From f4cfe820a3bfbe633efd2b0964be0d975afeeebd Mon Sep 17 00:00:00 2001
From: Anton Manakin <anton.manakin@gmail.com>
Date: Wed, 8 May 2024 06:57:08 +0300
Subject: [PATCH] record links with empty span context

---
 CHANGELOG.md            |  1 +
 sdk/trace/span.go       |  6 ++-
 sdk/trace/trace_test.go | 88 ++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 93 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0db3cf6ce6f..98915b58e26 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,6 +24,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - De-duplicate map attributes added to a `Record` in `go.opentelemetry.io/otel/sdk/log`. (#5230)
 - The `go.opentelemetry.io/otel/exporters/stdout/stdoutlog` exporter won't print `AttributeValueLengthLimit` and `AttributeCountLimit` fields now, instead it prints the `DroppedAttributes` field. (#5272)
 - Improved performance in the `Stringer` implementation of `go.opentelemetry.io/otel/baggage.Member` by reducing the number of allocations. (#5286)
+- Span in `go.opentelemetry.io/otel/sdk/trace` will record links without span context if either non-empty `TraceState` or attributes are provided. (#5315)
 
 ## [1.26.0/0.48.0/0.2.0-alpha] 2024-04-24
 
diff --git a/sdk/trace/span.go b/sdk/trace/span.go
index c44f6b926aa..7acfd3fe9f4 100644
--- a/sdk/trace/span.go
+++ b/sdk/trace/span.go
@@ -630,7 +630,11 @@ func (s *recordingSpan) Resource() *resource.Resource {
 }
 
 func (s *recordingSpan) AddLink(link trace.Link) {
-	if !s.IsRecording() || !link.SpanContext.IsValid() {
+	if !s.IsRecording() {
+		return
+	}
+	if !link.SpanContext.IsValid() && len(link.Attributes) == 0 &&
+		link.SpanContext.TraceState().Len() == 0 {
 		return
 	}
 
diff --git a/sdk/trace/trace_test.go b/sdk/trace/trace_test.go
index 2ec6cbb57a6..471c1731fad 100644
--- a/sdk/trace/trace_test.go
+++ b/sdk/trace/trace_test.go
@@ -1923,6 +1923,9 @@ func TestEmptyRecordingSpanDroppedAttributes(t *testing.T) {
 }
 
 func TestSpanAddLink(t *testing.T) {
+	ts, err := trace.ParseTraceState("k=v")
+	require.NoError(t, err)
+
 	tests := []struct {
 		name               string
 		attrLinkCountLimit int
@@ -1934,7 +1937,6 @@ func TestSpanAddLink(t *testing.T) {
 			attrLinkCountLimit: 128,
 			link: trace.Link{
 				SpanContext: trace.NewSpanContext(trace.SpanContextConfig{TraceID: trace.TraceID([16]byte{}), SpanID: [8]byte{}}),
-				Attributes:  []attribute.KeyValue{{Key: "k1", Value: attribute.StringValue("v1")}},
 			},
 			want: &snapshot{
 				name: "span0",
@@ -2002,6 +2004,50 @@ func TestSpanAddLink(t *testing.T) {
 				instrumentationScope: instrumentation.Scope{Name: "AddLinkWithMoreAttributesThanLimit"},
 			},
 		},
+		{
+			name:               "AddLinkWithAttributesEmptySpanContext",
+			attrLinkCountLimit: 128,
+			link: trace.Link{
+				Attributes: []attribute.KeyValue{{Key: "k1", Value: attribute.StringValue("v1")}},
+			},
+			want: &snapshot{
+				name: "span0",
+				spanContext: trace.NewSpanContext(trace.SpanContextConfig{
+					TraceID:    tid,
+					TraceFlags: 0x1,
+				}),
+				parent: sc.WithRemote(true),
+				links: []Link{
+					{
+						Attributes: []attribute.KeyValue{{Key: "k1", Value: attribute.StringValue("v1")}},
+					},
+				},
+				spanKind:             trace.SpanKindInternal,
+				instrumentationScope: instrumentation.Scope{Name: "AddLinkWithAttributesEmptySpanContext"},
+			},
+		},
+		{
+			name:               "AddLinkWithTraceStateEmptySpanContext",
+			attrLinkCountLimit: 128,
+			link: trace.Link{
+				SpanContext: trace.SpanContext{}.WithTraceState(ts),
+			},
+			want: &snapshot{
+				name: "span0",
+				spanContext: trace.NewSpanContext(trace.SpanContextConfig{
+					TraceID:    tid,
+					TraceFlags: 0x1,
+				}),
+				parent: sc.WithRemote(true),
+				links: []Link{
+					{
+						SpanContext: trace.SpanContext{}.WithTraceState(ts),
+					},
+				},
+				spanKind:             trace.SpanKindInternal,
+				instrumentationScope: instrumentation.Scope{Name: "AddLinkWithTraceStateEmptySpanContext"},
+			},
+		},
 	}
 
 	for _, tc := range tests {
@@ -2026,3 +2072,43 @@ func TestSpanAddLink(t *testing.T) {
 		})
 	}
 }
+
+func TestAddLinkToNonRecordingSpan(t *testing.T) {
+	te := NewTestExporter()
+	sl := NewSpanLimits()
+	tp := NewTracerProvider(
+		WithSpanLimits(sl),
+		WithSyncer(te),
+		WithResource(resource.Empty()),
+	)
+
+	attrs := []attribute.KeyValue{{Key: "k", Value: attribute.StringValue("v")}}
+
+	span := startSpan(tp, "AddLinkToNonRecordingSpan")
+	_, err := endSpan(te, span)
+	require.NoError(t, err)
+
+	// Add link after ending span
+	span.AddLink(trace.Link{
+		SpanContext: sc,
+		Attributes:  attrs,
+	})
+
+	require.Equal(t, 1, te.Len())
+	got := te.Spans()[0]
+	want := &snapshot{
+		name: "span0",
+		spanContext: trace.NewSpanContext(trace.SpanContextConfig{
+			TraceID:    tid,
+			TraceFlags: 0x1,
+		}),
+		parent:               sc.WithRemote(true),
+		links:                nil,
+		spanKind:             trace.SpanKindInternal,
+		instrumentationScope: instrumentation.Scope{Name: "AddLinkToNonRecordingSpan"},
+	}
+
+	if diff := cmpDiff(got, want); diff != "" {
+		t.Errorf("AddLinkToNonRecordingSpan: -got +want %s", diff)
+	}
+}