ddtrace/tracer: Ensure SpanLink access is safe from corruption (#2975)

ddtrace/ddtrace.go

@@ -38,11 +38,8 @@ type SpanContextW3C interface {
 type SpanContextWithLinks interface {
 	SpanContext
 
-	// SpanLinks returns the span links on the SpanContext.
+	// SpanLinks returns a copy of the span links on the SpanContext.
 	SpanLinks() []SpanLink
-
-	// Setlinks takes in a slice of SpanLinks and sets them to the SpanContext.
-	SetLinks(links []SpanLink)
 }
 
 // Tracer specifies an implementation of the Datadog tracer which allows starting

ddtrace/tracer/context_test.go

@@ -84,14 +84,12 @@ func TestStartSpanWithSpanLinks(t *testing.T) {
 	_, _, _, stop := startTestTracer(t)
 	defer stop()
 	spanLink := ddtrace.SpanLink{TraceID: 789, TraceIDHigh: 0, SpanID: 789, Attributes: map[string]string{"reason": "terminated_context", "context_headers": "datadog"}, Flags: 0}
-	var ctx ddtrace.SpanContext
-	ctx = &spanContext{spanID: 789, traceID: traceIDFrom64Bits(789), spanLinks: []ddtrace.SpanLink{spanLink}}
+	ctx := &spanContext{spanID: 789, traceID: traceIDFrom64Bits(789), spanLinks: []ddtrace.SpanLink{spanLink}}
 
 	t.Run("spanContext with spanLinks satisfies SpanContextWithLinks interface", func(t *testing.T) {
-		ctxLink, ok := ctx.(ddtrace.SpanContextWithLinks)
-		assert.True(t, ok)
-		assert.Equal(t, len(ctxLink.SpanLinks()), 1)
-		assert.Equal(t, ctxLink.SpanLinks()[0], spanLink)
+		var _ ddtrace.SpanContextWithLinks = ctx
+		assert.Equal(t, len(ctx.SpanLinks()), 1)
+		assert.Equal(t, ctx.SpanLinks()[0], spanLink)
 	})
 
 	t.Run("create span from spancontext with links", func(t *testing.T) {

ddtrace/tracer/spancontext.go

@@ -183,12 +183,11 @@ func (c *spanContext) TraceID128Bytes() [16]byte {
 	return c.traceID
 }
 
+// SpanLinks implements ddtrace.SpanContextWithLinks
 func (c *spanContext) SpanLinks() []ddtrace.SpanLink {
-	return c.spanLinks
-}
-
-func (c *spanContext) SetLinks(links []ddtrace.SpanLink) {
-	c.spanLinks = links
+	cp := make([]ddtrace.SpanLink, len(c.spanLinks))
+	copy(cp, c.spanLinks)
+	return cp
 }
 
 // ForeachBaggageItem implements ddtrace.SpanContext.