Added overwrite warning for db_export_metadefs.

The bug in [1] proposed to always require a directory option for
glance-manage db_export_metadefs command. The rationale being that
silently overwriting the default path /etc/glance/metadefs can
be undesired. However, the feedback was that this command should
stay silent in case it's called from cron jobs or automated tasks,
and that it's better to simply log a warning and proceed, as well
as update the documentation with a cautionary note. This patch
addresses that request.

[1] https://bugs.launchpad.net/glance/+bug/1367011

Change-Id: Ie8cc4bb3769a6347fd25e2235a72c6358af70d42
Closes-Bug: #1367011

doc/source/man/glancemanage.rst

@@ -57,11 +57,13 @@ COMMANDS
         Place a database under migration control and upgrade, creating
         it first if necessary.
 
-  **db_export_metadefs**
+  **db_export_metadefs [PATH | PREFIX]**
         Export the metadata definitions into json format. By default the
         definitions are exported to /etc/glance/metadefs directory.
+        **Note: this command will overwrite existing files in the supplied or
+        default path.**
 
-  **db_load_metadefs**
+  **db_load_metadefs [PATH]**
         Load the metadata definitions into glance database. By default the
         definitions are imported from /etc/glance/metadefs directory.
 

glance/db/sqlalchemy/metadata.py

@@ -464,6 +464,8 @@ def _export_data_to_file(meta, path):
 
         try:
             file_name = ''.join([path, namespace_file_name, '.json'])
+            if isfile(file_name):
+                LOG.info(_LI("Overwriting: %s"), file_name)
             with open(file_name, 'w') as json_file:
                 json_file.write(json.dumps(values))
         except Exception as e: