cmd/trace: change span id computation for trace view use

golang.org/cl/102697 attempted to fix the span presentation by utilizing
the position of the span in the span slices of a task. But it is
not complete either.

First, id=0 is omitted in json encoding and the trace viewer silently
drops entries with the missing id field, so we must avoid zero-value id.
Second, it is possible that a goroutine handles multiple tasks. Then,
id collisions will happen.

This takes a simpler approach - have a counter that increments for every
emitSpan call, and use the value as the id value.

Change-Id: Idaa9505634acf6d327c6f00af32d8260955b85e1
Reviewed-on: https://go-review.googlesource.com/106755Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/trace/trace.go

@@ -404,6 +404,8 @@ type traceContext struct {
 	heapStats, prevHeapStats     heapStats
 	threadStats, prevThreadStats threadStats
 	gstates, prevGstates         [gStateCount]int64
+
+	spanID int // last emitted span id. incremented in each emitSpan call.
 }
 
 type heapStats struct {
@@ -758,8 +760,8 @@ func generateTrace(params *traceParams, consumer traceConsumer) error {
 			// If we are in goroutine-oriented mode, we draw spans.
 			// TODO(hyangah): add this for task/P-oriented mode (i.e., focustask view) too.
 			if ctx.mode&modeGoroutineOriented != 0 {
-				for i, s := range task.spans {
-					ctx.emitSpan(s, i)
+				for _, s := range task.spans {
+					ctx.emitSpan(s)
 				}
 			}
 		}
@@ -857,10 +859,13 @@ func (ctx *traceContext) emitSlice(ev *trace.Event, name string) *ViewerEvent {
 	return sl
 }
 
-func (ctx *traceContext) emitSpan(s spanDesc, spanID int) {
+func (ctx *traceContext) emitSpan(s spanDesc) {
 	if s.Name == "" {
 		return
 	}
+	ctx.spanID++
+	spanID := ctx.spanID
+
 	id := s.TaskID
 	scopeID := fmt.Sprintf("%x", id)