Improve opentracing for incoming HTTP requests (#11618)

* remove `start_active_span_from_request`

Instead, pull out a separate function, `span_context_from_request`, to extract
the parent span, which we can then pass into `start_active_span` as
normal. This seems to be clearer all round.

* Remove redundant tags from `incoming-federation-request`

These are all wrapped up inside a parent span generated in AsyncResource, so
there's no point duplicating all the tags that are set there.

* Leave request spans open until the request completes

It may take some time for the response to be encoded into JSON, and that JSON
to be streamed back to the client, and really we want that inside the top-level
span, so let's hand responsibility for closure to the SynapseRequest.

* opentracing logs for HTTP request events

* changelog

4 files changed, 65 insertions, 73 deletions

diff --git a/changelog.d/11618.misc b/changelog.d/11618.misc
new file mode 100644
index 0000000000..4076b30bf7
--- /dev/null
+++ b/changelog.d/11618.misc
@@ -0,0 +1 @@
+Improve opentracing support for incoming HTTP requests.
diff --git a/synapse/federation/transport/server/_base.py b/synapse/federation/transport/server/_base.py
index dc39e3537b..da1fbf8b63 100644
--- a/synapse/federation/transport/server/_base.py
+++ b/synapse/federation/transport/server/_base.py
@@ -22,13 +22,11 @@ from synapse.api.urls import FEDERATION_V1_PREFIX
 from synapse.http.server import HttpServer, ServletCallback
 from synapse.http.servlet import parse_json_object_from_request
 from synapse.http.site import SynapseRequest
-from synapse.logging import opentracing
 from synapse.logging.context import run_in_background
 from synapse.logging.opentracing import (
-    SynapseTags,
-    start_active_span,
-    start_active_span_from_request,
-    tags,
+    set_tag,
+    span_context_from_request,
+    start_active_span_follows_from,
     whitelisted_homeserver,
 )
 from synapse.server import HomeServer
@@ -279,30 +277,19 @@ class BaseFederationServlet:
                 logger.warning("authenticate_request failed: %s", e)
                 raise
 
-            request_tags = {
-                SynapseTags.REQUEST_ID: request.get_request_id(),
-                tags.SPAN_KIND: tags.SPAN_KIND_RPC_SERVER,
-                tags.HTTP_METHOD: request.get_method(),
-                tags.HTTP_URL: request.get_redacted_uri(),
-                tags.PEER_HOST_IPV6: request.getClientIP(),
-                "authenticated_entity": origin,
-                "servlet_name": request.request_metrics.name,
-            }
-
-            # Only accept the span context if the origin is authenticated
-            # and whitelisted
+            # update the active opentracing span with the authenticated entity
+            set_tag("authenticated_entity", origin)
+
+            # if the origin is authenticated and whitelisted, link to its span context
+            context = None
             if origin and whitelisted_homeserver(origin):
-                scope = start_active_span_from_request(
-                    request, "incoming-federation-request", tags=request_tags
-                )
-            else:
-                scope = start_active_span(
-                    "incoming-federation-request", tags=request_tags
-                )
+                context = span_context_from_request(request)
 
-            with scope:
-                opentracing.inject_response_headers(request.responseHeaders)
+            scope = start_active_span_follows_from(
+                "incoming-federation-request", contexts=(context,) if context else ()
+            )
 
+            with scope:
                 if origin and self.RATELIMIT:
                     with ratelimiter.ratelimit(origin) as d:
                         await d
diff --git a/synapse/http/site.py b/synapse/http/site.py
index 9f68d7e191..80f7a2ff58 100644
--- a/synapse/http/site.py
+++ b/synapse/http/site.py
@@ -14,7 +14,7 @@
 import contextlib
 import logging
 import time
-from typing import Any, Generator, Optional, Tuple, Union
+from typing import TYPE_CHECKING, Any, Generator, Optional, Tuple, Union
 
 import attr
 from zope.interface import implementer
@@ -35,6 +35,9 @@ from synapse.logging.context import (
 )
 from synapse.types import Requester
 
+if TYPE_CHECKING:
+    import opentracing
+
 logger = logging.getLogger(__name__)
 
 _next_request_seq = 0
@@ -81,6 +84,10 @@ class SynapseRequest(Request):
         # server name, for client requests this is the Requester object.
         self._requester: Optional[Union[Requester, str]] = None
 
+        # An opentracing span for this request. Will be closed when the request is
+        # completely processed.
+        self._opentracing_span: "Optional[opentracing.Span]" = None
+
         # we can't yet create the logcontext, as we don't know the method.
         self.logcontext: Optional[LoggingContext] = None
 
@@ -148,6 +155,13 @@ class SynapseRequest(Request):
         # If there's no authenticated entity, it was the requester.
         self.logcontext.request.authenticated_entity = authenticated_entity or requester
 
+    def set_opentracing_span(self, span: "opentracing.Span") -> None:
+        """attach an opentracing span to this request
+
+        Doing so will cause the span to be closed when we finish processing the request
+        """
+        self._opentracing_span = span
+
     def get_request_id(self) -> str:
         return "%s-%i" % (self.get_method(), self.request_seq)
 
