Merge ~cjwatson/charm-postfix-relay:ensure-mailname into charm-postfix-relay:master

[Colin Watson]

Ensure that /etc/mailname exists

While Ubuntu LXD containers come up with `/etc/mailname` by default, not all image providers do the same - for example OpenStack VMs using Ubuntu cloud images don't. Ensure that it exists with some minimally-reasonable contents, since the generated `/etc/postfix/main.cf` requires it.

[🤖 Canonical IS Merge Bot (canonical-is-mergebot)]

This merge proposal is being monitored by mergebot. Change the status to Approved to merge.

[Nick Moffitt (nick-moffitt)]

This mirrors a workaround we've done by hand in the past. We used `uname -n` instead of `socket.getfqdn()`, but I've verified on those systems that the result is the same.

review: Approve

[🤖 prod-jenkaas-bootstack (prod-jenkaas-bootstack)]

PASSED: Continuous integration, rev:7c23020114f7e355998ef6fb789aefad193f0598
https://jenkins.canonical.com/bootstack/job/lp-charm-postfix-relay-ci/7/
Executed test runs:
    SUCCESS: https://jenkins.canonical.com/bootstack/job/lp-charm-test-functest/1170/
    None: https://jenkins.canonical.com/bootstack/job/lp-update-mp/346/

Click here to trigger a rebuild:
https://jenkins.canonical.com/bootstack/job/lp-charm-postfix-relay-ci/7//rebuild

review: Approve (continuous-integration)

[Gabriel Cocenza (gabrielcocenza)]

LGTM

review: Approve

[Tianqi Xiao (txiao)]

review: Approve

[🤖 Canonical IS Merge Bot (canonical-is-mergebot)]

Change successfully merged at revision 9a44eb5794433800cf04674b4c5195de53326388

diff --git a/src/lib/charm/postfix/postfix_relay.py b/src/lib/charm/postfix/postfix_relay.py
index 2ca0ac6..ca916a2 100644
--- a/src/lib/charm/postfix/postfix_relay.py
+++ b/src/lib/charm/postfix/postfix_relay.py
@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License
 import os
+import socket
 import subprocess
 from base64 import b64decode
 
@@ -36,6 +37,7 @@ def write_configs():
         cfg_template = Template(t.read())
     with open(MAIN_CFG, "w+") as f:
         f.write(cfg_template.render(PostfixContext()()))
+    ensure_mailname()
     if config("smtp_auth_username") and config("smtp_auth_password"):
         write_auth_file()
     if config("domain_rewrite_map"):
@@ -47,6 +49,13 @@ def write_configs():
     restart_postfix()
 
 
+def ensure_mailname():
+    """Ensure that /etc/mailname exists."""
+    if not os.path.exists("/etc/mailname"):
+        with open("/etc/mailname", "w") as mailname:
+            mailname.write(socket.getfqdn() + "\n")
+
+
 def write_auth_file():
     """Render sasl_passwd file."""
     auth_template_path = os.path.join(TEMPLATES, "sasl_passwd")
diff --git a/src/tests/unit/test_postfix_relay.py b/src/tests/unit/test_postfix_relay.py
index 74cf14c..46685f8 100644
--- a/src/tests/unit/test_postfix_relay.py
+++ b/src/tests/unit/test_postfix_relay.py
@@ -1,6 +1,7 @@
 """Unit tests for charm.postfix.postfix_relay."""
 
 import os
+import socket
 import subprocess
 from unittest import TestCase, mock
 
@@ -9,6 +10,26 @@ from systemfixtures import FakeFilesystem, FakeProcesses
 from charm.postfix import postfix_relay
 
 
+class EnsureMailnameTests(TestCase):
+    def test_already_exists(self):
+        with FakeFilesystem() as fs:
+            fs.add("/etc")
+            os.mkdir("/etc")
+            with open("/etc/mailname", "w") as mailname:
+                mailname.write("unit.example.com\n")
+            postfix_relay.ensure_mailname()
+            with open("/etc/mailname") as mailname:
+                self.assertEqual("unit.example.com\n", mailname.read())
+
+    def test_creates(self):
+        with FakeFilesystem() as fs:
+            fs.add("/etc")
+            os.mkdir("/etc")
+            postfix_relay.ensure_mailname()
+            with open("/etc/mailname") as mailname:
+                self.assertEqual(socket.getfqdn() + "\n", mailname.read())
+
+
 class WriteRelayhostMapTests(TestCase):
     @mock.patch("charm.postfix.postfix_relay.config")
     def test_single_sender(self, mock_config):