Merge ~raharper/cloud-init:sysconfig-resolvconf-duplicate-header-lp1701420 into cloud-init:master

[Ryan Harper]

sysconfig: Dont repeat header when rendering resolv.conf

The sysconfig renderer duplicates the cloud-init header string
when rendering resolv.conf file. This leads to resolv.conf file
growing with every reboot of a system. Fix this by checking for
the header when loading content from existing file.

Update one of the sysconfig unittests with multiple render calls
to simulate the reboot to check that we don't repeat the header.

LP: #1701420

[Server Team CI bot (server-team-bot)]

FAILED: Continuous integration, rev:491585acd21b21dc18defc90cc1e00489ea00769
https://jenkins.ubuntu.com/server/job/cloud-init-ci/106/
Executed test runs:
    SUCCESS: Checkout
    SUCCESS: Unit & Style Tests
    SUCCESS: Ubuntu LTS: Build
    SUCCESS: Ubuntu LTS: Integration
    FAILED: CentOS 6 & 7: Build & Test

Click here to trigger a rebuild:
https://jenkins.ubuntu.com/server/job/cloud-init-ci/106/rebuild

review: Needs Fixing (continuous-integration)

[Scott Moser (smoser)]

I think this is fine. It makes sense.
the other way to fix this is to fix
 https://bugs.launchpad.net/cloud-init/+bug/1693265
 to fix that we will stop writing resolv.conf but rather delegating that to sysconfig.

diff --git a/cloudinit/net/sysconfig.py b/cloudinit/net/sysconfig.py
index a550f97..f572796 100644
--- a/cloudinit/net/sysconfig.py
+++ b/cloudinit/net/sysconfig.py
@@ -484,7 +484,11 @@ class Renderer(renderer.Renderer):
             content.add_nameserver(nameserver)
         for searchdomain in network_state.dns_searchdomains:
             content.add_search_domain(searchdomain)
-        return "\n".join([_make_header(';'), str(content)])
+        header = _make_header(';')
+        content_str = str(content)
+        if not content_str.startswith(header):
+            content_str = header + '\n' + content_str
+        return content_str
 
     @staticmethod
     def _render_networkmanager_conf(network_state):
diff --git a/tests/unittests/test_net.py b/tests/unittests/test_net.py
index e49abcc..4653be1 100644
--- a/tests/unittests/test_net.py
+++ b/tests/unittests/test_net.py
@@ -1683,6 +1683,9 @@ USERCTL=no
             ns = network_state.parse_net_config_data(network_cfg,
                                                      skip_broken=False)
             renderer = sysconfig.Renderer()
+            # render a multiple times to simulate reboots
+            renderer.render_network_state(ns, render_dir)
+            renderer.render_network_state(ns, render_dir)
             renderer.render_network_state(ns, render_dir)
             for fn, expected_content in os_sample.get('out_sysconfig', []):
                 with open(os.path.join(render_dir, fn)) as fh: