Add config option turn_shared_secret_path (#17690)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/17690.feature

@@ -0,0 +1 @@
+Add config option `turn_shared_secret_path`.

docs/usage/configuration/config_documentation.md

@@ -2315,6 +2315,22 @@ Example configuration:
 ```yaml
 turn_shared_secret: "YOUR_SHARED_SECRET"
 ```
+---
+### `turn_shared_secret_path`
+
+An alternative to [`turn_shared_secret`](#turn_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.
+Synapse reads the shared secret from the given file once at startup.
+
+Example configuration:
+```yaml
+turn_shared_secret_path: /path/to/secrets/file
+```
+
+_Added in Synapse 1.116.0._
+
 ---
 ### `turn_username` and `turn_password`
 

synapse/config/voip.py

@@ -23,7 +23,12 @@ from typing import Any
 
 from synapse.types import JsonDict
 
-from ._base import Config
+from ._base import Config, ConfigError, read_file
+
+CONFLICTING_SHARED_SECRET_OPTS_ERROR = """\
+You have configured both `turn_shared_secret` and `turn_shared_secret_path`.
+These are mutually incompatible.
+"""
 
 
 class VoipConfig(Config):
@@ -32,6 +37,13 @@ class VoipConfig(Config):
     def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         self.turn_uris = config.get("turn_uris", [])
         self.turn_shared_secret = config.get("turn_shared_secret")
+        turn_shared_secret_path = config.get("turn_shared_secret_path")
+        if turn_shared_secret_path:
+            if self.turn_shared_secret:
+                raise ConfigError(CONFLICTING_SHARED_SECRET_OPTS_ERROR)
+            self.turn_shared_secret = read_file(
+                turn_shared_secret_path, ("turn_shared_secret_path",)
+            ).strip()
         self.turn_username = config.get("turn_username")
         self.turn_password = config.get("turn_password")
         self.turn_user_lifetime = self.parse_duration(