[receiver/skywalking]: fix parentSpanId lost when crossing segments (open-telemetry#21799)

In Skywalking, parentSpanId == -1 indicates that the span is [the first
span within the current
segment](https://github.com/apache/skywalking-data-collect-protocol/blob/0da9c8b3e111fb51c9f8854cae16d4519462ecfe/language-agent/Tracing.proto#L119),
rather than within the entire trace. Leaving parentSpanId blank can
cause the call relationship to be lost when crossing segments.

Author: linfn

.chloggen/fix-skywalking-receiver-parent-span-id.yaml

@@ -0,0 +1,21 @@
+# Use this changelog template to create an entry for release notes.
+# If your change doesn't affect end users, such as a test fix or a tooling change,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: receiver/skywalking
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix the issue where `ParentSpanId` is lost when crossing segments.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [21799]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+

receiver/skywalkingreceiver/internal/trace/skywalkingproto_to_traces.go

@@ -106,9 +106,13 @@ func swSpanToSpan(traceID string, segmentID string, span *agentV3.SpanObject, de
 	// so use segmentId to convert to an unique otel-span
 	dest.SetSpanID(segmentIDToSpanID(segmentID, uint32(span.GetSpanId())))
 
-	// parent spanid = -1, means(root span) no parent span in skywalking,so just make otlp's parent span id empty.
+	// parent spanid = -1, means(root span) no parent span in current skywalking segment, so it is necessary to search for the parent segment.
 	if span.ParentSpanId != -1 {
 		dest.SetParentSpanID(segmentIDToSpanID(segmentID, uint32(span.GetParentSpanId())))
+	} else if len(span.Refs) == 1 {
+		// TODO: SegmentReference references usually have only one element, but in batch consumer case, such as in MQ or async batch process, it could be multiple.
+		// We only handle one element for now.
+		dest.SetParentSpanID(segmentIDToSpanID(span.Refs[0].GetParentTraceSegmentId(), uint32(span.Refs[0].GetParentSpanId())))
 	}
 
 	dest.SetName(span.OperationName)

receiver/skywalkingreceiver/internal/trace/skywalkingproto_to_traces_test.go

@@ -301,6 +301,41 @@ func Test_segmentIdToSpanId_Unique(t *testing.T) {
 	assert.NotEqual(t, results[0], results[1])
 }
 
+func Test_swSpanToSpan_ParentSpanId(t *testing.T) {
+	type args struct {
+		span *agentV3.SpanObject
+	}
+	tests := []struct {
+		name string
+		args args
+		want pcommon.SpanID
+	}{
+		{
+			name: "mock-sw-span-with-parent-segment",
+			args: args{span: &agentV3.SpanObject{
+				ParentSpanId: -1,
+				Refs: []*agentV3.SegmentReference{{
+					ParentTraceSegmentId: "4f2f27748b8e44ecaf18fe0347194e86.33.16560607369950066",
+					ParentSpanId:         123,
+				}}}},
+			want: [8]byte{233, 196, 85, 168, 37, 66, 48, 106},
+		},
+		{
+			name: "mock-sw-span-without-parent-segment",
+			args: args{span: &agentV3.SpanObject{Refs: []*agentV3.SegmentReference{{
+				ParentSpanId: -1,
+			}}}},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			dest := ptrace.NewSpan()
+			swSpanToSpan("de5980b8-fce3-4a37-aab9-b4ac3af7eedd", "", tt.args.span, dest)
+			assert.Equal(t, tt.want, dest.ParentSpanID())
+		})
+	}
+}
+
 func generateTracesOneEmptyResourceSpans() ptrace.Span {
 	td := ptrace.NewTraces()
 	resourceSpan := td.ResourceSpans().AppendEmpty()