fix(tracing): Only propagate headers from spans within transactions

This change ensures that we only propagate trace headers from spans that are within a transaction. This fixes a bug where any child transactions of a span created outside a transaction are missing a dynamic sampling context and are part of a trace missing a root transaction (because the root is the span).

Also, remove/modify tests that were asserting the old behavior.

Fixes #3068

sentry_sdk/tracing.py

@@ -426,12 +426,18 @@ def iter_headers(self):
         If the span's containing transaction doesn't yet have a ``baggage`` value,
         this will cause one to be generated and stored.
         """
+        if not self.containing_transaction:
+            # Do not propagate headers if there is no containing transaction. Otherwise, this
+            # span ends up being the root span of a new trace, and since it does not get sent
+            # to Sentry, the trace will be missing a root transaction. The dynamic sampling
+            # context will also be missing, breaking dynamic sampling & traces.
+            return
+
         yield SENTRY_TRACE_HEADER_NAME, self.to_traceparent()
 
-        if self.containing_transaction:
-            baggage = self.containing_transaction.get_baggage().serialize()
-            if baggage:
-                yield BAGGAGE_HEADER_NAME, baggage
+        baggage = self.containing_transaction.get_baggage().serialize()
+        if baggage:
+            yield BAGGAGE_HEADER_NAME, baggage
 
     @classmethod
     def from_traceparent(

tests/integrations/aiohttp/test_aiohttp.py

@@ -404,13 +404,17 @@ async def hello(request):
     # The aiohttp_client is instrumented so will generate the sentry-trace header and add request.
     # Get the sentry-trace header from the request so we can later compare with transaction events.
     client = await aiohttp_client(app)
-    resp = await client.get("/")
+    with start_transaction():
+        # Headers are only added to the span if there is an active transaction
+        resp = await client.get("/")
+
     sentry_trace_header = resp.request_info.headers.get("sentry-trace")
     trace_id = sentry_trace_header.split("-")[0]
 
     assert resp.status == 500
 
-    msg_event, error_event, transaction_event = events
+    # Last item is the custom transaction event wrapping `client.get("/")`
+    msg_event, error_event, transaction_event, _ = events
 
     assert msg_event["contexts"]["trace"]
     assert "trace_id" in msg_event["contexts"]["trace"]

tests/integrations/celery/test_update_celery_task_headers.py

@@ -77,33 +77,6 @@ def test_span_with_transaction(sentry_init):
             )
 
 
-def test_span_with_no_transaction(sentry_init):
-    sentry_init(enable_tracing=True)
-    headers = {}
-
-    with sentry_sdk.start_span(op="test_span") as span:
-        updated_headers = _update_celery_task_headers(headers, span, False)
-
-        assert updated_headers["sentry-trace"] == span.to_traceparent()
-        assert updated_headers["headers"]["sentry-trace"] == span.to_traceparent()
-        assert "baggage" not in updated_headers.keys()
-        assert "baggage" not in updated_headers["headers"].keys()
-
-
-def test_custom_span(sentry_init):
-    sentry_init(enable_tracing=True)
-    span = sentry_sdk.tracing.Span()
-    headers = {}
-
-    with sentry_sdk.start_transaction(name="test_transaction"):
-        updated_headers = _update_celery_task_headers(headers, span, False)
-
-        assert updated_headers["sentry-trace"] == span.to_traceparent()
-        assert updated_headers["headers"]["sentry-trace"] == span.to_traceparent()
-        assert "baggage" not in updated_headers.keys()
-        assert "baggage" not in updated_headers["headers"].keys()
-
-
 def test_span_with_transaction_custom_headers(sentry_init):
     sentry_init(enable_tracing=True)
     headers = {
@@ -137,36 +110,3 @@ def test_span_with_transaction_custom_headers(sentry_init):
             assert updated_headers["headers"]["baggage"] == combined_baggage.serialize(
                 include_third_party=True
             )
-
-
-def test_span_with_no_transaction_custom_headers(sentry_init):
-    sentry_init(enable_tracing=True)
-    headers = {
-        "baggage": BAGGAGE_VALUE,
-        "sentry-trace": SENTRY_TRACE_VALUE,
-    }
-
-    with sentry_sdk.start_span(op="test_span") as span:
-        updated_headers = _update_celery_task_headers(headers, span, False)
-
-        assert updated_headers["sentry-trace"] == span.to_traceparent()
-        assert updated_headers["headers"]["sentry-trace"] == span.to_traceparent()
-        assert updated_headers["baggage"] == headers["baggage"]
-        assert updated_headers["headers"]["baggage"] == headers["baggage"]
-
-
-def test_custom_span_custom_headers(sentry_init):
-    sentry_init(enable_tracing=True)
-    span = sentry_sdk.tracing.Span()
-    headers = {
-        "baggage": BAGGAGE_VALUE,
-        "sentry-trace": SENTRY_TRACE_VALUE,
-    }
-
-    with sentry_sdk.start_transaction(name="test_transaction"):
-        updated_headers = _update_celery_task_headers(headers, span, False)
-
-        assert updated_headers["sentry-trace"] == span.to_traceparent()
-        assert updated_headers["headers"]["sentry-trace"] == span.to_traceparent()
-        assert updated_headers["baggage"] == headers["baggage"]
-        assert updated_headers["headers"]["baggage"] == headers["baggage"]

tests/integrations/httpx/test_httpx.py

@@ -5,6 +5,7 @@
 import pytest
 import responses
 
+import sentry_sdk
 from sentry_sdk import capture_message, start_transaction
 from sentry_sdk.consts import MATCH_ALL, SPANDATA
 from sentry_sdk.integrations.httpx import HttpxIntegration
@@ -258,10 +259,11 @@ def test_option_trace_propagation_targets(
         integrations=[HttpxIntegration()],
     )
 
-    if asyncio.iscoroutinefunction(httpx_client.get):
-        asyncio.get_event_loop().run_until_complete(httpx_client.get(url))
-    else:
-        httpx_client.get(url)
+    with sentry_sdk.start_transaction():  # Must be in a transaction to propagate headers
+        if asyncio.iscoroutinefunction(httpx_client.get):
+            asyncio.get_event_loop().run_until_complete(httpx_client.get(url))
+        else:
+            httpx_client.get(url)
 
     request_headers = httpx_mock.get_request().headers
 
@@ -271,6 +273,22 @@ def test_option_trace_propagation_targets(
         assert "sentry-trace" not in request_headers
 
 
+def test_do_not_propagate_outside_transaction(sentry_init, httpx_mock):
+    httpx_mock.add_response()
+
+    sentry_init(
+        traces_sample_rate=1.0,
+        trace_propagation_targets=[MATCH_ALL],
+        integrations=[HttpxIntegration()],
+    )
+
+    httpx_client = httpx.Client()
+    httpx_client.get("http://example.com/")
+
+    request_headers = httpx_mock.get_request().headers
+    assert "sentry-trace" not in request_headers
+
+
 @pytest.mark.tests_internal_exceptions
 def test_omit_url_data_if_parsing_fails(sentry_init, capture_events):
     sentry_init(integrations=[HttpxIntegration()])

tests/tracing/test_propagation.py

@@ -0,0 +1,40 @@
+import sentry_sdk
+import pytest
+
+
+def test_standalone_span_iter_headers(sentry_init):
+    sentry_init(enable_tracing=True)
+
+    with sentry_sdk.start_span(op="test") as span:
+        with pytest.raises(StopIteration):
+            # We should not have any propagation headers
+            next(span.iter_headers())
+
+
+def test_span_in_span_iter_headers(sentry_init):
+    sentry_init(enable_tracing=True)
+
+    with sentry_sdk.start_span(op="test"):
+        with sentry_sdk.start_span(op="test2") as span_inner:
+            with pytest.raises(StopIteration):
+                # We should not have any propagation headers
+                next(span_inner.iter_headers())
+
+
+def test_span_in_transaction(sentry_init):
+    sentry_init(enable_tracing=True)
+
+    with sentry_sdk.start_transaction(op="test"):
+        with sentry_sdk.start_span(op="test2") as span:
+            # Ensure the headers are there
+            next(span.iter_headers())
+
+
+def test_span_in_span_in_transaction(sentry_init):
+    sentry_init(enable_tracing=True)
+
+    with sentry_sdk.start_transaction(op="test"):
+        with sentry_sdk.start_span(op="test2"):
+            with sentry_sdk.start_span(op="test3") as span_inner:
+                # Ensure the headers are there
+                next(span_inner.iter_headers())