Be able to correlate timeouts in reverse-proxy layer in front of Synapse (pull request ID from header)

docs/usage/configuration/config_documentation.md

@@ -436,6 +436,12 @@ Sub-options for each listener include:
 * `x_forwarded`: Only valid for an 'http' listener. Set to true to use the X-Forwarded-For header as the client IP. Useful when Synapse is
    behind a reverse-proxy.
 
+* `request_id_header`: The header used as the basis for the request ID which
+   is used in logs and tracing to correlate requests. Useful when Synapse is
+   behind a reverse-proxy. For example, if you use Cloudflare in front of
+   Synapse, you can set this to `"cf-ray"` to match up requests even when the
+   Cloudflare layer times out before Synapse is done.
+
 * `resources`: Only valid for an 'http' listener. A list of resources to host
    on this port. Sub-options for each resource are:
 

synapse/config/server.py

@@ -206,6 +206,7 @@ class HttpListenerConfig:
     resources: List[HttpResourceConfig] = attr.Factory(list)
     additional_resources: Dict[str, dict] = attr.Factory(dict)
     tag: Optional[str] = None
+    request_id_header: Optional[str] = None
 
 
 @attr.s(slots=True, frozen=True, auto_attribs=True)
@@ -887,7 +888,7 @@ def read_gc_thresholds(
         )
 
 
-def parse_listener_def(num: int, listener: Any) -> ListenerConfig:
+def parse_listener_def(num: int, listener: JsonDict) -> ListenerConfig:
     """parse a listener config from the config file"""
     listener_type = listener["type"]
     # Raise a helpful error if direct TCP replication is still configured.
@@ -928,6 +929,7 @@ def parse_listener_def(num: int, listener: Any) -> ListenerConfig:
             resources=resources,
             additional_resources=listener.get("additional_resources", {}),
             tag=listener.get("tag"),
+            request_id_header=listener.get("request_id_header"),
         )
 
     return ListenerConfig(port, bind_addresses, listener_type, tls, http_config)

synapse/http/site.py

@@ -72,10 +72,12 @@ def __init__(
         site: "SynapseSite",
         *args: Any,
         max_request_body_size: int = 1024,
+        request_id_header: Optional[str],
         **kw: Any,
     ):
         super().__init__(channel, *args, **kw)
         self._max_request_body_size = max_request_body_size
+        self.request_id_header = request_id_header
         self.synapse_site = site
         self.reactor = site.reactor
         self._channel = channel  # this is used by the tests
@@ -172,7 +174,13 @@ def set_opentracing_span(self, span: "opentracing.Span") -> None:
         self._opentracing_span = span
 
     def get_request_id(self) -> str:
-        return "%s-%i" % (self.get_method(), self.request_seq)
+        if self.request_id_header:
+            request_id_value = self.getHeader(self.request_id_header)
+
+        if not request_id_value:
+            request_id_value = self.request_seq
+
+        return "%s-%i" % (self.get_method(), request_id_value)
 
     def get_redacted_uri(self) -> str:
         """Gets the redacted URI associated with the request (or placeholder if the URI
@@ -617,6 +625,7 @@ def request_factory(channel: HTTPChannel, queued: bool) -> Request:
                 self,
                 max_request_body_size=max_request_body_size,
                 queued=queued,
+                request_id_header=config.http_options.request_id_header,
             )
 
         self.requestFactory = request_factory  # type: ignore