ddtrace/tracer: don't set empty tracestate propagation tag (#1910)

ddtrace/tracer/textmap.go

@@ -896,6 +896,11 @@ func parseTraceparent(ctx *spanContext, header string) error {
 // `origin` = `o`
 // `_dd.p.` prefix = `t.`
 func parseTracestate(ctx *spanContext, header string) error {
+	if header == "" {
+		// The W3C spec says tracestate can be empty but should avoid sending it.
+		// https://www.w3.org/TR/trace-context-1/#tracestate-header-field-values
+		return nil
+	}
 	// if multiple headers are present, they must be combined and stored
 	setPropagatingTag(ctx, tracestateHeader, header)
 	list := strings.Split(strings.Trim(header, "\t "), ",")

ddtrace/tracer/textmap_test.go

@@ -899,6 +899,16 @@ func TestEnvVars(t *testing.T) {
 				origin          string
 				propagatingTags map[string]string
 			}{
+				{
+					in: TextMapCarrier{
+						traceparentHeader: "00-00000000000000001111111111111111-2222222222222222-01",
+						// no tracestate header, shouldn't put an empty tracestate in propagatingTags
+					},
+					tid:             traceIDFrom64Bits(1229782938247303441),
+					out:             []uint64{2459565876494606882, 1},
+					origin:          "",
+					propagatingTags: *(new(map[string]string)),
+				},
 				{
 					in: TextMapCarrier{
 						traceparentHeader: "00-00000000000000001111111111111111-2222222222222222-01",