support different and user-suppliable merging algorithms for cloud-config

This adds a very useful mechanism for merging cloud-config, allowing
the user to append to lists (ie, just add more 'run_cmd') or other
things.

See doc/merging.txt for more information, it is intended to be backwards
compatible by default.

LP: #1023179

1 files changed, 48 insertions, 0 deletions

diff --git a/cloudinit/mergers/dict.py b/cloudinit/mergers/dict.py
new file mode 100644
index 00000000..45a7d3a5
--- /dev/null
+++ b/cloudinit/mergers/dict.py
@@ -0,0 +1,48 @@
+# vi: ts=4 expandtab
+#
+#    Copyright (C) 2012 Yahoo! Inc.
+#
+#    Author: Joshua Harlow <harlowja@yahoo-inc.com>
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License version 3, as
+#    published by the Free Software Foundation.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+class Merger(object):
+    def __init__(self, merger, opts):
+        self._merger = merger
+        self._overwrite = 'overwrite' in opts
+
+    # This merging algorithm will attempt to merge with
+    # another dictionary, on encountering any other type of object
+    # it will not merge with said object, but will instead return
+    # the original value
+    #
+    # On encountering a dictionary, it will create a new dictionary
+    # composed of the original and the one to merge with, if 'overwrite'
+    # is enabled then keys that exist in the original will be overwritten
+    # by keys in the one to merge with (and associated values). Otherwise
+    # if not in overwrite mode the 2 conflicting keys themselves will
+    # be merged.
+    def _on_dict(self, value, merge_with):
+        if not isinstance(merge_with, (dict)):
+            return value
+        merged = dict(value)
+        for (k, v) in merge_with.items():
+            if k in merged:
+                if not self._overwrite:
+                    merged[k] = self._merger.merge(merged[k], v)
+                else:
+                    merged[k] = v
+            else:
+                merged[k] = v
+        return merged