Support registration_shared_secret in a file (#13614)

A new `registration_shared_secret_path` option. This is kinda handy for k8s deployments and things.

changelog.d/13614.feature

@@ -0,0 +1 @@
+Support setting the registration shared secret in a file, via a new `registration_shared_secret_path` configuration option.

docs/usage/configuration/config_documentation.md

@@ -2124,10 +2124,28 @@ registration_requires_token: true
 If set, allows registration of standard or admin accounts by anyone who
 has the shared secret, even if registration is otherwise disabled.
 
+See also [`registration_shared_secret_path`](#registration_shared_secret_path).
+
 Example configuration:
 ```yaml
 registration_shared_secret: <PRIVATE STRING>
 ```
+
+---
+### `registration_shared_secret_path`
+
+An alternative to [`registration_shared_secret`](#registration_shared_secret):
+allows the shared secret to be specified in an external file.
+
+The file should be a plain text file, containing only the shared secret.
+
+Example configuration:
+```yaml
+registration_shared_secret_file: /path/to/secrets/file
+```
+
+_Added in Synapse 1.67.0._
+
 ---
 ### `bcrypt_rounds`
 

synapse/_scripts/register_new_matrix_user.py

@@ -1,6 +1,6 @@
 # Copyright 2015, 2016 OpenMarket Ltd
 # Copyright 2018 New Vector
-# Copyright 2021 The Matrix.org Foundation C.I.C.
+# Copyright 2021-22 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -25,6 +25,15 @@ from typing import Any, Callable, Dict, Optional
 import requests
 import yaml
 
+_CONFLICTING_SHARED_SECRET_OPTS_ERROR = """\
+Conflicting options 'registration_shared_secret' and 'registration_shared_secret_path'
+are both defined in config file.
+"""
+
+_NO_SHARED_SECRET_OPTS_ERROR = """\
+No 'registration_shared_secret' or 'registration_shared_secret_path' defined in config.
+"""
+
 _DEFAULT_SERVER_URL = "http://localhost:8008"
 
 
@@ -222,9 +231,15 @@ def main() -> None:
         # argparse should check that we have either config or shared secret
         assert config
 
-        secret = config.get("registration_shared_secret", None)
+        secret = config.get("registration_shared_secret")
+        secret_file = config.get("registration_shared_secret_path")
+        if secret_file:
+            if secret:
+                print(_CONFLICTING_SHARED_SECRET_OPTS_ERROR, file=sys.stderr)
+                sys.exit(1)
+            secret = _read_file(secret_file, "registration_shared_secret_path").strip()
         if not secret:
-            print("No 'registration_shared_secret' defined in config.")
+            print(_NO_SHARED_SECRET_OPTS_ERROR, file=sys.stderr)
             sys.exit(1)
 
     if args.server_url:
@@ -254,6 +269,30 @@ def main() -> None:
     )
 
 
+def _read_file(file_path: Any, config_path: str) -> str:
+    """Check the given file exists, and read it into a string
+
+    If it does not, exit with an error indicating the problem
+
+    Args:
+        file_path: the file to be read
+        config_path: where in the configuration file_path came from, so that a useful
+           error can be emitted if it does not exist.
+    Returns:
+        content of the file.
+    """
+    if not isinstance(file_path, str):
+        print(f"{config_path} setting is not a string", file=sys.stderr)
+        sys.exit(1)
+
+    try:
+        with open(file_path) as file_stream:
+            return file_stream.read()
+    except OSError as e:
+        print(f"Error accessing file {file_path}: {e}", file=sys.stderr)
+        sys.exit(1)
+
+
 def _find_client_listener(config: Dict[str, Any]) -> Optional[str]:
     # try to find a listener in the config. Returns a host:port pair
     for listener in config.get("listeners", []):

synapse/config/registration.py

@@ -13,10 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import argparse
-from typing import Any, Optional
+from typing import Any, Dict, Optional
 
 from synapse.api.constants import RoomCreationPreset
-from synapse.config._base import Config, ConfigError
+from synapse.config._base import Config, ConfigError, read_file
 from synapse.types import JsonDict, RoomAlias, UserID
 from synapse.util.stringutils import random_string_with_symbols, strtobool
 
@@ -27,6 +27,11 @@ password resets, configure Synapse with an SMTP server via the `email` setting,
 remove `account_threepid_delegates.email`.
 """
 
+CONFLICTING_SHARED_SECRET_OPTS_ERROR = """\
+You have configured both `registration_shared_secret` and
+`registration_shared_secret_path`. These are mutually incompatible.
+"""
+
 
 class RegistrationConfig(Config):
     section = "registration"
@@ -53,7 +58,16 @@ class RegistrationConfig(Config):
         self.enable_registration_token_3pid_bypass = config.get(
             "enable_registration_token_3pid_bypass", False
         )
+
+        # read the shared secret, either inline or from an external file
         self.registration_shared_secret = config.get("registration_shared_secret")
+        registration_shared_secret_path = config.get("registration_shared_secret_path")
+        if registration_shared_secret_path:
+            if self.registration_shared_secret:
+                raise ConfigError(CONFLICTING_SHARED_SECRET_OPTS_ERROR)
+            self.registration_shared_secret = read_file(
+                registration_shared_secret_path, ("registration_shared_secret_path",)
+            ).strip()
 
         self.bcrypt_rounds = config.get("bcrypt_rounds", 12)
 
@@ -218,6 +232,21 @@ class RegistrationConfig(Config):
         else:
             return ""
 
+    def generate_files(self, config: Dict[str, Any], config_dir_path: str) -> None:
+        # if 'registration_shared_secret_path' is specified, and the target file
+        # does not exist, generate it.
+        registration_shared_secret_path = config.get("registration_shared_secret_path")
+        if registration_shared_secret_path and not self.path_exists(
+            registration_shared_secret_path
+        ):
+            print(
+                "Generating registration shared secret file "
+                + registration_shared_secret_path
+            )
+            secret = random_string_with_symbols(50)
+            with open(registration_shared_secret_path, "w") as f:
+                f.write(f"{secret}\n")
+
     @staticmethod
     def add_arguments(parser: argparse.ArgumentParser) -> None:
         reg_group = parser.add_argument_group("registration")