add reading password from file

Signed-off-by: Florian Brandes <florian.brandes@posteo.de>

smtprd_ng/smtprd.py

@@ -107,6 +107,22 @@ class Config:
             email_certs[email] = cert
         return email_certs
 
+    @classmethod
+    def _read_from_file(cls, file: Union[Path, str]) -> str:
+        """Read string from file"""
+        if len(file) == 0:
+            # empty string (which is our fallback), so return empty string
+            return ""
+        # file is non-empty and should provide a path
+        file = Path(file)
+        if not file.exists():
+            raise OSError("File wasn't found: " + str(file))
+        with open(file, "r", encoding="utf8") as fp:
+            line = fp.readline().strip("\n")
+        if len(line) == 0:
+            raise OSError("Empty file supplied: " + str(file))
+        return line
+
     @classmethod
     def _from_config(cls, config: configparser.RawConfigParser) -> "Config":
         email_certs = cls._get_emails_and_certs(config.items("emails"))
@@ -120,7 +136,9 @@ class Config:
                 port=config.getint("client", "port"),
                 sender=config.get("client", "sender"),
                 username=config.get("client", "username"),
-                password=config.get("client", "password"),
+                password=cls._read_from_file(
+                    (config.get("client", "password_file", fallback=""))
+                ),
                 set_reply_to=config.getboolean(
                     "client", "set_reply_to", fallback=False
                 ),