[refactor] split out backend from couch database

First step of splitting classes across files on common.
backend.py holds SoledadBackend (generic backend logic)
couch/ is now a directory with old code inside __init__.py and
CouchServerState on state.py
Also removed mock IndexedSoledadBackend, since Soledad does not support
indexing due to encryption on server side.
Also fixed DesignDocUnknownError to show up what is the message of the
original exception. It was being lost.

1 files changed, 746 insertions, 0 deletions

diff --git a/common/src/leap/soledad/common/couch/__init__.py b/common/src/leap/soledad/common/couch/__init__.py
new file mode 100644
index 00000000..727f033f
--- /dev/null
+++ b/common/src/leap/soledad/common/couch/__init__.py
@@ -0,0 +1,746 @@
+# -*- coding: utf-8 -*-
+# couch.py
+# Copyright (C) 2013 LEAP
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+
+"""A U1DB backend that uses CouchDB as its persistence layer."""
+
+
+import json
+import re
+import uuid
+import binascii
+import time
+
+
+from StringIO import StringIO
+from urlparse import urljoin
+from contextlib import contextmanager
+from multiprocessing.pool import ThreadPool
+
+
+from couchdb.client import Server, Database
+from couchdb.multipart import MultipartWriter
+from couchdb.http import (
+    ResourceConflict,
+    ResourceNotFound,
+    ServerError,
+    Session,
+    urljoin as couch_urljoin,
+    Resource,
+)
+from u1db.errors import (
+    DatabaseDoesNotExist,
+    InvalidGeneration,
+    RevisionConflict,
+)
+from u1db.remote import http_app
+
+
+from leap.soledad.common import ddocs
+from leap.soledad.common.errors import raise_server_error
+from leap.soledad.common.errors import raise_missing_design_doc_error
+from leap.soledad.common.errors import InvalidURLError
+from leap.soledad.common.document import CouchDocument
+from leap.soledad.common.backend import SoledadBackend
+
+
+COUCH_TIMEOUT = 120  # timeout for transfers between Soledad server and Couch
+
+
+def list_users_dbs(couch_url):
+    """
+    Retrieves a list with all databases that starts with 'user-' on CouchDB.
+    Those databases belongs to users. So, the list will contain all the
+    database names in the form of 'user-{uuid4}'.
+
+    :param couch_url: The couch url with needed credentials
+    :type couch_url: str
+
+    :return: The list of all database names from users.
+    :rtype: [str]
+    """
+    with couch_server(couch_url) as server:
+        users = [dbname for dbname in server if dbname.startswith('user-')]
+    return users
+
+
+# monkey-patch the u1db http app to use CouchDocument
+http_app.Document = CouchDocument
+
+
+@contextmanager
+def couch_server(url):
+    """
+    Provide a connection to a couch server and cleanup after use.
+
+    For database creation and deletion we use an ephemeral connection to the
+    couch server. That connection has to be properly closed, so we provide it
+    as a context manager.
+
+    :param url: The URL of the Couch server.
+    :type url: str
+    """
+    session = Session(timeout=COUCH_TIMEOUT)
+    server = Server(url=url, full_commit=False, session=session)
+    yield server
+
+
+THREAD_POOL = ThreadPool(20)
+
+
+class CouchDatabase(object):
+    """
+    Holds CouchDB related code.
+    This class gives methods to encapsulate database operations and hide
+    CouchDB details from backend code.
+    """
+
+    @classmethod
+    def open_database(cls, url, create, ensure_ddocs=False, replica_uid=None,
+                      database_security=None):
+        """
+        Open a U1DB database using CouchDB as backend.
+
+        :param url: the url of the database replica
+        :type url: str
+        :param create: should the replica be created if it does not exist?
+        :type create: bool
+        :param replica_uid: an optional unique replica identifier
+        :type replica_uid: str
+        :param ensure_ddocs: Ensure that the design docs exist on server.
