Repository: getsentry/sentry-python

Issue:
Dynamic sampling context might be missing from Celery Beat transactions
### How do you use Sentry?

Sentry Saas (sentry.io)

### Version

Latest 2.x

### Steps to Reproduce

Update: [See here](https://github.com/szokeasaurusrex/issue-reproductions/tree/main/getsentry/sentry-python/3068) for a simple demonstration of the broken behavior.

Under investigation – if the dynamic sampling context is missing from transactions started in Celery Beat, this would explain strange behavior we observe in Sentry where the parent (Celery Beat) transaction is missing while the child transactions are all present

### Expected Result

Celery Beat transactions contain a dynamic sampling context

### Actual Result

In some cases, the dynamic sampling context might be missing


Discussion:

Resolving patch:
diff --git a/sentry_sdk/tracing.py b/sentry_sdk/tracing.py
--- a/sentry_sdk/tracing.py
+++ b/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(


Test patch:
diff --git a/tests/integrations/aiohttp/test_aiohttp.py b/tests/integrations/aiohttp/test_aiohttp.py
--- a/tests/integrations/aiohttp/test_aiohttp.py
+++ b/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"]
diff --git a/tests/integrations/celery/test_update_celery_task_headers.py b/tests/integrations/celery/test_update_celery_task_headers.py
--- a/tests/integrations/celery/test_update_celery_task_headers.py
+++ b/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"]
diff --git a/tests/integrations/httpx/test_httpx.py b/tests/integrations/httpx/test_httpx.py
--- a/tests/integrations/httpx/test_httpx.py
+++ b/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()])
diff --git a/tests/tracing/test_propagation.py b/tests/tracing/test_propagation.py
new file mode 100644
--- /dev/null
+++ b/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())