feat: support externally managed TLS via tls_external_cert_and_key option (#860)

Adds a new tls_external_cert_and_key config option for chatmail servers
that manage their own TLS certificates (e.g. via an external ACME client
or a load balancer).

A systemd path unit (tls-cert-reload.path) watches the certificate file
via inotify and automatically reloads dovecot and nginx when it changes.
Postfix reads certs per TLS handshake so needs no reload.

Also extracts openssl_selfsigned_args() so cert generation parameters
are shared between SelfSignedTlsDeployer and the e2e test.

chatmaild/src/chatmaild/config.py

@@ -60,10 +60,23 @@ class Config:
         self.privacy_pdo = params.get("privacy_pdo")
         self.privacy_supervisor = params.get("privacy_supervisor")
 
-        # TLS certificate management: derived from the domain name.
-        # Domains starting with "_" use self-signed certificates
-        # All other domains use ACME.
-        if self.mail_domain.startswith("_"):
+        # TLS certificate management.
+        # If tls_external_cert_and_key is set, use externally managed certs.
+        # Otherwise derived from the domain name:
+        # - Domains starting with "_" use self-signed certificates
+        # - All other domains use ACME.
+        external = params.get("tls_external_cert_and_key", "").strip()
+
+        if external:
+            parts = external.split()
+            if len(parts) != 2:
+                raise ValueError(
+                    "tls_external_cert_and_key must have two space-separated"
+                    " paths: CERT_PATH KEY_PATH"
+                )
+            self.tls_cert_mode = "external"
+            self.tls_cert_path, self.tls_key_path = parts
+        elif self.mail_domain.startswith("_"):
             self.tls_cert_mode = "self"
             self.tls_cert_path = "/etc/ssl/certs/mailserver.pem"
             self.tls_key_path = "/etc/ssl/private/mailserver.key"

chatmaild/src/chatmaild/ini/chatmail.ini.f

@@ -48,6 +48,13 @@ passthrough_senders =
 # (space-separated, item may start with "@" to whitelist whole recipient domains)
 passthrough_recipients =
 
+# Use externally managed TLS certificates instead of built-in acmetool.
+# Paths refer to files on the deployment server (not the build machine).
+# Both files must already exist before running cmdeploy.
+# Certificate renewal is your responsibility; changed files are
+# picked up automatically by all relay services.
+# tls_external_cert_and_key = /path/to/fullchain.pem /path/to/privkey.pem
+
 # path to www directory - documented here: https://chatmail.at/doc/relay/getting_started.html#custom-web-pages
 #www_folder = www
 

chatmaild/src/chatmaild/tests/test_config.py

@@ -87,3 +87,37 @@ def test_config_tls_self(make_config):
     assert config.tls_cert_mode == "self"
     assert config.tls_cert_path == "/etc/ssl/certs/mailserver.pem"
     assert config.tls_key_path == "/etc/ssl/private/mailserver.key"
+
+
+def test_config_tls_external(make_config):
+    config = make_config(
+        "chat.example.org",
+        {
+            "tls_external_cert_and_key": "/custom/fullchain.pem /custom/privkey.pem",
+        },
+    )
+    assert config.tls_cert_mode == "external"
+    assert config.tls_cert_path == "/custom/fullchain.pem"
+    assert config.tls_key_path == "/custom/privkey.pem"
+
+
+def test_config_tls_external_overrides_underscore(make_config):
+    config = make_config(
+        "_test.example.org",
+        {
+            "tls_external_cert_and_key": "/certs/fullchain.pem /certs/privkey.pem",
+        },
+    )
+    assert config.tls_cert_mode == "external"
+    assert config.tls_cert_path == "/certs/fullchain.pem"
+    assert config.tls_key_path == "/certs/privkey.pem"
+
+
+def test_config_tls_external_bad_format(make_config):
+    with pytest.raises(ValueError, match="two space-separated"):
+        make_config(
+            "chat.example.org",
+            {
+                "tls_external_cert_and_key": "/only/one/path.pem",
+            },
+        )