+        :type ensure_ddocs: bool
+
+        :return: the database instance
+        :rtype: SoledadBackend
+        """
+        # get database from url
+        m = re.match('(^https?://[^/]+)/(.+)$', url)
+        if not m:
+            raise InvalidURLError
+        url = m.group(1)
+        dbname = m.group(2)
+        with couch_server(url) as server:
+            try:
+                server[dbname]
+            except ResourceNotFound:
+                if not create:
+                    raise DatabaseDoesNotExist()
+                server.create(dbname)
+        db = cls(url,
+                 dbname, ensure_ddocs=ensure_ddocs,
+                 database_security=database_security)
+        return SoledadBackend(
+            db, replica_uid=replica_uid)
+
+    def __init__(self, url, dbname, ensure_ddocs=True,
+                 database_security=None):
+        self._session = Session(timeout=COUCH_TIMEOUT)
+        self._url = url
+        self._dbname = dbname
+        self._database = Database(
+            urljoin(url, dbname),
+            self._session)
+        self._database.info()
+        if ensure_ddocs:
+            self.ensure_ddocs_on_db()
+            self.ensure_security_ddoc(database_security)
+
+    def ensure_ddocs_on_db(self):
+        """
+        Ensure that the design documents used by the backend exist on the
+        couch database.
+        """
+        for ddoc_name in ['docs', 'syncs', 'transactions']:
+            try:
+                self._database.resource('_design',
+                                        ddoc_name, '_info').get_json()
+            except ResourceNotFound:
+                ddoc = json.loads(
+                    binascii.a2b_base64(
+                        getattr(ddocs, ddoc_name)))
+                self._database.save(ddoc)
+
+    def ensure_security_ddoc(self, security_config=None):
+        """
+        Make sure that only soledad user is able to access this database as
+        an unprivileged member, meaning that administration access will
+        be forbidden even inside an user database.
+        The goal is to make sure that only the lowest access level is given
+        to the unprivileged CouchDB user set on the server process.
+        This is achieved by creating a _security design document, see:
+        http://docs.couchdb.org/en/latest/api/database/security.html
+
+        :param database_security: security configuration parsed from conf file
+        :type cache: dict
+        """
+        security_config = security_config or {}
+        security = self._database.resource.get_json('_security')[2]
+        security['members'] = {'names': [], 'roles': []}
+        security['members']['names'] = security_config.get('members',
+                                                           ['soledad'])
+        security['members']['roles'] = security_config.get('members_roles', [])
+        security['admins'] = {'names': [], 'roles': []}
+        security['admins']['names'] = security_config.get('admins', [])
+        security['admins']['roles'] = security_config.get('admins_roles', [])
+        self._database.resource.put_json('_security', body=security)
+
+    def delete_database(self):
+        """
+        Delete a U1DB CouchDB database.
+        """
+        with couch_server(self._url) as server:
+            del(server[self._dbname])
+
+    def set_replica_uid(self, replica_uid):
+        """
+        Force the replica uid to be set.
+
+        :param replica_uid: The new replica uid.
+        :type replica_uid: str
+        """
+        try:
+            # set on existent config document
+            doc = self._database['u1db_config']
+            doc['replica_uid'] = replica_uid
+        except ResourceNotFound:
+            # or create the config document
+            doc = {
+                '_id': 'u1db_config',
+                'replica_uid': replica_uid,
+            }
+        self._database.save(doc)
+
+    def get_replica_uid(self):
+        """
+        Get the replica uid.
+
+        :return: The replica uid.
+        :rtype: str
+        """
+        try:
+            # grab replica_uid from server
+            doc = self._database['u1db_config']
+            replica_uid = doc['replica_uid']
+            return replica_uid
+        except ResourceNotFound:
+            # create a unique replica_uid
+            replica_uid = uuid.uuid4().hex
+            self.set_replica_uid(replica_uid)
+            return replica_uid
+
+    def close(self):
+        self._database = None
+
+    def get_all_docs(self, include_deleted=False):
+        """
+        Get the JSON content for all documents in the database.
+
+        :param include_deleted: If set to True, deleted documents will be
+                                returned with empty content. Otherwise deleted
+                                documents will not be included in the results.
+        :type include_deleted: bool
+
+        :return: (generation, [CouchDocument])
+            The current generation of the database, followed by a list of all
+            the documents in the database.
+        :rtype: (int, [CouchDocument])
+        """
+
+        generation, _ = self._get_generation_info()
+        results = list(self.get_docs(self._database,
+                                     include_deleted=include_deleted))
+        return (generation, results)
+
+    def get_docs(self, doc_ids, check_for_conflicts=True,
+                 include_deleted=False):
+        """
+        Get the JSON content for many documents.
