Use libyaml parsing when available

Nodepool periodically reloads its config. On systems with large
configs this can cause a substantial amount of cpu usage. Our
profiling of an idle provider revealed that the yaml parsing accounted
for ~15% cpu load while the provider was idle.

This could be improved by utilizing the native libyaml bindings when
available.

Change-Id: Icbed393c5aa00abd52d1e13ccbbad8546ec6cf97

bindep.txt

@@ -9,6 +9,10 @@ libffi6 [platform:dpkg]
 libffi [platform:apk]
 libressl-dev [compile test platform:apk]
 libssl-dev [compile test platform:dpkg]
+libyaml-0-2 [platform:dpkg platform:suse]
+libyaml [platform:redhat]
+libyaml-dev [platform:dpkg compile test]
+libyaml-devel [platform:rpm compile test]
 linux-headers [compile test platform:apk]
 make [compile test platform:apk platform:dpkg]
 musl-dev [compile test platform:apk]

nodepool/config.py

@@ -24,6 +24,11 @@ from nodepool import zk
 from nodepool.driver import ConfigValue
 from nodepool.driver import Drivers
 
+try:
+    from yaml import CSafeLoader as SafeLoader
+except ImportError:
+    from yaml import SafeLoader
+
 
 class Config(ConfigValue):
     '''
@@ -314,7 +319,8 @@ def openConfig(path, env):
     while True:
         try:
             with open(path) as f:
-                return yaml.safe_load(substitute_env_vars(f.read(), env))
+                return yaml.load(
+                    substitute_env_vars(f.read(), env), SafeLoader)
         except IOError as e:
             if e.errno == 2:
                 retry = retry - 1