Fix sloppy initialization of the new disk ops semaphore.

Some tests weren't calling init_host, so the semaphore was None.
This caused the smoke to come out of nova's tests in ways that
would be less confusing if they'd failed during the testing of
the implementing patch.

Instead, set the semaphore to being unbounded, and then override
that later if the user has in fact specified a limit. This relies
on init_host being called very early, but that should be true
already.

Change-Id: If144be253f78b14cef60200a46aefc02c0e19ced
Closes-Bug: #1806123

nova/compute/manager.py

@@ -1192,14 +1192,12 @@ def init_host(self):
 
         nova.conf.neutron.register_dynamic_opts(CONF)
 
-        # one-time initialization
+        # Override the number of concurrent disk operations allowed if the
+        # user has specified a limit.
         if CONF.compute.max_concurrent_disk_ops != 0:
             compute_utils.disk_ops_semaphore = \
                 eventlet.semaphore.BoundedSemaphore(
                     CONF.compute.max_concurrent_disk_ops)
-        else:
-            compute_utils.disk_ops_semaphore = \
-                compute_utils.UnlimitedSemaphore()
 
         self.driver.init_host(host=self.host)
         context = nova.context.get_admin_context()

nova/compute/utils.py

@@ -54,11 +54,6 @@
 CONF = nova.conf.CONF
 LOG = log.getLogger(__name__)
 
-# This semaphore is used to enforce a limit on disk-IO-intensive operations
-# (image downloads, image conversions) at any given time.
-# It is initialized at ComputeManager.init_host()
-disk_ops_semaphore = None
-
 
 def exception_to_dict(fault, message=None):
     """Converts exceptions to a dict for use in notifications."""
@@ -1176,6 +1171,12 @@ def balance(self):
         return 0
 
 
+# This semaphore is used to enforce a limit on disk-IO-intensive operations
+# (image downloads, image conversions) at any given time.
+# It is initialized at ComputeManager.init_host()
+disk_ops_semaphore = UnlimitedSemaphore()
+
+
 @contextlib.contextmanager
 def notify_about_instance_delete(notifier, context, instance,
                                  delete_type='delete',