Record links with invalid SpanContext (#3917)

CHANGELOG.md

@@ -38,6 +38,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#3823] (https://github.com/open-telemetry/opentelemetry-python/pull/3823))
 - Add span flags to OTLP spans and links
   ([#3881](https://github.com/open-telemetry/opentelemetry-python/pull/3881))
+- Record links with invalid SpanContext if either attributes or TraceState are not empty
+  ([#3917](https://github.com/open-telemetry/opentelemetry-python/pull/3917/))
 - Add OpenTelemetry trove classifiers to PyPI packages
   ([#3913] (https://github.com/open-telemetry/opentelemetry-python/pull/3913))
 

opentelemetry-api/src/opentelemetry/trace/span.py

@@ -127,7 +127,8 @@ def add_link(  # pylint: disable=no-self-use
 
         Adds a single `Link` with the `SpanContext` of the span to link to and,
         optionally, attributes passed as arguments. Implementations may ignore
-        calls with an invalid span context.
+        calls with an invalid span context if both attributes and TraceState
+        are empty.
 
         Note: It is preferred to add links at span creation, instead of calling
         this method later since samplers can only consider information already

opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py

@@ -347,6 +347,12 @@ def wrapper(self, *args, **kwargs):
     return wrapper
 
 
+def _is_valid_link(context: SpanContext, attributes: types.Attributes) -> bool:
+    return bool(
+        context and (context.is_valid or (attributes or context.trace_state))
+    )
+
+
 class ReadableSpan:
     """Provides read-only access to span attributes.
 
@@ -867,7 +873,8 @@ def add_link(
         context: SpanContext,
         attributes: types.Attributes = None,
     ) -> None:
-        if context is None or not context.is_valid:
+
+        if not _is_valid_link(context, attributes):
             return
 
         attributes = BoundedAttributes(

opentelemetry-sdk/tests/trace/test_trace.py

@@ -941,9 +941,30 @@ def test_add_link_with_invalid_span_context(self):
 
         with self.tracer.start_as_current_span("root") as root:
             root.add_link(other_context)
-
+            root.add_link(None)
             self.assertEqual(len(root.links), 0)
 
+    def test_add_link_with_invalid_span_context_with_attributes(self):
+        invalid_context = trace_api.INVALID_SPAN_CONTEXT
+
+        with self.tracer.start_as_current_span("root") as root:
+            root.add_link(invalid_context, {"name": "neighbor"})
+            self.assertEqual(len(root.links), 1)
+            self.assertEqual(root.links[0].attributes, {"name": "neighbor"})
+
+    def test_add_link_with_invalid_span_context_with_tracestate(self):
+        invalid_context = trace.SpanContext(
+            trace_api.INVALID_TRACE_ID,
+            trace_api.INVALID_SPAN_ID,
+            is_remote=False,
+            trace_state="foo=bar",
+        )
+
+        with self.tracer.start_as_current_span("root") as root:
+            root.add_link(invalid_context)
+            self.assertEqual(len(root.links), 1)
+            self.assertEqual(root.links[0].context.trace_state, "foo=bar")
+
     def test_update_name(self):
         with self.tracer.start_as_current_span("root") as root:
             # name