Experimental Unix socket support (#15353)

* Add IReactorUNIX to ISynapseReactor type hint.

* Create listen_unix().

Two options, 'path' to the file and 'mode' of permissions(not umask, recommend 666 as default as
nginx/other reverse proxies write to it and it's setup as user www-data)

For the moment, leave the option to always create a PID lockfile turned on by default

* Create UnixListenerConfig and wire it up.

Rename ListenerConfig to TCPListenerConfig, then Union them together into ListenerConfig.
This spidered around a bit, but I think I got it all. Metrics and manhole have been placed
behind a conditional in case of accidental putting them onto a unix socket.

Use new helpers to get if a listener is configured for TLS, and to help create a site tag
for logging.

There are 2 TODO things in parse_listener_def() to finish up at a later point.

* Refactor SynapseRequest to handle logging correctly when using a unix socket.

This prevents an exception when an IP address can not be retrieved for a request.

* Make the 'Synapse now listening on Unix socket' log line a little prettier.

* No silent failures on generic workers when trying to use a unix socket with metrics or manhole.

* Inline variables in app/_base.py

* Update docstring for listen_unix() to remove reference to a hardcoded permission of 0o666 and add a few comments saying where the default IS declared.

* Disallow both a unix socket and a ip/port combo on the same listener resource

* Linting

* Changelog

* review: simplify how listen_unix returns(and get rid of a type: ignore)

* review: fix typo from ConfigError in app/homeserver.py

* review: roll conditional for http_options.tag into get_site_tag() helper(and add docstring)

* review: enhance the conditionals for checking if a port or path is valid, remove a TODO line

* review: Try updating comment in get_client_ip_if_available to clarify what is being retrieved and why

* Pretty up how 'Synapse now listening on Unix Socket' looks by decoding the byte string.

* review: In parse_listener_def(), raise ConfigError if neither socket_path nor port is declared(and fix a typo)

synapse/app/generic_worker.py

@@ -38,7 +38,7 @@ from synapse.app._base import (
 from synapse.config._base import ConfigError
 from synapse.config.homeserver import HomeServerConfig
 from synapse.config.logger import setup_logging
-from synapse.config.server import ListenerConfig
+from synapse.config.server import ListenerConfig, TCPListenerConfig
 from synapse.federation.transport.server import TransportLayerServer
 from synapse.http.server import JsonResource, OptionsResource
 from synapse.logging.context import LoggingContext
@@ -236,12 +236,18 @@ class GenericWorkerServer(HomeServer):
             if listener.type == "http":
                 self._listen_http(listener)
             elif listener.type == "manhole":
-                _base.listen_manhole(
-                    listener.bind_addresses,
-                    listener.port,
-                    manhole_settings=self.config.server.manhole_settings,
-                    manhole_globals={"hs": self},
-                )
+                if isinstance(listener, TCPListenerConfig):
+                    _base.listen_manhole(
+                        listener.bind_addresses,
+                        listener.port,
+                        manhole_settings=self.config.server.manhole_settings,
+                        manhole_globals={"hs": self},
+                    )
+                else:
+                    raise ConfigError(
+                        "Can not using a unix socket for manhole at this time."
+                    )
+
             elif listener.type == "metrics":
                 if not self.config.metrics.enable_metrics:
                     logger.warning(
@@ -249,10 +255,16 @@ class GenericWorkerServer(HomeServer):
                         "enable_metrics is not True!"
                     )
                 else:
-                    _base.listen_metrics(
-                        listener.bind_addresses,
-                        listener.port,
-                    )
+                    if isinstance(listener, TCPListenerConfig):
+                        _base.listen_metrics(
+                            listener.bind_addresses,
+                            listener.port,
+                        )
+                    else:
+                        raise ConfigError(
+                            "Can not use a unix socket for metrics at this time."
+                        )
+
             else:
                 logger.warning("Unsupported listener type: %s", listener.type)