Move selective patching of open() to nova.test for reuse

Several existing tests patch open() to fake the contents of a file
outside the test virtualenv, whilst avoiding interfering with reading
and writing of other files inside the test virtualenv.  Currently they
do it by duplicating logic.

Furthermore, in the near future, more tests (specifically, some SEV
tests) will want to do the same selective patching, and similarly will
need to avoid impacting reads of other files within the test
virtualenv, e.g. placement-policy.yaml.

So create new patch_open() context manager / decorator in nova.test
for selectively patching open based on the path parameter, and reuse
this for existing tests.

Also add unit tests for all these cases.

mock >= 3.0.0 is required because configparser.RawConfigParser._read()
uses enumerate() to iterate over the lines of the (mocked)
/etc/nova/release config file, and this uses __iter__() under the hood
which was not supported via mock_open until a bug was fixed and
backported to the external mock library for 3.0.0:

    https://bugs.python.org/issue21258
    https://bugs.python.org/issue32933
    https://github.com/testing-cabal/mock/commit/73f6eed0d6867299fa2543b88a07cd8f12198361

Change-Id: I19f49c923d2c41eab0c7b4cab28c50498dc07046

1 files changed, 1 insertions, 1 deletions

diff --git a/test-requirements.txt b/test-requirements.txt
index 318863b7cc..60f8fb3acd 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -7,7 +7,7 @@ contextlib2>=0.5.5;python_version<'3.0'  # PSF License
 coverage!=4.4,>=4.0 # Apache-2.0
 ddt>=1.0.1 # MIT
 fixtures>=3.0.0 # Apache-2.0/BSD
-mock>=2.0.0 # BSD
+mock>=3.0.0 # BSD
 mox3>=0.20.0 # Apache-2.0
 psycopg2>=2.7 # LGPL/ZPL
 PyMySQL>=0.7.6 # MIT License