cmd/go/internal/trace: add function to distinguish goroutines

trace.StartGoroutine will associate the trace information on the context
with a new chrome profiler thread id. The chrome profiler doesn't
expect multiple trace events to have the same thread id, so this
will allow us to display concurrent events on the trace.

Updates #38714

Change-Id: I888b0cce15a5a01db66366716fdd85bf86c832cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/248319
Run-TryBot: Michael Matloob <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: matloob

src/cmd/go/internal/trace/trace.go

@@ -34,20 +34,33 @@ func StartSpan(ctx context.Context, name string) (context.Context, *Span) {
 	if !ok {
 		return ctx, nil
 	}
-	childSpan := &Span{t: tc.t, name: name, start: time.Now()}
+	childSpan := &Span{t: tc.t, name: name, tid: tc.tid, start: time.Now()}
 	tc.t.writeEvent(&traceviewer.Event{
 		Name:  childSpan.name,
 		Time:  float64(childSpan.start.UnixNano()) / float64(time.Microsecond),
+		TID:   childSpan.tid,
 		Phase: "B",
 	})
-	ctx = context.WithValue(ctx, traceKey{}, traceContext{tc.t})
+	ctx = context.WithValue(ctx, traceKey{}, traceContext{tc.t, tc.tid})
 	return ctx, childSpan
 }
 
+// Goroutine associates the context with a new Thread ID. The Chrome trace viewer associates each
+// trace event with a thread, and doesn't expect events with the same thread id to happen at the
+// same time.
+func Goroutine(ctx context.Context) context.Context {
+	tc, ok := getTraceContext(ctx)
+	if !ok {
+		return ctx
+	}
+	return context.WithValue(ctx, traceKey{}, traceContext{tc.t, tc.t.getNextTID()})
+}
+
 type Span struct {
 	t *tracer
 
 	name  string
+	tid   uint64
 	start time.Time
 	end   time.Time
 }
@@ -60,12 +73,15 @@ func (s *Span) Done() {
 	s.t.writeEvent(&traceviewer.Event{
 		Name:  s.name,
 		Time:  float64(s.end.UnixNano()) / float64(time.Microsecond),
+		TID:   s.tid,
 		Phase: "E",
 	})
 }
 
 type tracer struct {
 	file chan traceFile // 1-buffered
+
+	nextTID uint64
 }
 
 func (t *tracer) writeEvent(ev *traceviewer.Event) error {
@@ -103,12 +119,17 @@ func (t *tracer) Close() error {
 	return firstErr
 }
 
+func (t *tracer) getNextTID() uint64 {
+	return atomic.AddUint64(&t.nextTID, 1)
+}
+
 // traceKey is the context key for tracing information. It is unexported to prevent collisions with context keys defined in
 // other packages.
 type traceKey struct{}
 
 type traceContext struct {
-	t *tracer
+	t   *tracer
+	tid uint64
 }
 
 // Start starts a trace which writes to the given file.