Use sendmail instead of SMTP

- remove the SMTP requirement to be able to work with /usr/lib/sendmail
  instead
- use f-strings where appropriate to improve readability
- use text-parameter to subprocess.run to avoid extra decode calls

update-zones.py

@@ -8,7 +8,6 @@ import tempfile
 from subprocess import CalledProcessError, run
 from sys import stderr
 from email.message import EmailMessage, MIMEPart
-from smtplib import SMTP
 
 REFERENCE_BRANCH = "provisioned"
 DEFAULT_BRANCH = "origin/main"
@@ -28,12 +27,13 @@ def git_changed_files(reference_branch, target_branch):
                 "git",
                 "diff",
                 "--name-only",
-                "{}..{}".format(reference_branch, target_branch),
+                f"{reference_branch}..{target_branch}",
             ],
             check=True,
             capture_output=True,
+            text=True,
         )
-        output = git_diff.stdout.decode("utf-8")
+        output = git_diff.stdout.strip()
     except CalledProcessError as e:
         print("git diff returned", e.returncode, file=stderr)
         return []
@@ -46,13 +46,15 @@ def git_changed_files(reference_branch, target_branch):
 
 def pdns_managed_zones():
     try:
-        all_zones = run(["pdnsutil", "list-all-zones"], check=True, capture_output=True)
-        zones = all_zones.stdout.decode("utf-8").strip().splitlines()
+        all_zones = run(
+            ["pdnsutil", "list-all-zones"], check=True, capture_output=True, text=True
+        )
+        zones = all_zones.stdout.strip().splitlines()
     except CalledProcessError as e:
         print(
             "could not get list of zones from pdnsutil",
             e.returncode,
-            e.stderr.decode("utf-8"),
+            e.stderr,
             file=stderr,
         )
         return []
@@ -75,11 +77,12 @@ def calculate_changed_zones(files, zones):
 
 def generate_diff(zone, reference_branch, target_branch):
     diffresult = run(
-        ["git", "diff", "{}..{}".format(reference_branch, target_branch), "--", zone],
+        ["git", "diff", f"{reference_branch}..{target_branch}", "--", zone],
         check=True,
         capture_output=True,
+        text=True,
     )
-    return diffresult.stdout.decode("utf-8").strip()
+    return diffresult.stdout.strip()
 
 
 def get_zone_data(zone, branch):
@@ -87,19 +90,21 @@ def get_zone_data(zone, branch):
     Get the zone data for the zone from target branch excluding SOA record.
     """
     result = run(
-        ["git", "show", "{}:{}".format(branch, zone)], check=True, capture_output=True
+        ["git", "show", f"{branch}:{zone}"], check=True, capture_output=True, text=True
     )
 
-    lines = result.stdout.decode("utf-8").strip().splitlines()
+    lines = result.stdout.strip().splitlines()
     soa = "\n".join([l for l in lines if "SOA" in l])
     non_soa = "\n".join([l for l in lines if not "SOA" in l])
     return non_soa
 
 
 def list_zone(zone):
-    result = run(["pdnsutil", "list-zone", zone], check=True, capture_output=True)
+    result = run(
+        ["pdnsutil", "list-zone", zone], check=True, capture_output=True, text=True
+    )
 
-    lines = result.stdout.decode("utf-8").strip().splitlines()
+    lines = result.stdout.strip().splitlines()
     soa = "\n".join([l for l in lines if "SOA" in l])
     non_soa = "\n".join([l for l in lines if not "SOA" in l])
     return soa, non_soa
@@ -116,46 +121,73 @@ def load_zone(zone, zonedata):
                 ["pdnsutil", "load-zone", zone, zonefile.name],
                 check=True,
                 capture_output=True,
+                text=True,
             )
-            print(p.stdout.decode("utf-8").strip())
+            print(p.stdout.strip())
         finally:
             os.unlink(zonefile.name)
 
 
 def check_zone(zone):
-    p = run(["pdnsutil", "check-zone", zone], check=True, capture_output=True)
-    print(p.stdout.decode("utf-8").strip())
+    p = run(
+        ["pdnsutil", "check-zone", zone], check=True, capture_output=True, text=True
+    )
+    print(p.stdout.strip())
 
 
 def increase_serial(zone):
-    p = run(["pdnsutil", "increase-serial", zone], check=True, capture_output=True)
-    print(p.stdout.decode("utf-8").strip())
+    p = run(
+        ["pdnsutil", "increase-serial", zone],
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    print(p.stdout.strip())
 
 
 def rectify_zone(zone):
-    p = run(["pdnsutil", "rectify-zone", zone], check=True, capture_output=True)
-    print(p.stdout.decode("utf-8").strip())
+    p = run(
+        ["pdnsutil", "rectify-zone", zone], check=True, capture_output=True, text=True
+    )
+    print(p.stdout.strip())
 
 
-def send_audit_mail(diffs, audit_email_address, audit_sender_address):
+def send_audit_mail(diffs, audit_email_address, audit_sender_address, changelog):
     message = EmailMessage()
     message["Subject"] = "DNS changes applied"
     message["To"] = audit_email_address
     message["From"] = audit_sender_address
 
-    body = """The following zones have been changed:
+    body = """A DNS change has been applied from git.
+
+The following zones have been changed:
 """
     for zone, _ in diffs:
-        body += "\n- {}".format(zone)
+        body += f"\n  - {zone}"
+
+    body += f"""
+
+This is the change log for the applied commits:
+
+{changelog}
+"""
 
     message.set_content(body)
 
     for zone, diff in diffs:
-        message.add_attachment(diff, filename="{}.diff".format(zone))
+        message.add_attachment(diff, filename=f"{zone}.diff")
 
-    server = SMTP("localhost")
-    server.send_message(message)
-    server.quit()
+    run(["/usr/lib/sendmail", "-t", "-oi"], input=message.as_bytes(), check=True)
+
+
+def get_changelog(reference_branch, target_branch):
+    r = run(
+        ["git", "log", f"{reference_branch}..{target_branch}"],
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    return r.stdout.strip()
 
 
 def main(reference_branch, target_branch, audit_email_address, audit_sender_address):
@@ -195,7 +227,7 @@ def main(reference_branch, target_branch, audit_email_address, audit_sender_addr
         soa, old_zonedata = list_zone(zone)
 
         zonedata = get_zone_data(zone=zone, branch=target_branch)
-        zonedata += "\n{}".format(soa)
+        zonedata += f"\n{soa}"
 
         try:
             load_zone(zone, zonedata)
@@ -205,17 +237,19 @@ def main(reference_branch, target_branch, audit_email_address, audit_sender_addr
         except CalledProcessError as pe:
             print(
                 "process {} failed with return code {}:\n{}".format(
-                    " ".join(pe.cmd), pe.returncode, pe.stderr.decode("utf-8")
+                    " ".join(pe.cmd), pe.returncode, pe.stderr
                 ),
                 file=stderr,
             )
-            print("reverting zone {}".format(zone), file=stderr)
-            load_zone(zone, "{}\n{}".format(old_zonedata, soa))
+            print(f"reverting zone {zone}", file=stderr)
+            load_zone(zone, f"{old_zonedata}\n{soa}")
+
+    changelog = get_changelog(reference_branch, target_branch)
 
     run(["git", "branch", "-d", reference_branch], check=True)
     run(["git", "branch", reference_branch, target_branch], check=True)
 
-    send_audit_mail(diffs, audit_email_address, audit_sender_address)
+    send_audit_mail(diffs, audit_email_address, audit_sender_address, changelog)
 
 
 if __name__ == "__main__":