+
+        :param doc_ids: A list of document identifiers or None for all.
+        :type doc_ids: list
+        :param check_for_conflicts: If set to False, then the conflict check
+                                    will be skipped, and 'None' will be
+                                    returned instead of True/False.
+        :type check_for_conflicts: bool
+        :param include_deleted: If set to True, deleted documents will be
+                                returned with empty content. Otherwise deleted
+                                documents will not be included in the results.
+        :return: iterable giving the Document object for each document id
+                 in matching doc_ids order.
+        :rtype: iterable
+        """
+        # Workaround for:
+        #
+        #   http://bugs.python.org/issue7980
+        #   https://leap.se/code/issues/5449
+        #
+        # python-couchdb uses time.strptime, which is not thread safe. In
+        # order to avoid the problem described on the issues above, we preload
+        # strptime here by evaluating the conversion of an arbitrary date.
+        # This will not be needed when/if we switch from python-couchdb to
+        # paisley.
+        time.strptime('Mar 8 1917', '%b %d %Y')
+        get_one = lambda doc_id: self._get_doc(doc_id, check_for_conflicts)
+        docs = [THREAD_POOL.apply_async(get_one, [doc_id])
+                for doc_id in doc_ids]
+        for doc in docs:
+            doc = doc.get()
+            if not doc or not include_deleted and doc.is_tombstone():
+                continue
+            yield doc
+
+    def _get_doc(self, doc_id, check_for_conflicts=False):
+        """
+        Extract the document from storage.
+
+        This can return None if the document doesn't exist.
+
+        :param doc_id: The unique document identifier
+        :type doc_id: str
+        :param check_for_conflicts: If set to False, then the conflict check
+                                    will be skipped.
+        :type check_for_conflicts: bool
+
+        :return: The document.
+        :rtype: CouchDocument
+        """
+        # get document with all attachments (u1db content and eventual
+        # conflicts)
+        try:
+            result = \
+                self._database.resource(doc_id).get_json(
+                    attachments=True)[2]
+        except ResourceNotFound:
+            return None
+        return self.__parse_doc_from_couch(result, doc_id, check_for_conflicts)
+
+    def __parse_doc_from_couch(self, result, doc_id,
+                               check_for_conflicts=False):
+        # restrict to u1db documents
+        if 'u1db_rev' not in result:
+            return None
+        doc = CouchDocument(doc_id, result['u1db_rev'])
+        # set contents or make tombstone
+        if '_attachments' not in result \
+                or 'u1db_content' not in result['_attachments']:
+            doc.make_tombstone()
+        else:
+            doc.content = json.loads(
+                binascii.a2b_base64(
+                    result['_attachments']['u1db_content']['data']))
+        # determine if there are conflicts
+        if check_for_conflicts \
+                and '_attachments' in result \
+                and 'u1db_conflicts' in result['_attachments']:
+            doc.set_conflicts(
+                self._build_conflicts(
+                    doc.doc_id,
+                    json.loads(binascii.a2b_base64(
+                        result['_attachments']['u1db_conflicts']['data']))))
+        # store couch revision
+        doc.couch_rev = result['_rev']
+        # store transactions
+        doc.transactions = result['u1db_transactions']
+        return doc
+
+    def _build_conflicts(self, doc_id, attached_conflicts):
+        """
+        Build the conflicted documents list from the conflicts attachment
+        fetched from a couch document.
+
+        :param attached_conflicts: The document's conflicts as fetched from a
+                                   couch document attachment.
+        :type attached_conflicts: dict
+        """
+        conflicts = []
+        for doc_rev, content in attached_conflicts:
+            doc = CouchDocument(doc_id, doc_rev)
+            if content is None:
+                doc.make_tombstone()
+            else:
+                doc.content = content
+            conflicts.append(doc)
+        return conflicts
+
+    def _get_trans_id_for_gen(self, generation):
+        """
+        Get the transaction id corresponding to a particular generation.
+
+        :param generation: The generation for which to get the transaction id.
+        :type generation: int
+
+        :return: The transaction id for C{generation}.
+        :rtype: str
+
+        :raise InvalidGeneration: Raised when the generation does not exist.
+        :raise MissingDesignDocError: Raised when tried to access a missing
+                                      design document.
