Append to existing aiohttp baggage

sentry_sdk/integrations/aiohttp.py

@@ -12,7 +12,11 @@
     _filter_headers,
     request_body_within_bounds,
 )
-from sentry_sdk.tracing import SOURCE_FOR_STYLE, TRANSACTION_SOURCE_ROUTE
+from sentry_sdk.tracing import (
+    BAGGAGE_HEADER_NAME,
+    SOURCE_FOR_STYLE,
+    TRANSACTION_SOURCE_ROUTE,
+)
 from sentry_sdk.tracing_utils import should_propagate_trace
 from sentry_sdk.utils import (
     capture_internal_exceptions,
@@ -220,7 +224,13 @@ async def on_request_start(session, trace_config_ctx, params):
                         key=key, value=value, url=params.url
                     )
                 )
-                params.headers[key] = value
+                if key == BAGGAGE_HEADER_NAME and params.headers.get(
+                    BAGGAGE_HEADER_NAME
+                ):
+                    # do not overwrite any existing baggage, just append to it
+                    params.headers[key] += "," + value
+                else:
+                    params.headers[key] = value
 
         trace_config_ctx.span = span
 

tests/integrations/aiohttp/test_aiohttp.py

@@ -505,3 +505,32 @@ async def handler(request):
             parent_span_id=request_span.span_id,
             sampled=1,
         )
+
+
+@pytest.mark.asyncio
+async def test_outgoing_trace_headers_append_to_baggage(
+    sentry_init, aiohttp_raw_server, aiohttp_client
+):
+    sentry_init(
+        integrations=[AioHttpIntegration()],
+        traces_sample_rate=1.0,
+        release="d08ebdb9309e1b004c6f52202de58a09c2268e42",
+    )
+
+    async def handler(request):
+        return web.Response(text="OK")
+
+    raw_server = await aiohttp_raw_server(handler)
+
+    with start_transaction(
+        name="/interactions/other-dogs/new-dog",
+        op="greeting.sniff",
+        trace_id="0123456789012345678901234567890",
+    ):
+        client = await aiohttp_client(raw_server)
+        resp = await client.get("/", headers={"bagGage": "custom=value"})
+
+        assert (
+            resp.request_info.headers["baggage"]
+            == "custom=value,sentry-trace_id=0123456789012345678901234567890,sentry-environment=production,sentry-release=d08ebdb9309e1b004c6f52202de58a09c2268e42,sentry-transaction=/interactions/other-dogs/new-dog,sentry-sample_rate=1.0"
+        )