Generate missing configuration files at startup (#13615)

If things like the signing key file are missing, let's just try to generate
them on startup.

Again, this is useful for k8s-like deployments where we just want to generate
keys on the first run.

changelog.d/13615.feature

@@ -0,0 +1 @@
+Change the default startup behaviour so that any missing "additional" configuration files (signing key, etc) are generated automatically.

docs/usage/configuration/config_documentation.md

@@ -2139,6 +2139,9 @@ allows the shared secret to be specified in an external file.
 
 The file should be a plain text file, containing only the shared secret.
 
+If this file does not exist, Synapse will create a new signing
+key on startup and store it in this file.
+
 Example configuration:
 ```yaml
 registration_shared_secret_file: /path/to/secrets/file
@@ -2555,7 +2558,10 @@ Config options relating to signing keys
 ---
 ### `signing_key_path`
 
-Path to the signing key to sign messages with.
+Path to the signing key to sign events and federation requests with.
+
+*New in Synapse 1.67*: If this file does not exist, Synapse will create a new signing
+key on startup and store it in this file.
 
 Example configuration:
 ```yaml

synapse/config/_base.py

@@ -20,6 +20,7 @@
 import os
 import re
 from collections import OrderedDict
+from enum import Enum, auto
 from hashlib import sha256
 from textwrap import dedent
 from typing import (
@@ -603,18 +604,44 @@ def load_or_generate_config(
             " may specify directories containing *.yaml files.",
         )
 
-        generate_group = parser.add_argument_group("Config generation")
-        generate_group.add_argument(
+        # we nest the mutually-exclusive group inside another group so that the help
+        # text shows them in their own group.
+        generate_mode_group = parser.add_argument_group(
+            "Config generation mode",
+        )
+        generate_mode_exclusive = generate_mode_group.add_mutually_exclusive_group()
+        generate_mode_exclusive.add_argument(
+            # hidden option to make the type and default work
+            "--generate-mode",
+            help=argparse.SUPPRESS,
+            type=_ConfigGenerateMode,
+            default=_ConfigGenerateMode.GENERATE_MISSING_AND_RUN,
+        )
+        generate_mode_exclusive.add_argument(
             "--generate-config",
-            action="store_true",
             help="Generate a config file, then exit.",
+            action="store_const",
+            const=_ConfigGenerateMode.GENERATE_EVERYTHING_AND_EXIT,
+            dest="generate_mode",
         )
-        generate_group.add_argument(
+        generate_mode_exclusive.add_argument(
             "--generate-missing-configs",
             "--generate-keys",
-            action="store_true",
             help="Generate any missing additional config files, then exit.",
+            action="store_const",
+            const=_ConfigGenerateMode.GENERATE_MISSING_AND_EXIT,
+            dest="generate_mode",
         )
+        generate_mode_exclusive.add_argument(
+            "--generate-missing-and-run",
+            help="Generate any missing additional config files, then run. This is the "
+            "default behaviour.",
+            action="store_const",
+            const=_ConfigGenerateMode.GENERATE_MISSING_AND_RUN,
+            dest="generate_mode",
+        )
+
+        generate_group = parser.add_argument_group("Details for --generate-config")
         generate_group.add_argument(
             "-H", "--server-name", help="The server name to generate a config file for."
         )
@@ -670,11 +697,12 @@ def load_or_generate_config(
         config_dir_path = os.path.abspath(config_dir_path)
         data_dir_path = os.getcwd()
 
-        generate_missing_configs = config_args.generate_missing_configs
-
         obj = cls(config_files)
 
-        if config_args.generate_config:
+        if (
+            config_args.generate_mode
+            == _ConfigGenerateMode.GENERATE_EVERYTHING_AND_EXIT
+        ):
             if config_args.report_stats is None:
                 parser.error(
                     "Please specify either --report-stats=yes or --report-stats=no\n\n"
@@ -732,11 +760,14 @@ def load_or_generate_config(
                     )
                     % (config_path,)
                 )
-                generate_missing_configs = True
 
         config_dict = read_config_files(config_files)
-        if generate_missing_configs:
-            obj.generate_missing_files(config_dict, config_dir_path)
+        obj.generate_missing_files(config_dict, config_dir_path)
+
+        if config_args.generate_mode in (
+            _ConfigGenerateMode.GENERATE_EVERYTHING_AND_EXIT,
+            _ConfigGenerateMode.GENERATE_MISSING_AND_EXIT,
+        ):
             return None
 
         obj.parse_config_dict(
@@ -965,6 +996,12 @@ def read_file(file_path: Any, config_path: Iterable[str]) -> str:
         raise ConfigError("Error accessing file %r" % (file_path,), config_path) from e
 
 
+class _ConfigGenerateMode(Enum):
+    GENERATE_MISSING_AND_RUN = auto()
+    GENERATE_MISSING_AND_EXIT = auto()
+    GENERATE_EVERYTHING_AND_EXIT = auto()
+
+
 __all__ = [
     "Config",
     "RootConfig",