Add `client_secret_path` as alternative for `client_secret` for OIDC config (#16030)

changelog.d/16030.feature

+Allow specifying `client_secret_path` as alternative to `client_secret` for OIDC providers. This avoids leaking the client secret in the homeserver config. Contributed by @Ma27.

docs/usage/configuration/config_documentation.md

@@ -3204,6 +3204,14 @@ Options for each entry include:
 
 * `client_secret`: oauth2 client secret to use. May be omitted if
   `client_secret_jwt_key` is given, or if `client_auth_method` is 'none'.
+  Must be omitted if `client_secret_path` is specified.
+
+* `client_secret_path`: path to the oauth2 client secret to use. With that
+   it's not necessary to leak secrets into the config file itself.
+   Mutually exclusive with `client_secret`. Can be omitted if
+   `client_secret_jwt_key` is specified.
+
+   *Added in Synapse 1.91.0.*
 
 * `client_secret_jwt_key`: Alternative to client_secret: details of a key used
    to create a JSON Web Token to be used as an OAuth2 client secret. If

synapse/config/oidc.py

@@ -280,6 +280,20 @@ def _parse_oidc_config_dict(
         for x in oidc_config.get("attribute_requirements", [])
     ]
 
+    # Read from either `client_secret_path` or `client_secret`. If both exist, error.
+    client_secret = oidc_config.get("client_secret")
+    client_secret_path = oidc_config.get("client_secret_path")
+    if client_secret_path is not None:
+        if client_secret is None:
+            client_secret = read_file(
+                client_secret_path, config_path + ("client_secret_path",)
+            ).rstrip("\n")
+        else:
+            raise ConfigError(
+                "Cannot specify both client_secret and client_secret_path",
+                config_path + ("client_secret",),
+            )
+
     return OidcProviderConfig(
         idp_id=idp_id,
         idp_name=oidc_config.get("idp_name", "OIDC"),
@@ -288,7 +302,7 @@ def _parse_oidc_config_dict(
         discover=oidc_config.get("discover", True),
         issuer=oidc_config["issuer"],
         client_id=oidc_config["client_id"],
-        client_secret=oidc_config.get("client_secret"),
+        client_secret=client_secret,
         client_secret_jwt_key=client_secret_jwt_key,
         client_auth_method=oidc_config.get("client_auth_method", "client_secret_basic"),
         pkce_method=oidc_config.get("pkce_method", "auto"),