[feature] add priorities for blob transfers

Closes: #8691

4 files changed, 411 insertions, 63 deletions

diff --git a/src/leap/soledad/client/_db/blobs/__init__.py b/src/leap/soledad/client/_db/blobs/__init__.py
index 4699e0a0..df801850 100644
--- a/src/leap/soledad/client/_db/blobs/__init__.py
+++ b/src/leap/soledad/client/_db/blobs/__init__.py
@@ -24,6 +24,7 @@ import os
 import json
 import base64
 
+from collections import defaultdict
 from io import BytesIO
 
 from twisted.logger import Logger
@@ -44,6 +45,7 @@ from leap.soledad.client._pipes import TruncatedTailPipe
 from leap.soledad.client._pipes import PreamblePipe
 
 from .sql import SyncStatus
+from .sql import Priority
 from .sql import SQLiteBlobBackend
 from .sync import BlobsSynchronizer
 from .errors import (
@@ -139,6 +141,7 @@ class BlobManager(BlobsSynchronizer):
         self.user = user
         self._client = HTTPClient(user, token, cert_file)
         self.semaphore = defer.DeferredSemaphore(self.concurrent_writes_limit)
+        self.locks = defaultdict(defer.DeferredLock)
 
     def close(self):
         if hasattr(self, 'local') and self.local:
@@ -207,7 +210,8 @@ class BlobManager(BlobsSynchronizer):
     def local_list_status(self, status, namespace=''):
         return self.local.list_status(status, namespace)
 
-    def put(self, doc, size, namespace='', local_only=False):
+    def put(self, doc, size, namespace='', local_only=False,
+            priority=Priority.DEFAULT):
         """
         Put a blob in local storage and upload it to server.
 
@@ -220,12 +224,15 @@ class BlobManager(BlobsSynchronizer):
         :param namespace:
             Optional parameter to restrict operation to a given namespace.
         :type namespace: str
+
+        :return: A deferred that fires when the blob has been put.
+        :rtype: twisted.internet.defer.Deferred
         """
         return self.semaphore.run(
-            self._put, doc, size, namespace, local_only=local_only)
+            self._put, doc, size, namespace, local_only, priority)
 
     @defer.inlineCallbacks
-    def _put(self, doc, size, namespace, local_only=False):
+    def _put(self, doc, size, namespace, local_only, priority):
         if (yield self.local.exists(doc.blob_id, namespace=namespace)):
             error_message = "Blob already exists: %s" % doc.blob_id
             raise BlobAlreadyExistsError(error_message)
@@ -233,17 +240,31 @@ class BlobManager(BlobsSynchronizer):
         # TODO this is a tee really, but ok... could do db and upload
         # concurrently. not sure if we'd gain something.
         yield self.local.put(doc.blob_id, fd, size=size, namespace=namespace)
+
         if local_only:
             yield self.local.update_sync_status(
-                doc.blob_id, SyncStatus.LOCAL_ONLY)
+                doc.blob_id, SyncStatus.LOCAL_ONLY, namespace=namespace)
             defer.returnValue(None)
+
         yield self.local.update_sync_status(
-            doc.blob_id, SyncStatus.PENDING_UPLOAD)
+            doc.blob_id, SyncStatus.PENDING_UPLOAD, namespace=namespace,
+            priority=priority)
+        yield self._send(doc.blob_id, namespace, 1, 1)
+
+    def _send(self, blob_id, namespace, i, total):
+        lock = self.locks[blob_id]
+        d = lock.run(self.__send, blob_id, namespace, i, total)
+        return d
+
+    @defer.inlineCallbacks
+    def __send(self, blob_id, namespace, i, total):
+        logger.info("Sending blob to server (%d/%d): %s"
+                    % (i, total, blob_id))
         # In fact, some kind of pipe is needed here, where each write on db
         # handle gets forwarded into a write on the connection handle
-        fd = yield self.local.get(doc.blob_id, namespace=namespace)
-        yield self._encrypt_and_upload(doc.blob_id, fd, namespace=namespace)
-        yield self.local.update_sync_status(doc.blob_id, SyncStatus.SYNCED)
+        fd = yield self.local.get(blob_id, namespace=namespace)
+        yield self._encrypt_and_upload(blob_id, fd)
+        yield self.local.update_sync_status(blob_id, SyncStatus.SYNCED)
 
     def set_flags(self, blob_id, flags, namespace=''):
         """
@@ -294,7 +315,7 @@ class BlobManager(BlobsSynchronizer):
         defer.returnValue((yield response.json()))
 
     @defer.inlineCallbacks
-    def get(self, blob_id, namespace=''):
+    def get(self, blob_id, namespace='', priority=Priority.DEFAULT):
         """
         Get the blob from local storage or, if not available, from the server.
 
@@ -304,6 +325,10 @@ class BlobManager(BlobsSynchronizer):
         :param namespace:
             Optional parameter to restrict operation to a given namespace.
         :type namespace: str
+
+        :return: A deferred that fires with the file descriptor for the
+                 contents of the blob.
+        :rtype: twisted.internet.defer.Deferred
         """
         local_blob = yield self.local.get(blob_id, namespace=namespace)
         if local_blob:
@@ -311,8 +336,19 @@ class BlobManager(BlobsSynchronizer):
             defer.returnValue(local_blob)
 
         yield self.local.update_sync_status(
-            blob_id, SyncStatus.PENDING_DOWNLOAD, namespace=namespace)
+            blob_id, SyncStatus.PENDING_DOWNLOAD, namespace=namespace,
+            priority=priority)
+
+        fd = yield self._fetch(blob_id, namespace)
+        defer.returnValue(fd)
+
+    def _fetch(self, blob_id, namespace):
+        lock = self.locks[blob_id]
+        d = lock.run(self.__fetch, blob_id, namespace)
+        return d
 
+    @defer.inlineCallbacks
+    def __fetch(self, blob_id, namespace):
         try:
             result = yield self._download_and_decrypt(blob_id, namespace)
         except Exception as e:
@@ -347,6 +383,8 @@ class BlobManager(BlobsSynchronizer):
             logger.info("Got decrypted blob of type: %s" % type(blob))
             blob.seek(0)
             yield self.local.put(blob_id, blob, size=size, namespace=namespace)
+            yield self.local.update_sync_status(blob_id, SyncStatus.SYNCED,
+                                                namespace=namespace)
             local_blob = yield self.local.get(blob_id, namespace=namespace)
             defer.returnValue(local_blob)
         else:
@@ -435,3 +473,38 @@ class BlobManager(BlobsSynchronizer):
         response = yield self._client.delete(uri, params=params)
         check_http_status(response.code, blob_id=blob_id)
         defer.returnValue(response)
+
+    def set_priority(self, blob_id, priority, namespace=''):
+        """
+        Set the transfer priority for a certain blob.
+
+        :param blob_id: Unique identifier of a blob.
+        :type blob_id: str
+        :param priority: The priority to be set.
+        :type priority: int
+        :param namespace: Optional parameter to restrict operation to a given
+                          namespace.
+        :type namespace: str
+
+        :return: A deferred that fires after the priority has been set.
+        :rtype: twisted.internet.defer.Deferred
+        """
+        d = self.local.update_priority(blob_id, priority, namespace=namespace)
+        return d
+
+    def get_priority(self, blob_id, namespace=''):
+        """
+        Get the transfer priority for a certain blob.
+
+        :param blob_id: Unique identifier of a blob.
+        :type blob_id: str
+        :param namespace: Optional parameter to restrict operation to a given
+                          namespace.
+        :type namespace: str
+
+        :return: A deferred that fires with the current transfer priority of
+                 the blob.
+        :rtype: twisted.internet.defer.Deferred
+        """
+        d = self.local.get_priority(blob_id, namespace=namespace)
+        return d
diff --git a/src/leap/soledad/client/_db/blobs/sql.py b/src/leap/soledad/client/_db/blobs/sql.py
index ebd6c095..a89802d8 100644
--- a/src/leap/soledad/client/_db/blobs/sql.py
+++ b/src/leap/soledad/client/_db/blobs/sql.py
@@ -46,6 +46,14 @@ class SyncStatus:
     UNAVAILABLE_STATUSES = (3, 5)
 
 
+class Priority:
+    LOW = 1
+    MEDIUM = 2
+    HIGH = 3
+    URGENT = 4
+    DEFAULT = 2
+
+
 class SQLiteBlobBackend(object):
 
     concurrency_limit = 10
@@ -130,7 +138,7 @@ class SQLiteBlobBackend(object):
     @defer.inlineCallbacks
     def list_status(self, sync_status, namespace=''):
         query = 'select blob_id from sync_state where sync_status = ?'
-        query += 'AND namespace = ?'
+        query += 'AND namespace = ? ORDER BY priority DESC'
         values = (sync_status, namespace,)
         result = yield self.dbpool.runQuery(query, values)
         if result:
@@ -139,21 +147,42 @@ class SQLiteBlobBackend(object):
             defer.returnValue([])
 
     @defer.inlineCallbacks
-    def update_sync_status(self, blob_id, sync_status, namespace=""):
-        query = 'SELECT sync_status FROM sync_state WHERE blob_id = ?'
-        result = yield self.dbpool.runQuery(query, (blob_id,))
+    def update_sync_status(self, blob_id, sync_status, namespace="",
+                           priority=None):
+        retries = '(SELECT retries from sync_state' \
+                  ' WHERE blob_id="%s" and namespace="%s")' \
+                  % (blob_id, namespace)
+        if not priority:
+            priority = '(SELECT priority FROM sync_state' \
+                       ' WHERE blob_id="%s" AND namespace="%s")' \
+                       % (blob_id, namespace)
+        fields = 'blob_id, namespace, sync_status, retries, priority'
+        markers = '?, ?, ?, %s, %s' % (retries, priority)
+        values = [blob_id, namespace, sync_status]
+        insert = 'INSERT or REPLACE INTO sync_state (%s) VALUES (%s)' \
+                 % (fields, markers)
+        yield self.dbpool.runOperation(insert, tuple(values))
 
+    @defer.inlineCallbacks
+    def get_priority(self, blob_id, namespace=""):
+        query = 'SELECT priority FROM sync_state WHERE blob_id = ?'
+        result = yield self.dbpool.runQuery(query, (blob_id,))
         if not result:
-            insert = 'INSERT INTO sync_state'
-            insert += ' (blob_id, namespace, sync_status)'
-            insert += ' VALUES (?, ?, ?)'
-            values = (blob_id, namespace, sync_status)
-            yield self.dbpool.runOperation(insert, values)
-            return
+            defer.returnValue(None)
+        priority = result.pop()[0]
+        defer.returnValue(priority)
 
-        update = 'UPDATE sync_state SET sync_status = ? WHERE blob_id = ?'
-        values = (sync_status, blob_id,)
-        result = yield self.dbpool.runOperation(update, values)
+    @defer.inlineCallbacks
+    def update_priority(self, blob_id, priority, namespace=""):
+        old_priority = self.get_priority(blob_id, namespace=namespace)
+        if not old_priority:
+            logger.error("Can't update priority of %s: no sync status entry.")
+            return
+        if old_priority == priority:
+            return
+        update = 'UPDATE sync_state SET priority = ? WHERE blob_id = ?'
+        values = (priority, blob_id,)
+        yield self.dbpool.runOperation(update, values)
 
     def update_batch_sync_status(self, blob_id_list, sync_status,
                                  namespace=''):
@@ -161,11 +190,12 @@ class SQLiteBlobBackend(object):
             return
         insert = 'INSERT or REPLACE INTO sync_state'
         first_blob_id, blob_id_list = blob_id_list[0], blob_id_list[1:]
-        insert += ' (blob_id, namespace, sync_status) VALUES (?, ?, ?)'
-        values = (first_blob_id, namespace, sync_status)
+        insert += ' (blob_id, namespace, sync_status, priority)'
+        insert += ' VALUES (?, ?, ?, ?)'
+        values = (first_blob_id, namespace, sync_status, Priority.DEFAULT)
         for blob_id in blob_id_list:
-            insert += ', (?, ?, ?)'
-            values += (blob_id, namespace, sync_status)
+            insert += ', (?, ?, ?, ?)'
+            values += (blob_id, namespace, sync_status, Priority.DEFAULT)
         return self.dbpool.runQuery(insert, values)
 
     def increment_retries(self, blob_id):
@@ -212,9 +242,11 @@ def _init_sync_table(conn):
         blob_id PRIMARY KEY,
         namespace TEXT,
         sync_status INT default %s,
+        priority INT default %d,
         retries INT default 0)"""
     default_status = SyncStatus.PENDING_UPLOAD
-    maybe_create %= default_status
+    default_priority = Priority.DEFAULT
+    maybe_create %= (default_status, default_priority)
     conn.execute(maybe_create)
 
 
diff --git a/src/leap/soledad/client/_db/blobs/sync.py b/src/leap/soledad/client/_db/blobs/sync.py
index 3ee60305..e6397ede 100644
--- a/src/leap/soledad/client/_db/blobs/sync.py
+++ b/src/leap/soledad/client/_db/blobs/sync.py
@@ -17,12 +17,15 @@
 """
 Synchronization between blobs client/server
 """
+from collections import defaultdict
 from twisted.internet import defer
 from twisted.internet import reactor
 from twisted.logger import Logger
 from twisted.internet import error
 from .sql import SyncStatus
 from .errors import RetriableTransferError
+
+
 logger = Logger()
 
