Expose opentracing trace id in response headers (#10199)

Fixes: #9480

changelog.d/10199.misc

@@ -0,0 +1 @@
+Expose opentracing trace id in response headers.

synapse/federation/transport/server.py

@@ -35,6 +35,7 @@
     parse_string_from_args,
     parse_strings_from_args,
 )
+from synapse.logging import opentracing
 from synapse.logging.context import run_in_background
 from synapse.logging.opentracing import (
     SynapseTags,
@@ -345,6 +346,8 @@ async def new_func(request, *args, **kwargs):
                 )
 
             with scope:
+                opentracing.inject_response_headers(request.responseHeaders)
+
                 if origin and self.RATELIMIT:
                     with ratelimiter.ratelimit(origin) as d:
                         await d

synapse/logging/opentracing.py

@@ -173,6 +173,7 @@ def set_fates(clotho, lachesis, atropos, father="Zues", mother="Themis"):
 import attr
 
 from twisted.internet import defer
+from twisted.web.http_headers import Headers
 
 from synapse.config import ConfigError
 from synapse.util import json_decoder, json_encoder
@@ -668,6 +669,25 @@ def inject_header_dict(
         headers[key.encode()] = [value.encode()]
 
 
+def inject_response_headers(response_headers: Headers) -> None:
+    """Inject the current trace id into the HTTP response headers"""
+    if not opentracing:
+        return
+    span = opentracing.tracer.active_span
+    if not span:
+        return
+
+    # This is a bit implementation-specific.
+    #
+    # Jaeger's Spans have a trace_id property; other implementations (including the
+    # dummy opentracing.span.Span which we use if init_tracer is not called) do not
+    # expose it
+    trace_id = getattr(span, "trace_id", None)
+
+    if trace_id is not None:
+        response_headers.addRawHeader("Synapse-Trace-Id", f"{trace_id:x}")
+
+
 @ensure_active_span("get the active span context as a dict", ret={})
 def get_active_span_text_map(destination=None):
     """
@@ -843,6 +863,7 @@ def trace_servlet(request: "SynapseRequest", extract_context: bool = False):
         scope = start_active_span(request_name)
 
     with scope:
+        inject_response_headers(request.responseHeaders)
         try:
             yield
         finally: