1 files changed, 74 insertions, 7 deletions

diff --git a/synapse/config/_base.py b/synapse/config/_base.py
index 179aa7ff88..42364fc133 100644
--- a/synapse/config/_base.py
+++ b/synapse/config/_base.py
@@ -16,14 +16,18 @@
 
 import argparse
 import errno
+import logging
 import os
 from collections import OrderedDict
 from hashlib import sha256
 from textwrap import dedent
 from typing import (
     Any,
+    ClassVar,
+    Collection,
     Dict,
     Iterable,
+    Iterator,
     List,
     MutableMapping,
     Optional,
@@ -40,6 +44,8 @@ import yaml
 
 from synapse.util.templates import _create_mxc_to_http_filter, _format_ts_filter
 
+logger = logging.getLogger(__name__)
+
 
 class ConfigError(Exception):
     """Represents a problem parsing the configuration
@@ -55,6 +61,38 @@ class ConfigError(Exception):
         self.path = path
 
 
+def format_config_error(e: ConfigError) -> Iterator[str]:
+    """
+    Formats a config error neatly
+
+    The idea is to format the immediate error, plus the "causes" of those errors,
+    hopefully in a way that makes sense to the user. For example:
+
+        Error in configuration at 'oidc_config.user_mapping_provider.config.display_name_template':
+          Failed to parse config for module 'JinjaOidcMappingProvider':
+            invalid jinja template:
+              unexpected end of template, expected 'end of print statement'.
+
+    Args:
+        e: the error to be formatted
+
+    Returns: An iterator which yields string fragments to be formatted
+    """
+    yield "Error in configuration"
+
+    if e.path:
+        yield " at '%s'" % (".".join(e.path),)
+
+    yield ":\n  %s" % (e.msg,)
+
+    parent_e = e.__cause__
+    indent = 1
+    while parent_e:
+        indent += 1
+        yield ":\n%s%s" % ("  " * indent, str(parent_e))
+        parent_e = parent_e.__cause__
+
+
 # We split these messages out to allow packages to override with package
 # specific instructions.
 MISSING_REPORT_STATS_CONFIG_INSTRUCTIONS = """\
@@ -119,7 +157,7 @@ class Config:
             defined in subclasses.
     """
 
-    section: str
+    section: ClassVar[str]
 
     def __init__(self, root_config: "RootConfig" = None):
         self.root = root_config
@@ -309,9 +347,12 @@ class RootConfig:
     class, lower-cased and with "Config" removed.
     """
 
-    config_classes = []
+    config_classes: List[Type[Config]] = []
+
+    def __init__(self, config_files: Collection[str] = ()):
+        # Capture absolute paths here, so we can reload config after we daemonize.
+        self.config_files = [os.path.abspath(path) for path in config_files]
 
-    def __init__(self):
         for config_class in self.config_classes:
             if config_class.section is None:
                 raise ValueError("%r requires a section name" % (config_class,))
@@ -512,12 +553,10 @@ class RootConfig:
             object from parser.parse_args(..)`
         """
 
-        obj = cls()
-
         config_args = parser.parse_args(argv)
 
         config_files = find_config_files(search_paths=config_args.config_path)
-
+        obj = cls(config_files)
         if not config_files:
             parser.error("Must supply a config file.")
 
@@ -627,7 +666,7 @@ class RootConfig:
 
         generate_missing_configs = config_args.generate_missing_configs
 
-        obj = cls()
+        obj = cls(config_files)
 
         if config_args.generate_config:
             if config_args.report_stats is None:
@@ -727,6 +766,34 @@ class RootConfig:
     ) -> None:
         self.invoke_all("generate_files", config_dict, config_dir_path)
 
+    def reload_config_section(self, section_name: str) -> Config:
+        """Reconstruct the given config section, leaving all others unchanged.
+
+        This works in three steps:
+
+        1. Create a new instance of the relevant `Config` subclass.
+        2. Call `read_config` on that instance to parse the new config.
+        3. Replace the existing config instance with the new one.
+
+        :raises ValueError: if the given `section` does not exist.
+        :raises ConfigError: for any other problems reloading config.
+
+        :returns: the previous config object, which no longer has a reference to this
+            RootConfig.
+        """
+        existing_config: Optional[Config] = getattr(self, section_name, None)
+        if existing_config is None:
+            raise ValueError(f"Unknown config section '{section_name}'")
+        logger.info("Reloading config section '%s'", section_name)
+
+        new_config_data = read_config_files(self.config_files)
+        new_config = type(existing_config)(self)
+        new_config.read_config(new_config_data)
+        setattr(self, section_name, new_config)
+
+        existing_config.root = None
+        return existing_config
+
 
 def read_config_files(config_files: Iterable[str]) -> Dict[str, Any]:
     """Read the config files into a dict