 
@@ -56,6 +59,9 @@ def with_retry(func, *args, **kwargs):
 
 class BlobsSynchronizer(object):
 
+    def __init__(self):
+        self.locks = defaultdict(defer.DeferredLock)
+
     @defer.inlineCallbacks
     def refresh_sync_status_from_server(self, namespace=''):
         d1 = self.remote_list(namespace=namespace)
@@ -82,7 +88,6 @@ class BlobsSynchronizer(object):
             SyncStatus.SYNCED,
             namespace=namespace)
 
-    @defer.inlineCallbacks
     def send_missing(self, namespace=''):
         """
         Compare local and remote blobs and send what's missing in server.
@@ -90,30 +95,41 @@ class BlobsSynchronizer(object):
         :param namespace:
             Optional parameter to restrict operation to a given namespace.
         :type namespace: str
+
+        :return: A deferred that fires when all local blobs were sent to
+                 server.
+        :rtype: twisted.internet.defer.Deferred
         """
-        missing = yield self.local.list_status(
-            SyncStatus.PENDING_UPLOAD, namespace)
-        total = len(missing)
-        logger.info("Will send %d blobs to server." % total)
-        deferreds = []
-        semaphore = defer.DeferredSemaphore(self.concurrent_transfers_limit)
-
-        for i in xrange(total):
-            blob_id = missing.pop()
-            d = semaphore.run(
-                with_retry, self.__send_one, blob_id, namespace, i, total)
-            deferreds.append(d)
-        yield defer.gatherResults(deferreds, consumeErrors=True)
+        lock = self.locks['send_missing']
+        d = lock.run(self._send_missing, namespace)
+        return d
 
     @defer.inlineCallbacks
-    def __send_one(self, blob_id, namespace, i, total):
-        logger.info("Sending blob to server (%d/%d): %s"
-                    % (i, total, blob_id))
-        fd = yield self.local.get(blob_id, namespace=namespace)
-        yield self._encrypt_and_upload(blob_id, fd)
-        yield self.local.update_sync_status(blob_id, SyncStatus.SYNCED)
+    def _send_missing(self, namespace):
+        max_transfers = self.concurrent_transfers_limit
+        semaphore = defer.DeferredSemaphore(max_transfers)
+        # the list of blobs should be refreshed often, so we run as many
+        # concurrent transfers as we can and then refresh the list
+        while True:
+            d = self.local_list_status(SyncStatus.PENDING_UPLOAD, namespace)
+            missing = yield d
+
+            if not missing:
+                break
+
+            total = len(missing)
+            now = min(total, max_transfers)
+            logger.info("There are %d pending blob uploads." % total)
+            logger.info("Will send %d blobs to server now." % now)
+            missing = missing[:now]
+            deferreds = []
+            for i in xrange(now):
+                blob_id = missing.pop(0)
+                d = semaphore.run(
+                    with_retry, self._send, blob_id, namespace, i, total)
+                deferreds.append(d)
+            yield defer.gatherResults(deferreds, consumeErrors=True)
 
-    @defer.inlineCallbacks
     def fetch_missing(self, namespace=''):
         """
         Compare local and remote blobs and fetch what's missing in local
@@ -122,21 +138,41 @@ class BlobsSynchronizer(object):
         :param namespace:
             Optional parameter to restrict operation to a given namespace.
         :type namespace: str
+
+        :return: A deferred that fires when all remote blobs were received from
+                 server.
+        :rtype: twisted.internet.defer.Deferred
         """
-        # TODO: Use something to prioritize user requests over general new docs
-        d = self.local_list_status(SyncStatus.PENDING_DOWNLOAD, namespace)
-        docs_we_want = yield d
-        total = len(docs_we_want)
-        logger.info("Will fetch %d blobs from server." % total)
-        deferreds = []
-        semaphore = defer.DeferredSemaphore(self.concurrent_transfers_limit)
-
-        for i in xrange(len(docs_we_want)):
-            blob_id = docs_we_want.pop()
-            logger.info("Fetching blob (%d/%d): %s" % (i, total, blob_id))
-            d = semaphore.run(with_retry, self.get, blob_id, namespace)
-            deferreds.append(d)
-        yield defer.gatherResults(deferreds, consumeErrors=True)
+        lock = self.locks['fetch_missing']
+        d = lock.run(self._fetch_missing, namespace)
+        return d
+
+    @defer.inlineCallbacks
+    def _fetch_missing(self, namespace=''):
+        max_transfers = self.concurrent_transfers_limit
+        semaphore = defer.DeferredSemaphore(max_transfers)
+        # in order to make sure that transfer priorities will be met, the list
+        # of blobs to transfer should be refreshed often. What we do is run as
+        # many concurrent transfers as we can and then refresh the list
+        while True:
+            d = self.local_list_status(SyncStatus.PENDING_DOWNLOAD, namespace)
+            docs_we_want = yield d
+
+            if not docs_we_want:
+                break
+
+            total = len(docs_we_want)
+            now = min(total, max_transfers)
+            logger.info("There are %d pending blob downloads." % total)
+            logger.info("Will fetch %d blobs from server now." % now)
+            docs_we_want = docs_we_want[:now]
+            deferreds = []
+            for i in xrange(now):
+                blob_id = docs_we_want.pop(0)
+                logger.info("Fetching blob (%d/%d): %s" % (i, now, blob_id))
+                d = semaphore.run(with_retry, self._fetch, blob_id, namespace)
+                deferreds.append(d)
+            yield defer.gatherResults(deferreds, consumeErrors=True)
 
