Update globals safely

The right way to update these globals is to use a lock
and ensure that nobody else is updating them at the same
time. Also update a temporary dictionary before setting
the global one so that nobody sees partial updates to the
global one.

This should help fix the thread-safety of shade (and other
tooling built ontop of this library).

Change-Id: Ie0e0369d98ba6a01edcbf447378a786eec3f13f9

os_client_config/constructors.py

@@ -14,15 +14,23 @@
 
 import json
 import os
+import threading
 
 _json_path = os.path.join(
     os.path.dirname(os.path.realpath(__file__)), 'constructors.json')
 _class_mapping = None
+_class_mapping_lock = threading.Lock()
 
 
 def get_constructor_mapping():
     global _class_mapping
-    if not _class_mapping:
+    if _class_mapping is not None:
+        return _class_mapping.copy()
+    with _class_mapping_lock:
+        if _class_mapping is not None:
+            return _class_mapping.copy()
+        tmp_class_mapping = {}
         with open(_json_path, 'r') as json_file:
-            _class_mapping = json.load(json_file)
-    return _class_mapping
+            tmp_class_mapping.update(json.load(json_file))
+        _class_mapping = tmp_class_mapping
+        return tmp_class_mapping.copy()

os_client_config/defaults.py

@@ -14,19 +14,30 @@
 
 import json
 import os
+import threading
 
 _json_path = os.path.join(
     os.path.dirname(os.path.realpath(__file__)), 'defaults.json')
 _defaults = None
+_defaults_lock = threading.Lock()
 
 
 def get_defaults():
     global _defaults
-    if not _defaults:
+    if _defaults is not None:
+        return _defaults.copy()
+    with _defaults_lock:
+        if _defaults is not None:
+            # Did someone else just finish filling it?
+            return _defaults.copy()
         # Python language specific defaults
         # These are defaults related to use of python libraries, they are
         # not qualities of a cloud.
-        _defaults = dict(
+        #
+        # NOTE(harlowja): update a in-memory dict, before updating
+        # the global one so that other callers of get_defaults do not
+        # see the partially filled one.
+        tmp_defaults = dict(
             api_timeout=None,
             verify=True,
             cacert=None,
@@ -36,6 +47,6 @@ def get_defaults():
         with open(_json_path, 'r') as json_file:
             updates = json.load(json_file)
             if updates is not None:
-                _defaults.update(updates)
-
-    return _defaults.copy()
+                tmp_defaults.update(updates)
+        _defaults = tmp_defaults
+        return tmp_defaults.copy()