Correctly handle IO errors at policy file load

For now, policy file unaccessible due to access permission leads to
silent failure without any notifictaion. Side effects of it are also
affected by policy caching and might be quite confusing, like periodic
disappearing of Horizon panels. Proposed patch added handling of such
errors.

Closes-bug: #1836390
Change-Id: I0d67b6e7c2dcaa63d6bb807f013e5e7334efc715

oslo_policy/_cache_handler.py

@@ -13,9 +13,12 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import errno
 import logging
 import os
 
+from oslo_config import cfg
+
 LOG = logging.getLogger(__name__)
 
 
@@ -33,13 +36,33 @@ def read_cached_file(cache, filename, force_reload=False):
         delete_cached_file(cache, filename)
 
     reloaded = False
-    mtime = os.path.getmtime(filename)
+    try:
+        mtime = os.path.getmtime(filename)
+    except OSError as err:
+        msg = err.strerror
+        LOG.error('Config file not found %(filename)s: %(msg)s',
+                  {'filename': filename, 'msg': msg})
+        return True, {}
+
     cache_info = cache.setdefault(filename, {})
 
     if not cache_info or mtime > cache_info.get('mtime', 0):
         LOG.debug("Reloading cached file %s", filename)
-        with open(filename) as fap:
-            cache_info['data'] = fap.read()
+        try:
+            with open(filename) as fap:
+                cache_info['data'] = fap.read()
+        except IOError as err:
+            msg = err.strerror
+            err_code = err.errno
+            LOG.error('IO error loading %(filename)s: %(msg)s',
+                      {'filename': filename, 'msg': msg})
+            if err_code == errno.EACCES:
+                raise cfg.ConfigFilesPermissionDeniedError((filename,))
+        except OSError as err:
+            msg = err.strerror
+            LOG.error('Config file not found %(filename)s: %(msg)s',
+                      {'filename': filename, 'msg': msg})
+            raise cfg.ConfigFilesNotFoundError((filename,))
         cache_info['mtime'] = mtime
         reloaded = True
     return (reloaded, cache_info['data'])