write one resolv config

When systemd-resolved is enabled dns info is only written to
/etc/systemd/resolved.conf.  If systemd-resolved is not enabled dns info is
only written to /etc/resolv.conf.  We do not write to /etc/resolv.conf
if systemd-resolved is enabled because /etc/resolv.conf is a symlink pointing
to ../run/systemd/resolve/stub-resolv.conf.  This target file does not exist
before systemd-resolved is run so glean would fail with:
    'FileNotFoundError: [Errno 2] No such file or directory: '/etc/resolv.conf'

Change-Id: I644e0b50cfb7bb00a108160b99c0c1359d6a9dd4
Signed-off-by: Matthew Thode <mthode@mthode.org>

glean/cmd.py

@@ -945,15 +945,26 @@ def write_debian_interfaces(interfaces, sys_interfaces):
 
 
 def write_dns_info(dns_servers):
+    # will fail on non-systemd systems (what we want)
+    # will exit 1 if not enabled (what we want)
+    # will exit 0 if enabled (or indirectly enabled)
+    resolved_enabled = os.system('systemctl is-enabled systemd-resolved')
     resolve_confs = {}
-    resolv_nameservers = ""
-    for server in dns_servers:
-        resolv_nameservers += "nameserver {0}\n".format(server)
-    resolve_confs['/etc/resolv.conf'] = resolv_nameservers
-    # set up resolved if available
-    if os.path.isfile('/etc/systemd/resolved.conf'):
+    # write resolv.conf if the file can be written to (if symlink is pointing
+    # to a non-existant file, writing will fail), will return false if the
+    # pointer is incomplete
+    if resolved_enabled != 0:
+        log.debug("resolved not in use, writing to /etc/resolv.conf")
+        resolv_nameservers = ""
+        for server in dns_servers:
+            resolv_nameservers += "nameserver {0}\n".format(server)
+        resolve_confs['/etc/resolv.conf'] = resolv_nameservers
+    # set up resolved if enabled
+    if resolved_enabled == 0:
+        log.debug("resolved in use, writing to /etc/systemd/resolved.conf")
         # read the existing config  so we only overwrite what's needed
         resolved_conf = configparser.ConfigParser()
+        resolved_conf.optionxform = str
         resolved_conf.read('/etc/systemd/resolved.conf')
         # create config section if not created
         if not resolved_conf.has_section('Resolve'):

glean/tests/fixtures/test/liberty.networkd.network.out

@@ -1,6 +1,7 @@
-### Write /etc/resolv.conf
-nameserver 72.3.128.241
-nameserver 72.3.128.240
+### Write /etc/systemd/resolved.conf
+[Resolve]
+DNS = 72.3.128.241 72.3.128.240
+
 ### Write /etc/systemd/network/eth0.network
 # Automatically generated, do not edit
 [Match]

glean/tests/fixtures/test/rax-iad.networkd.network.out

@@ -1,6 +1,7 @@
-### Write /etc/resolv.conf
-nameserver 69.20.0.196
-nameserver 69.20.0.164
+### Write /etc/systemd/resolved.conf
+[Resolve]
+DNS = 69.20.0.196 69.20.0.164
+
 ### Write /etc/systemd/network/eth0.network
 # Automatically generated, do not edit
 [Match]

glean/tests/fixtures/test/rax-iad.networkd.network.out.dns

@@ -1,6 +1,7 @@
-### Write /etc/resolv.conf
-nameserver 69.20.0.196
-nameserver 69.20.0.164
+### Write /etc/systemd/resolved.conf
+[Resolve]
+DNS = 69.20.0.196 69.20.0.164
+
 ### Write /etc/systemd/network/eth0.network
 # Automatically generated, do not edit
 [Match]

glean/tests/fixtures/test/rax.networkd.network.out

@@ -1,6 +1,7 @@
-### Write /etc/resolv.conf
-nameserver 72.3.128.241
-nameserver 72.3.128.240
+### Write /etc/systemd/resolved.conf
+[Resolve]
+DNS = 72.3.128.241 72.3.128.240
+
 ### Write /etc/systemd/network/eth0.network
 # Automatically generated, do not edit
 [Match]

glean/tests/test_glean.py

@@ -142,6 +142,13 @@ class TestGlean(base.BaseTestCase):
             return False
         return real_path_exists(path)
 
+    def os_system_side_effect(self, distro, command):
+        if distro.lower() != 'networkd' and \
+                command == 'systemctl is-enabled systemd-resolved':
+            return 3
+        else:
+            return 0
+
     @mock.patch('subprocess.call', return_value=0, new_callable=mock.Mock)
     @mock.patch('os.fsync', return_value=0, new_callable=mock.Mock)
     @mock.patch('os.unlink', return_value=0, new_callable=mock.Mock)
@@ -183,6 +190,10 @@ class TestGlean(base.BaseTestCase):
             self.os_listdir_side_effect, provider)
         mock_open.side_effect = functools.partial(
             self.open_side_effect, provider)
+        # we want os.system to return False for specific commands if
+        # running networkd
+        mock_os_system.side_effect = functools.partial(
+            self.os_system_side_effect, distro)
 
         # default args
         argv = ['--hostname']
@@ -234,6 +245,9 @@ class TestGlean(base.BaseTestCase):
             if skip_dns and '/etc/resolv.conf' in dest:
                 self.assertNotIn(dest, self.file_handle_mocks)
                 continue
+            if skip_dns and '/etc/systemd/resolved.conf' in dest:
+                self.assertNotIn(dest, self.file_handle_mocks)
+                continue
             self.assertIn(dest, self.file_handle_mocks)
             write_handle = self.file_handle_mocks[dest].write
             write_handle.assert_called_once_with(content)
@@ -279,6 +293,10 @@ class TestGlean(base.BaseTestCase):
             self._assert_distro_provider(self.distro, self.style,
                                          'eth0', skip_dns=True)
 
+    def test_glean_systemd_resolved(self):
+        with mock.patch('glean.systemlock.Lock'):
+            self._assert_distro_provider(self.distro, self.style, 'eth0')
+
     def test_glean_skip_dns(self):
         with mock.patch('glean.systemlock.Lock'):
             self._assert_distro_provider(