Restore Python 2.6 compatibility in tests

contextlib.nested was removed in Python 3.0, because of problems
with exceptions raised in initialization of its individual context
managers. Its replacement, ExitStack, is not available in Python 2.

Emulate the old "nested" in terms of ExitStack if it's not available.

Partially reverts commit 973202b

tests/test_query_device_capacity.py

@@ -33,6 +33,24 @@
 import mock
 import struct
 
+try:
+    # Python 3.3+
+    from contextlib import ExitStack, contextmanager
+except ImportError:
+    # Python 2.x
+    from contextlib import nested
+else:
+    @contextmanager
+    def nested(*contexts):
+        """Emulation of contextlib.nested in terms of ExitStack
+
+        Has the problems for which "nested" was removed from Python; see:
+            https://docs.python.org/2/library/contextlib.html#contextlib.nested
+        But for mock.patch, these do not matter.
+        """
+        with ExitStack() as stack:
+            yield tuple(stack.enter_context(c) for c in contexts)
+
 device_file_no = mock.Mock(return_value=4)
 device = mock.MagicMock('file')
 device.fileno = device_file_no
@@ -45,10 +63,12 @@
 class TestQueryDeviceCapacity(TestCase):
     def test_query_device_capacity_linux_everything_is_wonderful(self):
         """query device capacity works on a happy Linux host"""
-        with mock.patch('os.stat') as os_stat,\
-                mock.patch('stat.S_ISBLK') as stat_is_block,\
-                mock.patch('platform.system') as plat_system,\
-                mock.patch('fcntl.ioctl') as ioctl:
+        with nested(
+            mock.patch('os.stat'),
+            mock.patch('stat.S_ISBLK'),
+            mock.patch('platform.system'),
+            mock.patch('fcntl.ioctl'),
+        ) as (os_stat, stat_is_block, plat_system, ioctl):
             os_stat.return_value = mock.Mock(st_mode=25008)
             stat_is_block.return_value = True
             plat_system.return_value = 'Linux'
@@ -64,10 +84,12 @@ def test_query_device_capacity_linux_everything_is_wonderful(self):
 
     def test_query_device_capacity_mac_everything_is_wonderful(self):
         """query device capacity works on a happy Mac OS X host"""
-        with mock.patch('os.stat') as os_stat,\
-                mock.patch('stat.S_ISBLK') as stat_is_block,\
-                mock.patch('platform.system') as plat_system,\
-                mock.patch('fcntl.ioctl') as ioctl:
+        with nested(
+            mock.patch('os.stat'),
+            mock.patch('stat.S_ISBLK'),
+            mock.patch('platform.system'),
+            mock.patch('fcntl.ioctl'),
+        ) as (os_stat, stat_is_block, plat_system, ioctl):
             # These are the struct.pack() equivalents of 244140625
             # (type: u64) and 4096 (type: u32). Multiplied together
             # they equal the number of bytes in 1 TB.
@@ -91,9 +113,11 @@ def side_effect(*args, **kwargs):
 
     def test_query_device_capacity_device_not_block(self):
         """query device capacity aborts if a non-block-device is provided"""
-        with mock.patch('os.stat') as os_stat,\
-                mock.patch('stat.S_ISBLK') as stat_is_block,\
-                mock.patch('fcntl.ioctl') as ioctl:
+        with nested(
+            mock.patch('os.stat'),
+            mock.patch('stat.S_ISBLK'),
+            mock.patch('fcntl.ioctl'),
+        ) as (os_stat, stat_is_block, ioctl):
             os_stat.return_value = mock.Mock(st_mode=33204)
             # Force ISBLK to reject the input 'device'
             stat_is_block.return_value = False