do not render systemd.link files

When fixing bug 1579130, we made cloud-init rename devices itself,
rather than relying on the systemd.link files to do that.
That was necessary to
 - rename devices in a container
 - rename devices on first boot or in any situation when the 
   link files in the initramfs were stale.

However, cloud-init was still writing .link files like:
 /etc/systemd/network/50-cloud-init-ens2.link

That leads to just a confusing situation as cloud-init will trump
any renaming systemd does in all cases.

Also added here is a header into the rendered ENI file:
 /etc/network/interfaces.d/50-cloud-init.cfg

that describes how to disable cloud-init networking.

LP: #1594546

3 files changed, 17 insertions, 2 deletions

diff --git a/cloudinit/distros/debian.py b/cloudinit/distros/debian.py
index 53f3aa4d..5ae9a509 100644
--- a/cloudinit/distros/debian.py
+++ b/cloudinit/distros/debian.py
@@ -43,6 +43,13 @@ APT_GET_WRAPPER = {
     'enabled': 'auto',
 }
 
+ENI_HEADER = """# This file is generated from information provided by
+# the datasource.  Changes to it will not persist across an instance.
+# To disable cloud-init's network configuration capabilities, write a file
+# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
+# network: {config: disabled}
+"""
+
 
 class Distro(distros.Distro):
     hostname_conf_fn = "/etc/hostname"
@@ -59,7 +66,8 @@ class Distro(distros.Distro):
         self.osfamily = 'debian'
         self._net_renderer = eni.Renderer({
             'eni_path': self.network_conf_fn,
-            'links_prefix_path': self.links_prefix,
+            'eni_header': ENI_HEADER,
+            'links_prefix_path': None,
             'netrules_path': None,
         })
 
diff --git a/cloudinit/net/__init__.py b/cloudinit/net/__init__.py
index 6959ad34..63e54f91 100644
--- a/cloudinit/net/__init__.py
+++ b/cloudinit/net/__init__.py
@@ -237,6 +237,11 @@ def _get_current_rename_info(check_downable=True):
 
 def _rename_interfaces(renames, strict_present=True, strict_busy=True,
                        current_info=None):
+
+    if not len(renames):
+        LOG.debug("no interfaces to rename")
+        return
+
     if current_info is None:
         current_info = _get_current_rename_info()
 
diff --git a/cloudinit/net/eni.py b/cloudinit/net/eni.py
index ccd16ba7..e5ed10fd 100644
--- a/cloudinit/net/eni.py
+++ b/cloudinit/net/eni.py
@@ -303,6 +303,7 @@ class Renderer(renderer.Renderer):
         if not config:
             config = {}
         self.eni_path = config.get('eni_path', 'etc/network/interfaces')
+        self.eni_header = config.get('eni_header', None)
         self.links_path_prefix = config.get(
             'links_path_prefix', 'etc/systemd/network/50-cloud-init-')
         self.netrules_path = config.get(
@@ -417,7 +418,8 @@ class Renderer(renderer.Renderer):
     def render_network_state(self, target, network_state):
         fpeni = os.path.join(target, self.eni_path)
         util.ensure_dir(os.path.dirname(fpeni))
-        util.write_file(fpeni, self._render_interfaces(network_state))
+        header = self.eni_header if self.eni_header else ""
+        util.write_file(fpeni, header + self._render_interfaces(network_state))
 
         if self.netrules_path:
             netrules = os.path.join(target, self.netrules_path)