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/http/site.py

@@ -19,6 +19,7 @@ from typing import TYPE_CHECKING, Any, Generator, Optional, Tuple, Union
 import attr
 from zope.interface import implementer
 
+from twisted.internet.address import UNIXAddress
 from twisted.internet.defer import Deferred
 from twisted.internet.interfaces import IAddress, IReactorTime
 from twisted.python.failure import Failure
@@ -257,7 +258,7 @@ class SynapseRequest(Request):
             request_id,
             request=ContextRequest(
                 request_id=request_id,
-                ip_address=self.getClientAddress().host,
+                ip_address=self.get_client_ip_if_available(),
                 site_tag=self.synapse_site.site_tag,
                 # The requester is going to be unknown at this point.
                 requester=None,
@@ -414,7 +415,7 @@ class SynapseRequest(Request):
 
         self.synapse_site.access_logger.debug(
             "%s - %s - Received request: %s %s",
-            self.getClientAddress().host,
+            self.get_client_ip_if_available(),
             self.synapse_site.site_tag,
             self.get_method(),
             self.get_redacted_uri(),
@@ -462,7 +463,7 @@ class SynapseRequest(Request):
             "%s - %s - {%s}"
             " Processed request: %.3fsec/%.3fsec (%.3fsec, %.3fsec) (%.3fsec/%.3fsec/%d)"
             ' %sB %s "%s %s %s" "%s" [%d dbevts]',
-            self.getClientAddress().host,
+            self.get_client_ip_if_available(),
             self.synapse_site.site_tag,
             requester,
             processing_time,
@@ -500,6 +501,26 @@ class SynapseRequest(Request):
 
         return True
 
+    def get_client_ip_if_available(self) -> str:
+        """Logging helper. Return something useful when a client IP is not retrievable
+        from a unix socket.
+
+        In practice, this returns the socket file path on a SynapseRequest if using a
+        unix socket and the normal IP address for TCP sockets.
+
+        """
+        # getClientAddress().host returns a proper IP address for a TCP socket. But
+        # unix sockets have no concept of IP addresses or ports and return a
+        # UNIXAddress containing a 'None' value. In order to get something usable for
+        # logs(where this is used) get the unix socket file. getHost() returns a
+        # UNIXAddress containing a value of the socket file and has an instance
+        # variable of 'name' encoded as a byte string containing the path we want.
+        # Decode to utf-8 so it looks nice.
+        if isinstance(self.getClientAddress(), UNIXAddress):
+            return self.getHost().name.decode("utf-8")
+        else:
+            return self.getClientAddress().host
+
 
 class XForwardedForRequest(SynapseRequest):
     """Request object which honours proxy headers