netplan: Don't render yaml aliases when dumping netplan

Cloud-init rendered netplan with duplicate aliases if a network config
included "global" nameserver/search values.  Netplan uses can read yaml
files which do use aliaes but cloud-init did not render a single yaml
dictionary, instead it combined yaml sections into a single document
which sometimes resulted in duplicate aliases being present.

This branch introduces a yaml SafeDumper class which can set the
'ignore_aliases' attribute.  This is not enabled by default but callers
to util.yaml_dumps can pass a boolean to toggle this.  The netplan
render uses noalias=True and the resulting yaml output does not contain
any aliases.

LP: #1815051

1 files changed, 7 insertions, 0 deletions

diff --git a/cloudinit/safeyaml.py b/cloudinit/safeyaml.py
index 7bcf9dd3..3bd5e03d 100644
--- a/cloudinit/safeyaml.py
+++ b/cloudinit/safeyaml.py
@@ -17,6 +17,13 @@ _CustomSafeLoader.add_constructor(
     _CustomSafeLoader.construct_python_unicode)
 
 
+class NoAliasSafeDumper(yaml.dumper.SafeDumper):
+    """A class which avoids constructing anchors/aliases on yaml dump"""
+
+    def ignore_aliases(self, data):
+        return True
+
+
 def load(blob):
     return(yaml.load(blob, Loader=_CustomSafeLoader))