Improve getting tmp download path

mycroft/util/download.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+from glob import glob
 import os
 from os.path import exists, dirname
 import subprocess
@@ -32,16 +33,11 @@ def _get_download_tmp(dest):
         (str) path to temporary download location
     """
     tmp_base = dest + '.part'
-    if not exists(tmp_base):
+    existing = glob(tmp_base + '*')
+    if len(existing) > 0:
+        return '{}.{}'.format(tmp_base, len(existing))
+    else:
         return tmp_base
-    else:
-        i = 1
-        while(True):
-            tmp = tmp_base + '.' + str(i)
-            if not exists(tmp):
-                return tmp
-            else:
-                i += 1
 
 
 class Downloader(Thread):

test/unittests/util/test_download.py

@@ -1,7 +1,8 @@
 from threading import Event
 from unittest import TestCase, mock
 
-from mycroft.util.download import download, _running_downloads
+from mycroft.util.download import (download, _running_downloads,
+                                   _get_download_tmp)
 
 TEST_URL = 'http://example.com/mycroft-test.tar.gz'
 TEST_DEST = '/tmp/file.tar.gz'
@@ -83,3 +84,22 @@ class TestDownload(TestCase):
         self.assertTrue(downloader is downloader2)
         transfer_done.set()
         downloader.join()
+
+
+@mock.patch('mycroft.util.download.glob')
+class TestGetTemp(TestCase):
+    def test_no_existing(self, mock_glob):
+        mock_glob.return_value = []
+        dest = '/tmp/test'
+        self.assertEqual(_get_download_tmp(dest), dest + '.part')
+
+    def test_existing(self, mock_glob):
+        mock_glob.return_value = ['/tmp/test.part']
+        dest = '/tmp/test'
+        self.assertEqual(_get_download_tmp(dest), dest + '.part.1')
+
+    def test_multiple_existing(self, mock_glob):
+        mock_glob.return_value = ['/tmp/test.part', '/tmp/test.part.1',
+                                  '/tmp/test.part.2']
+        dest = '/tmp/test'
+        self.assertEqual(_get_download_tmp(dest), dest + '.part.3')