction: transaction['transaction']['fulfillments']
                    .contains(lambda fulfillment: fulfillment['input'] == {'txid': txid, 'cid': cid})))

        transactions = list(response)

        # a transaction_id should have been spent at most one time
        if transactions:
            # determine if these valid transactions appear in more than one valid block
            num_valid_transactions = 0
            for transaction in transactions:
                # ignore invalid blocks
                # FIXME: Isn't there a faster solution than doing I/O again?
                if self.get_transaction(transaction['id']):
                    num_valid_transactions += 1
                if num_valid_transactions > 1:
                    raise exceptions.DoubleSpend('`{}` was spent more then once. There is a problem with the chain'.format(
                        txid))

            if num_valid_transactions:
                return Transaction.from_dict(transactions[0])
            else:
                # all queried transactions were invalid
                return None
        else:
            return None

    def get_owned_ids(self, owner):
        """Retrieve a list of `txids` that can we used has inputs.

        Args:
            owner (str): base58 encoded public key.

        Returns:
            list (TransactionLink): list of `txid`s and `cid`s pointing to
                                      another transaction's condition
        """

        # get all transactions in which owner is in the `owners_after` list
        response = self.connection.run(
                r.table('bigchain', read_mode=self.read_mode)
                .concat_map(lambda doc: doc['block']['transactions'])
                .filter(lambda tx: tx['transaction']['conditions']
                    .contains(lambda c: c['owners_after']
                        .contains(owner))))
        owned = []

        for tx in response:
            # disregard transactions from invalid blocks
            validity = self.get_blocks_status_containing_tx(tx['id'])
            if Bigchain.BLOCK_VALID not in validity.values():
                if Bigchain.BLOCK_UNDECIDED not in validity.values():
                    continue

            # NOTE: It's OK to not serialize the transaction here, as we do not
            #       use it after the execution of this function.
            # a transaction can contain multiple outputs (conditions) so we need to iterate over all of them
            # to get a list of outputs available to spend
            for index, cond in enumerate(tx['transaction']['conditions']):
                # for simple signature conditions there are no subfulfillments
                # check if the owner is in the condition `owners_after`
                if len(cond['owners_after']) == 1:
                    if cond['condition']['details']['public_key'] == owner:
                        tx_link = TransactionLink(tx['id'], index)
                else:
                    # for transactions with multiple `owners_after` there will be several subfulfillments nested
                    # in the condition. We need to iterate the subfulfillments to make sure there is a
                    # subfulfillment for `owner`
                    if util.condition_details_has_owner(cond['condition']['details'], owner):
                        tx_link = TransactionLink(tx['id'], index)
                # check if input was already spent
                if not self.get_spent(tx_link.txid, tx_link.cid):
                    owned.append(tx_link)

        return owned

    def create_block(self, validated_transactions):
        """Creates a block given a list of `validated_transactions`.

        Note that this method does not validate the transactions. Transactions
        should be validated before calling create_block.

        Args:
            validated_transactions (list(Transaction)): list of validated
                                                        transactions.

        Returns:
            Block: created block.
        """
        # Prevent the creation of empty blocks
        if len(validated_transactions) == 0:
            raise exceptions.OperationError('Empty block creation is not '
                                            'allowed')

        voters = self.nodes_except_me + [self.me]
        block = Block(validated_transactions, self.me, gen_timestamp(), voters)
        block = block.sign(self.me_private)

        return block

    # TODO: check that the votings structure is correctly constructed
    def validate_block(self, block):
        """Validate a block.

        Args:
            block (Block): block to validate.

        Returns:
            The block if the block is valid else it raises and exception
            describing the reason why the block is invalid.
        """
        return self.consensus.validate_block(self, block)

    def has_previous_vote(self, block_id, voters):
        """Check for previous votes from this node

        Args:
            block_id (str): the id of the block to check
            voters (list(str)): the voters of the block to check

        Returns:
            bool: :const:`True` if this block already has a
            valid vote from this node, :const:`False` otherwise.

        Raises:
            ImproperVoteError: If there is already a vote,
                but the vote is invalid.

        """
        votes = list(self.connection.run(
            r.table('votes', read_mode=self.read_mode)
            .get_all([block_id, self.me], index='block_and_voter')))

        if len(votes) > 1:
            raise exceptions.MultipleVotesError('Block {block_id} has {n_votes} votes from public key {me}'
                                                .format(block_id=block_id, n_votes=str(len(votes)), me=self.me))
        has_previous_vote = False
        if votes:
            if util.verify_vote_signature(voters, votes[0]):
                has_previous_vote = True
            else:
                raise exceptions.ImproperVoteError('Block {block_id} already has an incorrectly signed vote '
                                                   'from public key {me}'.format(block_id=block_id, me=self.me))

        return has_previous_vote

    def write_block(self, block, durability='soft'):
        """Write a block to bigchain.