+        :raise MissingDesignDocListFunctionError: Raised when trying to access
+                                                  a missing list function on a
+                                                  design document.
+        :raise MissingDesignDocNamedViewError: Raised when trying to access a
+                                               missing named view on a design
+                                               document.
+        :raise MissingDesignDocDeletedError: Raised when trying to access a
+                                             deleted design document.
+        :raise MissingDesignDocUnknownError: Raised when failed to access a
+                                             design document for an yet
+                                             unknown reason.
+        """
+        if generation == 0:
+            return ''
+        # query a couch list function
+        ddoc_path = [
+            '_design', 'transactions', '_list', 'trans_id_for_gen', 'log'
+        ]
+        res = self._database.resource(*ddoc_path)
+        try:
+            response = res.get_json(gen=generation)
+            if response[2] == {}:
+                raise InvalidGeneration
+            return response[2]['transaction_id']
+        except ResourceNotFound as e:
+            raise_missing_design_doc_error(e, ddoc_path)
+        except ServerError as e:
+            raise_server_error(e, ddoc_path)
+
+    def _get_replica_gen_and_trans_id(self, other_replica_uid):
+        """
+        Return the last known generation and transaction id for the other db
+        replica.
+
+        When you do a synchronization with another replica, the Database keeps
+        track of what generation the other database replica was at, and what
+        the associated transaction id was.  This is used to determine what data
+        needs to be sent, and if two databases are claiming to be the same
+        replica.
+
+        :param other_replica_uid: The identifier for the other replica.
+        :type other_replica_uid: str
+
+        :return: A tuple containing the generation and transaction id we
+                 encountered during synchronization. If we've never
+                 synchronized with the replica, this is (0, '').
+        :rtype: (int, str)
+        """
+        doc_id = 'u1db_sync_%s' % other_replica_uid
+        try:
+            doc = self._database[doc_id]
+        except ResourceNotFound:
+            doc = {
+                '_id': doc_id,
+                'generation': 0,
+                'transaction_id': '',
+            }
+            self._database.save(doc)
+        result = doc['generation'], doc['transaction_id']
+        return result
+
+    def get_doc_conflicts(self, doc_id, couch_rev=None):
+        """
+        Get the conflicted versions of a document.
+
+        If the C{couch_rev} parameter is not None, conflicts for a specific
+        document's couch revision are returned.
+
+        :param couch_rev: The couch document revision.
+        :type couch_rev: str
+
+        :return: A list of conflicted versions of the document.
+        :rtype: list
+        """
+        # request conflicts attachment from server
+        params = {}
+        conflicts = []
+        if couch_rev is not None:
+            params['rev'] = couch_rev  # restric document's couch revision
+        else:
+            # TODO: move into resource logic!
+            first_entry = self._get_doc(doc_id, check_for_conflicts=True)
+            conflicts.append(first_entry)
+        resource = self._database.resource(doc_id, 'u1db_conflicts')
+        try:
+            response = resource.get_json(**params)
+            return conflicts + self._build_conflicts(
+                doc_id, json.loads(response[2].read()))
+        except ResourceNotFound:
+            return []
+
+    def _do_set_replica_gen_and_trans_id(
+            self, other_replica_uid, other_generation, other_transaction_id,
+            number_of_docs=None, doc_idx=None, sync_id=None):
+        """
+        Set the last-known generation and transaction id for the other
+        database replica.
+
+        We have just performed some synchronization, and we want to track what
+        generation the other replica was at. See also
+        _get_replica_gen_and_trans_id.
+
+        :param other_replica_uid: The U1DB identifier for the other replica.
+        :type other_replica_uid: str
+        :param other_generation: The generation number for the other replica.
+        :type other_generation: int
+        :param other_transaction_id: The transaction id associated with the
+                                     generation.
+        :type other_transaction_id: str
+        :param number_of_docs: The total amount of documents sent on this sync
+                               session.
+        :type number_of_docs: int
+        :param doc_idx: The index of the current document being sent.
+        :type doc_idx: int
+        :param sync_id: The id of the current sync session.
+        :type sync_id: str
+        """
+        doc_id = 'u1db_sync_%s' % other_replica_uid
+        try:
+            doc = self._database[doc_id]
+        except ResourceNotFound:
+            doc = {'_id': doc_id}
+        doc['generation'] = other_generation
+        doc['transaction_id'] = other_transaction_id
+        self._database.save(doc)
+
+    def _get_transaction_log(self):
+        """
+        This is only for the test suite, it is not part of the api.
