Store the list of hosts to run on in runner object

This reduces the number of times inventory.list_hosts is called, which
can be costly. When coming from a playbook that data is already known.

lib/ansible/playbook/__init__.py

@@ -316,7 +316,8 @@ class PlayBook(object):
             check=self.check, diff=self.diff, environment=task.environment, complex_args=task.args,
             accelerate=task.play.accelerate, accelerate_port=task.play.accelerate_port,
             accelerate_ipv6=task.play.accelerate_ipv6,
-            error_on_undefined_vars=C.DEFAULT_UNDEFINED_VAR_BEHAVIOR
+            error_on_undefined_vars=C.DEFAULT_UNDEFINED_VAR_BEHAVIOR,
+            run_hosts=hosts
         )
 
         if task.async_seconds == 0:

lib/ansible/runner/__init__.py

@@ -141,6 +141,7 @@ class Runner(object):
         accelerate=False,                   # use accelerated connection
         accelerate_ipv6=False,              # accelerated connection w/ IPv6
         accelerate_port=None,               # port to use with accelerated connection
+        run_hosts=None,                     # an optional list of pre-calculated hosts to run on
         ):
 
         # used to lock multiprocess inputs and outputs at various levels
@@ -205,6 +206,10 @@ class Runner(object):
             # don't override subset when passed from playbook
             self.inventory.subset(subset)
 
+        # If we get a pre-built list of hosts to run on, from say a playbook, use them.
+        # Also where we will store the hosts to run on once discovered
+        self.run_hosts = run_hosts
+
         if self.transport == 'local':
             self.remote_user = pwd.getpwuid(os.geteuid())[0]
 
@@ -1002,7 +1007,7 @@ class Runner(object):
                 results2["dark"][host] = result.result
 
         # hosts which were contacted but never got a chance to return
-        for host in self.inventory.list_hosts(self.pattern):
+        for host in self.run_hosts:
             if not (host in results2['dark'] or host in results2['contacted']):
                 results2["dark"][host] = {}
         return results2
@@ -1013,7 +1018,9 @@ class Runner(object):
         ''' xfer & run module on all matched hosts '''
 
         # find hosts that match the pattern
-        hosts = self.inventory.list_hosts(self.pattern)
+        if not self.run_hosts:
+            self.run_hosts = self.inventory.list_hosts(self.pattern)
+        hosts = self.run_hosts
         if len(hosts) == 0:
             self.callbacks.on_no_hosts()
             return dict(contacted={}, dark={})