@@ -286,6 +300,9 @@ class SynapseRequest(Request):
             self._processing_finished_time = time.time()
             self._is_processing = False
 
+            if self._opentracing_span:
+                self._opentracing_span.log_kv({"event": "finished processing"})
+
             # if we've already sent the response, log it now; otherwise, we wait for the
             # response to be sent.
             if self.finish_time is not None:
@@ -299,6 +316,8 @@ class SynapseRequest(Request):
         """
         self.finish_time = time.time()
         Request.finish(self)
+        if self._opentracing_span:
+            self._opentracing_span.log_kv({"event": "response sent"})
         if not self._is_processing:
             assert self.logcontext is not None
             with PreserveLoggingContext(self.logcontext):
@@ -333,6 +352,11 @@ class SynapseRequest(Request):
         with PreserveLoggingContext(self.logcontext):
             logger.info("Connection from client lost before response was sent")
 
+            if self._opentracing_span:
+                self._opentracing_span.log_kv(
+                    {"event": "client connection lost", "reason": str(reason.value)}
+                )
+
             if not self._is_processing:
                 self._finished_processing()
 
@@ -421,6 +445,10 @@ class SynapseRequest(Request):
             usage.evt_db_fetch_count,
         )
 
+        # complete the opentracing span, if any.
+        if self._opentracing_span:
+            self._opentracing_span.finish()
+
         try:
             self.request_metrics.stop(self.finish_time, self.code, self.sentLength)
         except Exception as e:
diff --git a/synapse/logging/opentracing.py b/synapse/logging/opentracing.py
index 5d93ab07f1..6364290615 100644
--- a/synapse/logging/opentracing.py
+++ b/synapse/logging/opentracing.py
@@ -173,6 +173,7 @@ from typing import TYPE_CHECKING, Collection, Dict, List, Optional, Pattern, Typ
 import attr
 
 from twisted.internet import defer
+from twisted.web.http import Request
 from twisted.web.http_headers import Headers
 
 from synapse.config import ConfigError
@@ -490,48 +491,6 @@ def start_active_span_follows_from(
     return scope
 
 
-def start_active_span_from_request(
-    request,
-    operation_name,
-    references=None,
-    tags=None,
-    start_time=None,
-    ignore_active_span=False,
-    finish_on_close=True,
-):
-    """
-    Extracts a span context from a Twisted Request.
-    args:
-        headers (twisted.web.http.Request)
-
-        For the other args see opentracing.tracer
-
-    returns:
-        span_context (opentracing.span.SpanContext)
-    """
-    # Twisted encodes the values as lists whereas opentracing doesn't.
-    # So, we take the first item in the list.
-    # Also, twisted uses byte arrays while opentracing expects strings.
-
-    if opentracing is None:
-        return noop_context_manager()  # type: ignore[unreachable]
-
-    header_dict = {
-        k.decode(): v[0].decode() for k, v in request.requestHeaders.getAllRawHeaders()
-    }
-    context = opentracing.tracer.extract(opentracing.Format.HTTP_HEADERS, header_dict)
-
-    return opentracing.tracer.start_active_span(
-        operation_name,
-        child_of=context,
-        references=references,
-        tags=tags,
-        start_time=start_time,
-        ignore_active_span=ignore_active_span,
-        finish_on_close=finish_on_close,
-    )
-
-
 def start_active_span_from_edu(
     edu_content,
     operation_name,
@@ -743,6 +702,20 @@ def active_span_context_as_string():
     return json_encoder.encode(carrier)
 
 
+def span_context_from_request(request: Request) -> "Optional[opentracing.SpanContext]":
+    """Extract an opentracing context from the headers on an HTTP request
+
+    This is useful when we have received an HTTP request from another part of our
+    system, and want to link our spans to those of the remote system.
+    """
+    if not opentracing:
+        return None
+    header_dict = {
+        k.decode(): v[0].decode() for k, v in request.requestHeaders.getAllRawHeaders()
+    }
+    return opentracing.tracer.extract(opentracing.Format.HTTP_HEADERS, header_dict)
+
+
 @only_if_tracing
 def span_context_from_string(carrier):
     """
@@ -882,10 +855,13 @@ def trace_servlet(request: "SynapseRequest", extract_context: bool = False):
     }
 
     request_name = request.request_metrics.name
-    if extract_context:
-        scope = start_active_span_from_request(request, request_name)
-    else:
-        scope = start_active_span(request_name)
+    context = span_context_from_request(request) if extract_context else None
+
+    # we configure the scope not to finish the span immediately on exit, and instead
+    # pass the span into the SynapseRequest, which will finish it once we've finished
+    # sending the response to the client.
+    scope = start_active_span(request_name, child_of=context, finish_on_close=False)
+    request.set_opentracing_span(scope.span)
 
     with scope:
         inject_response_headers(request.responseHeaders)