chore(trace): add helper to get the top_level span of a service

Author: florentinl

ddtrace/_trace/span.py

@@ -128,6 +128,7 @@ class Span(object):
         "_context",
         "_parent_context",
         "_local_root_value",
+        "_service_entry_span_value",
         "_parent",
         "_ignored_exceptions",
         "_on_finish_callbacks",
@@ -221,6 +222,7 @@ def __init__(
         self._parent: Optional["Span"] = None
         self._ignored_exceptions: Optional[List[Type[Exception]]] = None
         self._local_root_value: Optional["Span"] = None  # None means this is the root span.
+        self._service_entry_span_value: Optional["Span"] = None  # None means this is the service entry span.
         self._store: Optional[Dict[str, Any]] = None
 
     def _update_tags_from_context(self) -> None:
@@ -725,6 +727,18 @@ def _local_root(self, value: "Span") -> None:
     def _local_root(self) -> None:
         del self._local_root_value
 
+    @property
+    def _service_entry_span(self) -> "Span":
+        return self._service_entry_span_value or self
+
+    @_service_entry_span.setter
+    def _service_entry_span(self, span: "Span") -> None:
+        self._service_entry_span_value = None if span is self else span
+
+    @_service_entry_span.deleter
+    def _service_entry_span(self) -> None:
+        del self._service_entry_span_value
+
     def link_span(self, context: Context, attributes: Optional[Dict[str, Any]] = None) -> None:
         """Defines a causal relationship between two spans"""
         if not context.trace_id or not context.span_id:

ddtrace/_trace/tracer.py

@@ -553,6 +553,8 @@ def _start_span(
             if parent:
                 span._parent = parent
                 span._local_root = parent._local_root
+                if span._parent.service == service:
+                    span._service_entry_span = parent._service_entry_span
 
             for k, v in _get_metas_to_propagate(context):
                 # We do not want to propagate AppSec propagation headers