+
+        :return: The complete transaction log.
+        :rtype: [(str, str)]
+
+        :raise MissingDesignDocError: Raised when tried to access a missing
+                                      design document.
+        :raise MissingDesignDocListFunctionError: Raised when trying to access
+                                                  a missing list function on a
+                                                  design document.
+        :raise MissingDesignDocNamedViewError: Raised when trying to access a
+                                               missing named view on a design
+                                               document.
+        :raise MissingDesignDocDeletedError: Raised when trying to access a
+                                             deleted design document.
+        :raise MissingDesignDocUnknownError: Raised when failed to access a
+                                             design document for an yet
+                                             unknown reason.
+        """
+        # query a couch view
+        ddoc_path = ['_design', 'transactions', '_view', 'log']
+        res = self._database.resource(*ddoc_path)
+        try:
+            response = res.get_json()
+            return map(
+                lambda row: (row['id'], row['value']),
+                response[2]['rows'])
+        except ResourceNotFound as e:
+            raise_missing_design_doc_error(e, ddoc_path)
+
+    def whats_changed(self, old_generation=0):
+        """
+        Return a list of documents that have changed since old_generation.
+
+        :param old_generation: The generation of the database in the old
+                               state.
+        :type old_generation: int
+
+        :return: (generation, trans_id, [(doc_id, generation, trans_id),...])
+                 The current generation of the database, its associated
+                 transaction id, and a list of of changed documents since
+                 old_generation, represented by tuples with for each document
+                 its doc_id and the generation and transaction id corresponding
+                 to the last intervening change and sorted by generation (old
+                 changes first)
+        :rtype: (int, str, [(str, int, str)])
+
+        :raise MissingDesignDocError: Raised when tried to access a missing
+                                      design document.
+        :raise MissingDesignDocListFunctionError: Raised when trying to access
+                                                  a missing list function on a
+                                                  design document.
+        :raise MissingDesignDocNamedViewError: Raised when trying to access a
+                                               missing named view on a design
+                                               document.
+        :raise MissingDesignDocDeletedError: Raised when trying to access a
+                                             deleted design document.
+        :raise MissingDesignDocUnknownError: Raised when failed to access a
+                                             design document for an yet
+                                             unknown reason.
+        """
+        # query a couch list function
+        ddoc_path = [
+            '_design', 'transactions', '_list', 'whats_changed', 'log'
+        ]
+        res = self._database.resource(*ddoc_path)
+        try:
+            response = res.get_json(old_gen=old_generation)
+            results = map(
+                lambda row:
+                    (row['generation'], row['doc_id'], row['transaction_id']),
+                response[2]['transactions'])
+            results.reverse()
+            cur_gen = old_generation
+            seen = set()
+            changes = []
+            newest_trans_id = ''
+            for generation, doc_id, trans_id in results:
+                if doc_id not in seen:
+                    changes.append((doc_id, generation, trans_id))
+                    seen.add(doc_id)
+            if changes:
+                cur_gen = changes[0][1]  # max generation
+                newest_trans_id = changes[0][2]
+                changes.reverse()
+            else:
+                cur_gen, newest_trans_id = self._get_generation_info()
+
+            return cur_gen, newest_trans_id, changes
+        except ResourceNotFound as e:
+            raise_missing_design_doc_error(e, ddoc_path)
+        except ServerError as e:
+            raise_server_error(e, ddoc_path)
+
+    def _get_generation_info(self):
+        """
+        Return the current generation.
+
+        :return: A tuple containing the current generation and transaction id.
+        :rtype: (int, str)
+
+        :raise MissingDesignDocError: Raised when tried to access a missing
+                                      design document.
+        :raise MissingDesignDocListFunctionError: Raised when trying to access
+                                                  a missing list function on a
+                                                  design document.
+        :raise MissingDesignDocNamedViewError: Raised when trying to access a
+                                               missing named view on a design
+                                               document.
+        :raise MissingDesignDocDeletedError: Raised when trying to access a
+                                             deleted design document.
+        :raise MissingDesignDocUnknownError: Raised when failed to access a
+                                             design document for an yet
+                                             unknown reason.
+        """
+        # query a couch list function
+        ddoc_path = ['_design', 'transactions', '_list', 'generation', 'log']
+        res = self._database.resource(*ddoc_path)
+        try:
+            response = res.get_json()
+            return (response[2]['generation'], response[2]['transaction_id'])
+        except ResourceNotFound as e:
+            raise_missing_design_doc_error(e, ddoc_path)
+        except ServerError as e:
+            raise_server_error(e, ddoc_path)
+
+    def save_document(self, old_doc, doc, transaction_id):
+        """
+        Put the document in the Couch backend database.
+
+        Note that C{old_doc} must have been fetched with the parameter
+        C{check_for_conflicts} equal to True, so we can properly update the
+        new document using the conflict information from the old one.
+
+        :param old_doc: The old document version.
+        :type old_doc: CouchDocument
+        :param doc: The document to be put.
+        :type doc: CouchDocument
+
+        :raise RevisionConflict: Raised when trying to update a document but
+                                 couch revisions mismatch.
+        :raise MissingDesignDocError: Raised when tried to access a missing
+                                      design document.
+        :raise MissingDesignDocListFunctionError: Raised when trying to access
+                                                  a missing list function on a
+                                                  design document.
+        :raise MissingDesignDocNamedViewError: Raised when trying to access a
+                                               missing named view on a design
+                                               document.
+        :raise MissingDesignDocDeletedError: Raised when trying to access a
+                                             deleted design document.
+        :raise MissingDesignDocUnknownError: Raised when failed to access a
+                                             design document for an yet
+                                             unknown reason.
+        """
+        attachments = {}  # we save content and conflicts as attachments
+        parts = []  # and we put it using couch's multipart PUT
+        # save content as attachment
+        if doc.is_tombstone() is False:
+            content = doc.get_json()
+            attachments['u1db_content'] = {
+                'follows': True,
+                'content_type': 'application/octet-stream',
+                'length': len(content),
+            }
+            parts.append(content)
+        # save conflicts as attachment
+        if doc.has_conflicts is True:
+            conflicts = json.dumps(
+                map(lambda cdoc: (cdoc.rev, cdoc.content),
+                    doc.get_conflicts()))
+            attachments['u1db_conflicts'] = {
+                'follows': True,
+                'content_type': 'application/octet-stream',
+                'length': len(conflicts),
+            }
+            parts.append(conflicts)
+        # store old transactions, if any
+        transactions = old_doc.transactions[:] if old_doc is not None else []
+        # create a new transaction id and timestamp it so the transaction log
+        # is consistent when querying the database.
+        transactions.append(
+            # here we store milliseconds to keep consistent with javascript
+            # Date.prototype.getTime() which was used before inside a couchdb
+            # update handler.
+            (int(time.time() * 1000),
+             transaction_id))
+        # build the couch document
+        couch_doc = {
+            '_id': doc.doc_id,
+            'u1db_rev': doc.rev,
+            'u1db_transactions': transactions,
+            '_attachments': attachments,
+        }
+        # if we are updating a doc we have to add the couch doc revision
+        if old_doc is not None:
+            couch_doc['_rev'] = old_doc.couch_rev
+        # prepare the multipart PUT
+        buf = StringIO()
+        headers = {}
+        envelope = MultipartWriter(buf, headers=headers, subtype='related')
+        envelope.add('application/json', json.dumps(couch_doc))
+        for part in parts:
+            envelope.add('application/octet-stream', part)
+        envelope.close()
+        # try to save and fail if there's a revision conflict
+        try:
+            resource = self._new_resource()
+            resource.put_json(
+                doc.doc_id, body=str(buf.getvalue()), headers=headers)
+        except ResourceConflict:
+            raise RevisionConflict()
+        return transactions[-1][1]
+
+    def _new_resource(self, *path):
+        """
+        Return a new resource for accessing a couch database.
+
+        :return: A resource for accessing a couch database.
+        :rtype: couchdb.http.Resource
+        """
+        # Workaround for: https://leap.se/code/issues/5448
+        url = couch_urljoin(self._database.resource.url, *path)
+        resource = Resource(url, Session(timeout=COUCH_TIMEOUT))
+        resource.credentials = self._database.resource.credentials
+        resource.headers = self._database.resource.headers.copy()
+        return resource