Use ThreadPoolExecutor for max_concurrent_live_migrations

This changes the max_concurrent_live_migrations handling
to use a ThreadPoolExecutor so that we can control a bounded
pool of Futures in order to cancel queued live migrations
later in this series.

There is a slight functional difference in the unlimited
case since starting in python 3.5, ThreadPoolExecutor will
default to ncpu * 5 concurrently running threads. However,
max_concurrent_live_migrations defaults to 1 and assuming
compute hosts run with 32 physical CPUs on average, you'd
be looking at a maximum of 160 concurrently running live
migrations, which is probably way above what anyone would
consider sane.

Co-Authored-By: Matt Riedemann <mriedem.os@gmail.com>

Part of blueprint abort-live-migration-in-queued-status

Change-Id: Ia9ea1e164fb3b4a386405538eed58d94ad115172

lower-constraints.txt

@@ -33,6 +33,7 @@ fixtures==3.0.0
 flake8==2.5.5
 future==0.16.0
 futurist==1.6.0
+futures==3.0.0
 gabbi==1.35.0
 gitdb2==2.0.3
 GitPython==2.1.8

nova/tests/fixtures.py

@@ -1010,6 +1010,26 @@ class SpawnIsSynchronousFixture(fixtures.Fixture):
             'nova.utils.spawn', _FakeGreenThread))
 
 
+class SynchronousThreadPoolExecutorFixture(fixtures.Fixture):
+    """Make ThreadPoolExecutor.submit() synchronous.
+
+    The function passed to submit() will be executed and a mock.Mock
+    object will be returned as the Future where Future.result() will
+    return the result of the call to the submitted function.
+    """
+    def setUp(self):
+        super(SynchronousThreadPoolExecutorFixture, self).setUp()
+
+        def fake_submit(_self, fn, *args, **kwargs):
+            result = fn(*args, **kwargs)
+            future = mock.Mock(spec='concurrent.futures.Future')
+            future.return_value.result.return_value = result
+            return future
+        self.useFixture(fixtures.MonkeyPatch(
+            'concurrent.futures.ThreadPoolExecutor.submit',
+            fake_submit))
+
+
 class BannedDBSchemaOperations(fixtures.Fixture):
     """Ban some operations for migrations"""
     def __init__(self, banned_resources=None):

requirements.txt

@@ -67,3 +67,4 @@ os-service-types>=1.2.0 # Apache-2.0
 taskflow>=2.16.0 # Apache-2.0
 python-dateutil>=2.5.3 # BSD
 zVMCloudConnector>=1.1.1;sys_platform!='win32'  # Apache 2.0 License
+futures>=3.0.0;python_version=='2.7' or python_version=='2.6' # PSF