removed the lock completely in finishedOne()

ddtrace/tracer/spancontext.go

@@ -288,6 +288,12 @@ func (t *trace) finishedOne(s *span) {
 		t.root.setMetric(keySamplingPriority, *t.priority)
 		t.locked = true
 	}
+	if len(t.spans) > 0 && s == t.spans[0] {
+		// first span in chunk finished, lock down the tags
+		for k, v := range t.tags {
+			s.setMeta(k, v)
+		}
+	}
 	if len(t.spans) != t.finished {
 		return
 	}
@@ -309,24 +315,5 @@ func (t *trace) finishedOne(s *span) {
 		}
 		return
 	}
-	t.populateTraceTags(s)
 	tr.pushTrace(t.spans)
 }
-
-// populateTraceTags puts trace tags in Meta of the first span.
-func (t *trace) populateTraceTags(s *span) {
-	if len(t.tags) == 0 {
-		return
-	}
-	if t.spans[0] != s {
-		t.spans[0].Lock()
-		defer t.spans[0].Unlock()
-	}
-	if t.spans[0].Meta == nil {
-		t.spans[0].Meta = t.tags
-		return
-	}
-	for k, v := range t.tags {
-		t.spans[0].Meta[k] = v
-	}
-}

ddtrace/tracer/spancontext_test.go

@@ -75,6 +75,21 @@ func TestAsyncSpanRace(t *testing.T) {
 				}
 			}()
 			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				select {
+				case <-done:
+					root.Finish()
+					for i := 0; i < 500; i++ {
+						for range root.(*span).Meta {
+							// this range simulates iterating over the meta map
+							// as we do when encoding msgpack upon flushing.
+						}
+					}
+					return
+				}
+			}()
+			wg.Add(1)
 			go func() {
 				defer wg.Done()
 				select {