cmd/go: add trace events for each action

This change adds a trace event for each action and also
annotates each of the action execution goroutines with trace.Goroutine
so that the actions eaxecuted by each goroutine appear on different threads in
the chrome trace viewer.

Updates #38714

Change-Id: I2e58dc5606b2e3f7f87076a61e1cc6a2014255c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/248320
Run-TryBot: Michael Matloob <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

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

@@ -45,10 +45,10 @@ func StartSpan(ctx context.Context, name string) (context.Context, *Span) {
 	return ctx, childSpan
 }
 
-// Goroutine associates the context with a new Thread ID. The Chrome trace viewer associates each
+// StartGoroutine 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 {
+func StartGoroutine(ctx context.Context) context.Context {
 	tc, ok := getTraceContext(ctx)
 	if !ok {
 		return ctx

src/cmd/go/internal/work/exec.go

@@ -115,13 +115,21 @@ func (b *Builder) Do(ctx context.Context, root *Action) {
 
 	// Handle runs a single action and takes care of triggering
 	// any actions that are runnable as a result.
-	handle := func(a *Action) {
+	handle := func(ctx context.Context, a *Action) {
 		if a.json != nil {
 			a.json.TimeStart = time.Now()
 		}
 		var err error
 		if a.Func != nil && (!a.Failed || a.IgnoreFail) {
+			// TODO(matloob): Better action descriptions
+			desc := "Executing action "
+			if a.Package != nil {
+				desc += "(" + a.Mode + " " + a.Package.Desc() + ")"
+			}
+			ctx, span := trace.StartSpan(ctx, desc)
+			_ = ctx
 			err = a.Func(b, a)
+			span.Done()
 		}
 		if a.json != nil {
 			a.json.TimeDone = time.Now()
@@ -169,6 +177,7 @@ func (b *Builder) Do(ctx context.Context, root *Action) {
 	for i := 0; i < par; i++ {
 		wg.Add(1)
 		go func() {
+			ctx := trace.StartGoroutine(ctx)
 			defer wg.Done()
 			for {
 				select {
@@ -181,7 +190,7 @@ func (b *Builder) Do(ctx context.Context, root *Action) {
 					b.exec.Lock()
 					a := b.ready.pop()
 					b.exec.Unlock()
-					handle(a)
+					handle(ctx, a)
 				case <-base.Interrupted:
 					base.SetExitStatus(1)
 					return