internal/trace/v2: redefine NoTask and add BackgroundTask

The v2 trace parser currently handles task inheritance and region task
association incorrectly. It assumes that a TaskID of 0 means that there
is no task. However, this is only true for task events. A TaskID of 0
means that a region gets assigned to the "background task." The parser
currently has no concept of a "background task."

Fix this by defining the background task as task ID 0 and redefining
NoTask to ^uint64(0). This aligns the TaskID values more closely with
other IDs in the parser and also enables disambiguating these two cases.

For #60773.
For #63960.

Change-Id: I09c8217b33b87c8f8f8ea3b0203ed83fd3b61e11
Reviewed-on: https://go-review.googlesource.com/c/go/+/543019
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>

src/internal/trace/goroutinesv2.go

@@ -299,9 +299,7 @@ func (s *GoroutineSummarizer) Event(ev *tracev2.Event) {
 				if creatorG := s.gs[ev.Goroutine()]; creatorG != nil && len(creatorG.activeRegions) > 0 {
 					regions := creatorG.activeRegions
 					s := regions[len(regions)-1]
-					if s.TaskID != tracev2.NoTask {
-						g.activeRegions = []*UserRegionSummary{{TaskID: s.TaskID, Start: ev}}
-					}
+					g.activeRegions = []*UserRegionSummary{{TaskID: s.TaskID, Start: ev}}
 				}
 				s.gs[g.ID] = g
 			case tracev2.GoRunning:

src/internal/trace/v2/event.go

@@ -203,8 +203,14 @@ type RangeAttribute struct {
 // are of the same type).
 type TaskID uint64
 
-// NoTask indicates the lack of a task.
-const NoTask = TaskID(0)
+const (
+	// NoTask indicates the lack of a task.
+	NoTask = TaskID(^uint64(0))
+
+	// BackgroundTask is the global task that events are attached to if there was
+	// no other task in the context at the point the event was emitted.
+	BackgroundTask = TaskID(0)
+)
 
 // Task provides details about a Task event.
 type Task struct {

src/internal/trace/v2/order.go

@@ -525,6 +525,13 @@ func (o *ordering) advance(ev *baseEvent, evt *evTable, m ThreadID, gen uint64)
 		// Get the parent ID, but don't validate it. There's no guarantee
 		// we actually have information on whether it's active.
 		parentID := TaskID(ev.args[1])
+		if parentID == BackgroundTask {
+			// Note: a value of 0 here actually means no parent, *not* the
+			// background task. Automatic background task attachment only
+			// applies to regions.
+			parentID = NoTask
+			ev.args[1] = uint64(NoTask)
+		}
 
 		// Validate the name and record it. We'll need to pass it through to
 		// EvUserTaskEnd.

src/internal/trace/v2/testdata/generators/go122-task-across-generations.go

@@ -29,7 +29,7 @@ func gen(t *testgen.Trace) {
 	b1 := g1.Batch(trace.ThreadID(0), 0)
 	b1.Event("ProcStatus", trace.ProcID(0), go122.ProcRunning)
 	b1.Event("GoStatus", trace.GoID(1), trace.ThreadID(0), go122.GoRunning)
-	b1.Event("UserTaskBegin", trace.TaskID(2), trace.NoTask, "my task", testgen.NoStack)
+	b1.Event("UserTaskBegin", trace.TaskID(2), trace.TaskID(0) /* 0 means no parent, not background */, "my task", testgen.NoStack)
 
 	g2 := t.Generation(2)
 

src/internal/trace/v2/testtrace/validation.go

@@ -256,9 +256,14 @@ func (v *Validator) Event(ev trace.Event) error {
 	case trace.EventTaskBegin:
 		// Validate task begin.
 		t := ev.Task()
-		if t.ID == trace.NoTask {
+		if t.ID == trace.NoTask || t.ID == trace.BackgroundTask {
+			// The background task should never have an event emitted for it.
 			e.Errorf("found invalid task ID for task of type %s", t.Type)
 		}
+		if t.Parent == trace.BackgroundTask {
+			// It's not possible for a task to be a subtask of the background task.
+			e.Errorf("found background task as the parent for task of type %s", t.Type)
+		}
 		// N.B. Don't check the task type. Empty string is a valid task type.
 		v.tasks[t.ID] = t.Type
 	case trace.EventTaskEnd:

src/internal/trace/v2/trace_test.go

@@ -35,7 +35,7 @@ func TestTraceAnnotations(t *testing.T) {
 			{trace.EventRegionEnd, trace.TaskID(1), []string{"region0"}},
 			{trace.EventTaskEnd, trace.TaskID(1), []string{"task0"}},
 			//  Currently, pre-existing region is not recorded to avoid allocations.
-			{trace.EventRegionBegin, trace.NoTask, []string{"post-existing region"}},
+			{trace.EventRegionBegin, trace.BackgroundTask, []string{"post-existing region"}},
 		}
 		r, err := trace.NewReader(bytes.NewReader(tb))
 		if err != nil {