     @defer.inlineCallbacks
     def sync(self, namespace=''):
diff --git a/tests/blobs/test_priorities.py b/tests/blobs/test_priorities.py
new file mode 100644
index 00000000..fca582e5
--- /dev/null
+++ b/tests/blobs/test_priorities.py
@@ -0,0 +1,207 @@
+# -*- coding: utf-8 -*-
+# test_priorities.py
+# Copyright (C) 2017 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/>.
+"""
+Tests for transfer priorities.
+"""
+import pytest
+
+from io import BytesIO
+from mock import Mock
+from twisted.internet import defer
+from twisted.trial import unittest
+from uuid import uuid4
+
+from leap.soledad.client._db.blobs import BlobManager
+from leap.soledad.client._db.blobs import Priority
+from leap.soledad.client._db.blobs import SyncStatus
+from leap.soledad.client._document import BlobDoc
+
+
+class BlobPrioritiesTests(unittest.TestCase):
+
+    def setUp(self):
+        self.cleartext = BytesIO('patriarchy is opression')
+        self.secret = 'A' * 96
+        self.manager = BlobManager(
+            self.tempdir, '',
+            'A' * 32, self.secret,
+            uuid4().hex, 'token', None)
+        self.addCleanup(self.manager.close)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_get_sets_default_priority(self):
+        self.manager._download_and_decrypt = Mock(return_value=None)
+        missing_blob_id = uuid4().hex
+        result = yield self.manager.get(missing_blob_id)
+        self.assertIsNone(result)
+        priority = yield self.manager.get_priority(missing_blob_id)
+        self.assertEqual(Priority.DEFAULT, priority)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_get_sets_priority(self):
+        self.manager._download_and_decrypt = Mock(return_value=None)
+        missing_blob_id = uuid4().hex
+        urgent = Priority.URGENT
+        result = yield self.manager.get(missing_blob_id, priority=urgent)
+        self.assertIsNone(result)
+        priority = yield self.manager.get_priority(missing_blob_id)
+        self.assertEqual(urgent, priority)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_put_sets_default_priority(self):
+        upload_failure = defer.fail(Exception())
+        self.manager._encrypt_and_upload = Mock(return_value=upload_failure)
+        content, blob_id = "Blob content", uuid4().hex
+        doc1 = BlobDoc(BytesIO(content), blob_id)
+        with pytest.raises(Exception):
+            yield self.manager.put(doc1, len(content))
+        priority = yield self.manager.get_priority(blob_id)
+        self.assertEqual(Priority.DEFAULT, priority)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_put_sets_priority(self):
+        upload_failure = defer.fail(Exception())
+        self.manager._encrypt_and_upload = Mock(return_value=upload_failure)
+        content, blob_id = "Blob content", uuid4().hex
+        doc1 = BlobDoc(BytesIO(content), blob_id)
+        urgent = Priority.URGENT
+        with pytest.raises(Exception):
+            yield self.manager.put(doc1, len(content), priority=urgent)
+        priority = yield self.manager.get_priority(blob_id)
+        self.assertEqual(urgent, priority)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_set_priority_sets_priority(self):
+        self.manager._download_and_decrypt = Mock(return_value=None)
+        missing_blob_id = uuid4().hex
+        result = yield self.manager.get(missing_blob_id)
+        self.assertIsNone(result)
+        urgent = Priority.URGENT
+        yield self.manager.set_priority(missing_blob_id, urgent)
+        priority = yield self.manager.get_priority(missing_blob_id)
+        self.assertEqual(urgent, priority)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_local_list_status_orders_by_priority(self):
+        self.manager._download_and_decrypt = Mock(return_value=None)
+
+        def _get(priority):
+            missing_blob_id = uuid4().hex
+            d = self.manager.get(missing_blob_id, priority=priority)
+            d.addCallback(lambda _: missing_blob_id)
+            return d
+
+        # get some blobs in arbitrary order
+        low = yield _get(Priority.LOW)
+        high = yield _get(Priority.HIGH)
+        medium = yield _get(Priority.MEDIUM)
+        urgent = yield _get(Priority.URGENT)
+
+        # make sure they are ordered by priority
+        status = SyncStatus.PENDING_DOWNLOAD
+        pending = yield self.manager.local_list_status(status)
+        self.assertEqual([urgent, high, medium, low], pending)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_refresh_sync_status_from_server_saves_default_priorities(self):
+        remote_ids = [uuid4().hex for _ in range(10)]
+        self.manager.remote_list = Mock(return_value=defer.succeed(remote_ids))
+        yield self.manager.refresh_sync_status_from_server()
+        for blob_id in remote_ids:
+            priority = yield self.manager.get_priority(blob_id)
+            self.assertEquals(Priority.DEFAULT, priority)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_fetch_missing_fetches_with_priority(self):
+
+        # pretend we have some pending downloads
+        deferreds = [
+            self.manager.local.update_sync_status(
+                'low', SyncStatus.PENDING_DOWNLOAD,
+                priority=Priority.LOW),
+            self.manager.local.update_sync_status(
+                'high', SyncStatus.PENDING_DOWNLOAD,
+                priority=Priority.HIGH),
+            self.manager.local.update_sync_status(
+                'medium', SyncStatus.PENDING_DOWNLOAD,
+                priority=Priority.MEDIUM),
+            self.manager.local.update_sync_status(
+                'urgent', SyncStatus.PENDING_DOWNLOAD,
+                priority=Priority.URGENT),
+        ]
+        yield defer.gatherResults(deferreds)
+
+        # make sure download "succeeds" so fetching works
+        content = 'vegan muffin'
+        fd = BytesIO(content)
+        size = len(content)
+        self.manager._download_and_decrypt = Mock(return_value=(fd, size))
+        self.manager.concurrent_transfers_limit = 1
+
+        # this is the operation we are interested in
+        yield self.manager.fetch_missing()
+
+        # retrieve the order in which blob transfers were made
+        calls = self.manager._download_and_decrypt.mock_calls
+        order = map(lambda c: c[1][0], calls)
+        self.assertEqual(['urgent', 'high', 'medium', 'low'], order)
+
+    @defer.inlineCallbacks
+    @pytest.mark.usefixtures("method_tmpdir")
+    def test_send_missing_sends_with_priority(self):
+
+        # pretend we have some pending uploads
+        _send = self.manager._send
+        self.manager._send = Mock(return_value=None)
+        content = "vegan cake"
+        length = len(content)
+        deferreds = [
+            self.manager.put(
+                BlobDoc(BytesIO(content), 'low'), length,
+                priority=Priority.LOW),
+            self.manager.put(
+                BlobDoc(BytesIO(content), 'high'), length,
+                priority=Priority.HIGH),
+            self.manager.put(
+                BlobDoc(BytesIO(content), 'medium'), length,
+                priority=Priority.MEDIUM),
+            self.manager.put(
+                BlobDoc(BytesIO(content), 'urgent'), length,
+                priority=Priority.URGENT),
+        ]
+        yield defer.gatherResults(deferreds)
+
+        # make sure upload "succeeds" so sending works
+        self.manager._send = _send
+        self.manager._encrypt_and_upload = Mock(return_value=None)
+
+        # this is the operation we are interested in
+        self.manager.concurrent_transfers_limit = 1
+        yield self.manager.send_missing()
+
+        # retrieve the order in which blob transfers were made
+        calls = self.manager._encrypt_and_upload.mock_calls
+        order = map(lambda c: c[1][0], calls)
+        self.assertEqual(['urgent', 'high', 'medium', 'low'], order)