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

3 files changed, 19 insertions, 8 deletions

diff --git a/cloudinit/net/netplan.py b/cloudinit/net/netplan.py
index 21517fda..e54a34e5 100644
--- a/cloudinit/net/netplan.py
+++ b/cloudinit/net/netplan.py
@@ -361,7 +361,8 @@ class Renderer(renderer.Renderer):
             if section:
                 dump = util.yaml_dumps({name: section},
                                        explicit_start=False,
-                                       explicit_end=False)
+                                       explicit_end=False,
+                                       noalias=True)
                 txt = util.indent(dump, ' ' * 4)
                 return [txt]
             return []
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))
 
diff --git a/cloudinit/util.py b/cloudinit/util.py
index a8a232b6..2be528a0 100644
--- a/cloudinit/util.py
+++ b/cloudinit/util.py
@@ -1596,14 +1596,17 @@ def json_dumps(data):
                       separators=(',', ': '), default=json_serialize_default)
 
 
-def yaml_dumps(obj, explicit_start=True, explicit_end=True):
+def yaml_dumps(obj, explicit_start=True, explicit_end=True, noalias=False):
     """Return data in nicely formatted yaml."""
-    return yaml.safe_dump(obj,
-                          line_break="\n",
-                          indent=4,
-                          explicit_start=explicit_start,
-                          explicit_end=explicit_end,
-                          default_flow_style=False)
+
+    return yaml.dump(obj,
+                     line_break="\n",
+                     indent=4,
+                     explicit_start=explicit_start,
+                     explicit_end=explicit_end,
+                     default_flow_style=False,
+                     Dumper=(safeyaml.NoAliasSafeDumper
+                             if noalias else yaml.dumper.Dumper))
 
 
 def ensure_dir(path, mode=None):