Fixes bug 723235

The XML templates have been converted into properties, thus we can
compare the mtime of the XML templates (libvirt and cpuinfo) each
time they are needed, checking if they have been modified and
reloading them. Added a function to read cached files.

Change-Id: I6cf0229c6435300e73f9d9a6b10b0bf9bf144a55

nova/tests/test_utils.py

@@ -14,6 +14,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import __builtin__
+import mox
 import datetime
 import os
 import tempfile
@@ -330,6 +332,33 @@ def test_generate_glance_url(self):
         actual_url = "http://%s:%d" % (FLAGS.glance_host, FLAGS.glance_port)
         self.assertEqual(generated_url, actual_url)
 
+    def test_read_cached_file(self):
+        self.mox.StubOutWithMock(os.path, "getmtime")
+        os.path.getmtime(mox.IgnoreArg()).AndReturn(1)
+        self.mox.ReplayAll()
+
+        cache_data = {"data": 1123, "mtime": 1}
+        data = utils.read_cached_file("/this/is/a/fake", cache_data)
+        self.assertEqual(cache_data["data"], data)
+
+    def test_read_modified_cached_file(self):
+        self.mox.StubOutWithMock(os.path, "getmtime")
+        self.mox.StubOutWithMock(__builtin__, 'open')
+
+        os.path.getmtime(mox.IgnoreArg()).AndReturn(2)
+
+        fake_contents = "lorem ipsum"
+        fake_file = self.mox.CreateMockAnything()
+        fake_file.read().AndReturn(fake_contents)
+        __builtin__.open(mox.IgnoreArg()).AndReturn(fake_file)
+
+        self.mox.ReplayAll()
+        cache_data = {"data": 1123, "mtime": 1}
+        data = utils.read_cached_file("/this/is/a/fake", cache_data)
+        self.mox.VerifyAll()
+        self.mox.UnsetStubs()
+        self.assertEqual(data, fake_contents)
+
 
 class IsUUIDLikeTestCase(test.TestCase):
     def assertUUIDLike(self, val, expected):

nova/utils.py

@@ -1110,3 +1110,17 @@ def sanitize_hostname(hostname):
     hostname = hostname.strip('.-')
 
     return hostname
+
+
+def read_cached_file(filename, cache_info):
+    """Return the contents of a file. If the file hasn't changed since the
+    last invocation, a cached version will be returned.
+    """
+    mtime = os.path.getmtime(filename)
+    if cache_info and mtime == cache_info.get('mtime', None):
+        return cache_info['data']
+
+    data = open(filename).read()
+    cache_info['data'] = data
+    cache_info['mtime'] = mtime
+    return data

nova/virt/libvirt/connection.py

@@ -185,8 +185,6 @@ class LibvirtConnection(driver.ComputeDriver):
     def __init__(self, read_only):
         super(LibvirtConnection, self).__init__()
 
-        self.libvirt_xml = open(FLAGS.libvirt_xml_template).read()
-        self.cpuinfo_xml = open(FLAGS.cpuinfo_xml_template).read()
         self._host_state = None
         self._wrapped_conn = None
         self.read_only = read_only
@@ -220,6 +218,22 @@ def init_host(self, host):
         # NOTE(nsokolov): moved instance restarting to ComputeManager
         pass
 
+    @property
+    def libvirt_xml(self):
+        if not hasattr(self, '_libvirt_xml_cache_info'):
+            self._libvirt_xml_cache_info = {}
+
+        return utils.read_cached_file(FLAGS.libvirt_xml_template,
+                self._libvirt_xml_cache_info)
+
+    @property
+    def cpuinfo_xml(self):
+        if not hasattr(self, '_cpuinfo_xml_cache_info'):
+            self._cpuinfo_xml_cache_info = {}
+
+        return utils.read_cached_file(FLAGS.cpuinfo_xml_template,
+                self._cpuinfo_xml_cache_info)
+
     def _get_connection(self):
         if not self._wrapped_conn or not self._test_connection():
             LOG.debug(_('Connecting to libvirt: %s'), self.uri)