Add error message when merge fails

If the source cfg directory contains a path (e.g.
/etc/pki/ca-trust/extracted/) which is also bind mounted into the
container as read-only, when it attempts to merge the paths together
the resulting exception doesn't really indicate what happened.

Change-Id: I73bbfd31586012e1fd64d30f9ce1d7eb2256d210
Related-Bug: #1860607

1 files changed, 9 insertions, 1 deletions

diff --git a/docker/base/set_configs.py b/docker/base/set_configs.py
index 9176a510d..93a758f1d 100644
--- a/docker/base/set_configs.py
+++ b/docker/base/set_configs.py
@@ -147,7 +147,15 @@ class ConfigFile(object):
             if not self.merge:
                 self._delete_path(dest)
             self._create_parent_dirs(dest)
-            self._merge_directories(source, dest)
+            try:
+                self._merge_directories(source, dest)
+            except OSError:
+                # If a source is tried to merge with a read-only mount, it
+                # may throw an OSError. Because we don't print the source or
+                # dest anywhere, let's catch the exception and log a better
+                # message to help with tracking down the issue.
+                LOG.error('Unable to merge %s with %s', source, dest)
+                raise
 
     def _cmp_file(self, source, dest):
         # check exsit