fix(tracing): Make unsampled transactions findable on the scope (getsentry#872)

Author: lobsterkatie

sentry_sdk/scope.py

@@ -144,14 +144,26 @@ def fingerprint(self, value):
     def transaction(self):
         # type: () -> Any
         # would be type: () -> Optional[Transaction], see https://github.com/python/mypy/issues/3004
-        """Return the transaction (root span) in the scope."""
-        if self._span is None or self._span._span_recorder is None:
-            return None
-        try:
-            return self._span._span_recorder.spans[0]
-        except (AttributeError, IndexError):
+        """Return the transaction (root span) in the scope, if any."""
+
+        # there is no span/transaction on the scope
+        if self._span is None:
             return None
 
+        # the span on the scope is itself a transaction
+        if isinstance(self._span, Transaction):
+            return self._span
+
+        # the span on the scope isn't a transaction but belongs to one
+        if self._span._containing_transaction:
+            return self._span._containing_transaction
+
+        # there's a span (not a transaction) on the scope, but it was started on
+        # its own, not as the descendant of a transaction (this is deprecated
+        # behavior, but as long as the start_span function exists, it can still
+        # happen)
+        return None
+
     @transaction.setter
     def transaction(self, value):
         # type: (Any) -> None

tests/tracing/test_sampling.py

@@ -1,6 +1,6 @@
 import pytest
 
-from sentry_sdk import start_span, start_transaction
+from sentry_sdk import Hub, start_span, start_transaction
 from sentry_sdk.tracing import _is_valid_sample_rate
 from sentry_sdk.utils import logger
 
@@ -73,3 +73,17 @@ def test_warns_on_invalid_sample_rate(rate, StringContaining):  # noqa: N803
         result = _is_valid_sample_rate(rate)
         logger.warning.assert_any_call(StringContaining("Given sample rate is invalid"))
         assert result is False
+
+
+@pytest.mark.parametrize("sampling_decision", [True, False])
+def test_get_transaction_and_span_from_scope_regardless_of_sampling_decision(
+    sentry_init, sampling_decision
+):
+    sentry_init(traces_sample_rate=1.0)
+
+    with start_transaction(name="/", sampled=sampling_decision):
+        with start_span(op="child-span"):
+            with start_span(op="child-child-span"):
+                scope = Hub.current.scope
+                assert scope.span.op == "child-child-span"
+                assert scope.transaction.name == "/"