        Args:
            block (Block): block to write to bigchain.
        """

        self.connection.run(
                r.table('bigchain')
                .insert(r.json(block.to_str()), durability=durability))

    def transaction_exists(self, transaction_id):
        response = self.connection.run(
                r.table('bigchain', read_mode=self.read_mode)\
                .get_all(transaction_id, index='transaction_id'))
        return len(response.items) > 0

    def prepare_genesis_block(self):
        """Prepare a genesis block."""

        metadata = {'message': 'Hello World from the BigchainDB'}
        # TODO: When updating the BDBC lib, change `payload` to `metadata`
        transaction = Transaction.create([self.me], [self.me],
                                         metadata=metadata)

        # NOTE: The transaction model doesn't expose an API to generate a
        #       GENESIS transaction, as this is literally the only usage.
        transaction.operation = 'GENESIS'
        transaction = transaction.sign([self.me_private])

        # create the block
        return self.create_block([transaction])

    def create_genesis_block(self):
        """Create the genesis block

        Block created when bigchain is first initialized. This method is not atomic, there might be concurrency
        problems if multiple instances try to write the genesis block when the BigchainDB Federation is started,
        but it's a highly unlikely scenario.
        """

        # 1. create one transaction
        # 2. create the block with one transaction
        # 3. write the block to the bigchain

        blocks_count = self.connection.run(
                r.table('bigchain', read_mode=self.read_mode)
                .count())

        if blocks_count:
            raise exceptions.GenesisBlockAlreadyExistsError('Cannot create the Genesis block')

        block = self.prepare_genesis_block()
        self.write_block(block, durability='hard')

        return block

    def vote(self, block_id, previous_block_id, decision, invalid_reason=None):
        """Cast your vote on the block given the previous_block_hash and the decision (valid/invalid)
        return the block to the updated in the database.

        Args:
            block_id (str): The id of the block to vote.
            previous_block_id (str): The id of the previous block.
            decision (bool): Whether the block is valid or invalid.
            invalid_reason (Optional[str]): Reason the block is invalid
        """

        if block_id == previous_block_id:
            raise exceptions.CyclicBlockchainError()

        vote = {
            'voting_for_block': block_id,
            'previous_block': previous_block_id,
            'is_block_valid': decision,
            'invalid_reason': invalid_reason,
            'timestamp': gen_timestamp()
        }

        vote_data = serialize(vote)
        signature = crypto.SigningKey(self.me_private).sign(vote_data)

        vote_signed = {
            'node_pubkey': self.me,
            'signature': signature,
            'vote': vote
        }

        return vote_signed

    def write_vote(self, vote):
        """Write the vote to the database."""

        self.connection.run(
                r.table('votes')
                .insert(vote))

    def get_last_voted_block(self):
        """Returns the last block that this node voted on."""

        try:
            # get the latest value for the vote timestamp (over all votes)
            max_timestamp = self.connection.run(
                    r.table('votes', read_mode=self.read_mode)
                    .filter(r.row['node_pubkey'] == self.me)
                    .max(r.row['vote']['timestamp']))['vote']['timestamp']

            last_voted = list(self.connection.run(
                r.table('votes', read_mode=self.read_mode)
                .filter(r.row['vote']['timestamp'] == max_timestamp)
                .filter(r.row['node_pubkey'] == self.me)))

        except r.ReqlNonExistenceError:
            # return last vote if last vote exists else return Genesis block
            res = self.connection.run(
                r.table('bigchain', read_mode=self.read_mode)
                .filter(util.is_genesis_block))
            block = list(res)[0]
            return Block.from_dict(block)

        # Now the fun starts. Since the resolution of timestamp is a second,
        # we might have more than one vote per timestamp. If this is the case
        # then we need to rebuild the chain for the blocks that have been retrieved
        # to get the last one.

        # Given a block_id, mapping returns the id of the block pointing at it.
        mapping = {v['vote']['previous_block']: v['vote']['voting_for_block']
                   for v in last_voted}

        # Since we follow the chain backwards, we can start from a random
        # point of the chain and "move up" from it.
        last_block_id = list(mapping.values())[0]

        # We must be sure to break the infinite loop. This happens when:
        # - the block we are currenty iterating is the one we are looking for.
        #   This will trigger a KeyError, breaking the loop
        # - we are visiting again a node we already explored, hence there is
        #   a loop. This might happen if a vote points both `previous_block`
        #   and `voting_for_block` to the same `block_id`
        explored = set()

        while True:
            try:
                if last_block_id in explored:
                    raise exceptions.CyclicBlockchainError()
                explored.add(last_block_id)
                last_block_id = mapping[last_block_id]
            except KeyError:
                break

        res = self.connection.run(
                r.table('bigchain', read_mode=self.read_mode)
                .get(last_block_id))

        return Block.from_dict(res)

    def get_unvoted_blocks(self):
        """Return all the blocks that have not been voted on by this node.

        Returns:
            :obj:`list` of :obj:`dict`: a list of unvoted blocks
        """

        unvoted = self.connection.run(
                r.table('bigchain', read_mode=self.read_mode)
                .filter(lambda block: r.table('votes', read_mode=self.read_mode)
                    .get_all([block['id'], self.me], index='block_and_voter')
                    .is_empty())
                .order_by(r.asc(r.row['block']['timestamp'])))

        # FIXME: I (@vrde) don't like this solution. Filtering should be done at a
        #        database level. Solving issue #444 can help untangling the situation
        unvoted_blocks = filter(lambda block: not util.is_genesis_block(block), unvoted)
        return unvoted_blocks

    def block_election_status(self, block_id, voters):
        """Tally the votes on a block, and return the status: valid, invalid, or undecided."""

        votes = self.connection.run(r.table('votes', read_mode=self.read_mode)
            .between([block_id, r.minval], [block_id, r.maxval], index='block_and_voter'))

        votes = list(votes)

        n_voters = len(voters)

        voter_counts = collections.Counter([vote['node_pubkey'] for vote in votes])
        for node in voter_counts:
            if voter_counts[node] > 1:
                raise exceptions.MultipleVotesError('Block {block_id} has multiple votes ({n_votes}) from voting node {node_id}'
                                                    .format(block_id=block_id, n_votes=str(voter_counts[node]), node_id=node))

        if len(votes) > n_voters:
            raise exceptions.MultipleVotesError('Block {block_id} has {n_votes} votes cast, but only {n_voters} voters'
                                                .format(block_id=block_id, n_votes=str(len(votes)), n_voters=str(n_voters)))

        # vote_cast is the list of votes e.g. [True, True, False]
        vote_cast = [vote['vote']['is_block_valid'] for vote in votes]
        # prev_block are the ids of the nominal prev blocks e.g.
        # ['block1_id', 'block1_id', 'block2_id']
        prev_block = [vote['vote']['previous_block'] for vote in votes]
        # vote_validity checks whether a vote is valid
        # or invalid, e.g. [False, True, True]
        vote_validity = [self.consensus.verify_vote_signature(voters, vote) for vote in votes]

        # element-wise product of stated vote and validity of vote
        # vote_cast = [True, True, False] and
        # vote_validity = [False, True, True] gives
        # [True, False]
        # Only the correctly signed votes are tallied.
        vote_list = list(compress(vote_cast, vote_validity))

        # Total the votes. Here, valid and invalid refer
        # to the vote cast, not whether the vote itself
        # is valid or invalid.
        n_valid_votes = sum(vote_list)
        n_invalid_votes = len(vote_cast) - n_valid_votes

        # The use of ceiling and floor is to account for the case of an
        # even number of voters where half the voters have voted 'invalid'
        # and half 'valid'. In this case, the block should be marked invalid
        # to avoid a tie. In the case of an odd number of voters this is not
        # relevant, since one side must be a majority.
        if n_invalid_votes >= math.ceil(n_voters / 2):
            return Bigchain.BLOCK_INVALID
        elif n_valid_votes > math.floor(n_voters / 2):
            # The block could be valid, but we still need to check if votes
            # agree on the previous block.
            #
            # First, only consider blocks with legitimate votes
            prev_block_list = list(compress(prev_block, vote_validity))
            # Next, only consider the blocks with 'yes' votes
            prev_block_valid_list = list(compress(prev_block_list, vote_list))
            counts = collections.Counter(prev_block_valid_list)
            # Make sure the majority vote agrees on previous node.
            # The majority vote must be the most common, by definition.
            # If it's not, there is no majority agreement on the previous
            # block.
            if counts.most_common()[0][1] > math.floor(n_voters / 2):
                return Bigchain.BLOCK_VALID
            else:
                return Bigchain.BLOCK_INVALID
        else:
            return Bigchain.BLOCK_UNDECIDED