internal/trace: Revert "remove backlinks from span/task end to start"

This reverts commit 16398894dccf06e34af8fb1c7cff6eed9b163057.
This broke TestUserTaskSpan test.

Change-Id: If5ff8bdfe84e8cb30787b03ead87205ece3d5601
Reviewed-on: https://go-review.googlesource.com/98235
Reviewed-by: Heschi Kreinick <heschi@google.com>

2 files changed, 38 insertions, 76 deletions

diff --git a/src/cmd/trace/annotations.go b/src/cmd/trace/annotations.go
index c7c0b637c7..91cdd4d198 100644
--- a/src/cmd/trace/annotations.go
+++ b/src/cmd/trace/annotations.go
@@ -152,65 +152,6 @@ type annotationAnalysisResult struct {
 	gcEvents []*trace.Event       // GCStartevents, sorted
 }
 
-type activeSpanTracker struct {
-	stacks map[uint64][]*trace.Event // goid to stack of active span start events
-}
-
-func (t *activeSpanTracker) top(goid uint64) *trace.Event {
-	if t.stacks == nil {
-		return nil
-	}
-	stk := t.stacks[goid]
-	if len(stk) == 0 {
-		return nil
-	}
-	return stk[len(stk)-1]
-}
-
-func (t *activeSpanTracker) addSpanEvent(ev *trace.Event, task *taskDesc) *spanDesc {
-	if ev.Type != trace.EvUserSpan {
-		return nil
-	}
-	if t.stacks == nil {
-		t.stacks = make(map[uint64][]*trace.Event)
-	}
-
-	goid := ev.G
-	stk := t.stacks[goid]
-
-	var sd *spanDesc
-	switch mode := ev.Args[1]; mode {
-	case 0: // span start
-		t.stacks[goid] = append(stk, ev) // push
-		sd = &spanDesc{
-			name:  ev.SArgs[0],
-			task:  task,
-			goid:  goid,
-			start: ev,
-			end:   ev.Link,
-		}
-	case 1: // span end
-		if n := len(stk); n > 0 {
-			stk = stk[:n-1] // pop
-		} else {
-			// There is no matching span start event; can happen if the span start was before tracing.
-			sd = &spanDesc{
-				name:  ev.SArgs[0],
-				task:  task,
-				goid:  goid,
-				start: nil,
-				end:   ev,
-			}
-		}
-		if len(stk) == 0 {
-			delete(t.stacks, goid)
-		} else {
-			t.stacks[goid] = stk
-		}
-	}
-	return sd
-}
-
 // analyzeAnnotations analyzes user annotation events and
 // returns the task descriptors keyed by internal task id.
 func analyzeAnnotations() (annotationAnalysisResult, error) {
@@ -225,14 +166,14 @@ func analyzeAnnotations() (annotationAnalysisResult, error) {
 	}
 
 	tasks := allTasks{}
+	activeSpans := map[uint64][]*trace.Event{} // goid to active span start events
 	var gcEvents []*trace.Event
-	var activeSpans activeSpanTracker
 
 	for _, ev := range events {
 		goid := ev.G
 
 		switch typ := ev.Type; typ {
-		case trace.EvUserTaskCreate, trace.EvUserTaskEnd, trace.EvUserLog:
+		case trace.EvUserTaskCreate, trace.EvUserTaskEnd, trace.EvUserLog, trace.EvUserSpan:
 			taskid := ev.Args[0]
 			task := tasks.task(taskid)
 			task.addEvent(ev)
@@ -248,13 +189,18 @@ func analyzeAnnotations() (annotationAnalysisResult, error) {
 				}
 			}
 
-		case trace.EvUserSpan:
-			taskid := ev.Args[0]
-			task := tasks.task(taskid)
-			task.addEvent(ev)
-			sd := activeSpans.addSpanEvent(ev, task)
-			if task != nil && sd != nil {
-				task.spans = append(task.spans, sd)
+			if typ == trace.EvUserSpan {
+				mode := ev.Args[1]
+				spans := activeSpans[goid]
+				if mode == 0 { // start
+					activeSpans[goid] = append(spans, ev) // push
+				} else { // end
+					if n := len(spans); n > 1 {
+						activeSpans[goid] = spans[:n-1] // pop
+					} else if n == 1 {
+						delete(activeSpans, goid)
+					}
+				}
 			}
 
 		case trace.EvGoCreate:
@@ -263,11 +209,11 @@ func analyzeAnnotations() (annotationAnalysisResult, error) {
 			//
 			// TODO(hyangah): the task info needs to propagate
 			// to all decendents, not only to the immediate child.
-			s := activeSpans.top(goid)
-			if s == nil {
+			spans := activeSpans[goid]
+			if len(spans) == 0 {
 				continue
 			}
-			taskid := s.Args[0]
+			taskid := spans[len(spans)-1].Args[0]
 			task := tasks.task(taskid)
 			task.addEvent(ev)
 
@@ -380,6 +326,21 @@ func (task *taskDesc) addEvent(ev *trace.Event) {
 		task.create = ev
 	case trace.EvUserTaskEnd:
 		task.end = ev
+	case trace.EvUserSpan:
+		if mode := ev.Args[1]; mode == 0 { // start
+			task.spans = append(task.spans, &spanDesc{
+				name:  ev.SArgs[0],
+				task:  task,
+				goid:  ev.G,
+				start: ev,
+				end:   ev.Link})
+		} else if ev.Link == nil { // span end without matching start
+			task.spans = append(task.spans, &spanDesc{
+				name: ev.SArgs[0],
+				task: task,
+				goid: ev.G,
+				end:  ev})
+		}
 	}
 }
 
diff --git a/src/internal/trace/parser.go b/src/internal/trace/parser.go
index 29ba73c761..155c23940a 100644
--- a/src/internal/trace/parser.go
+++ b/src/internal/trace/parser.go
@@ -56,7 +56,8 @@ type Event struct {
 	// for GoSysExit: the next GoStart
 	// for GCMarkAssistStart: the associated GCMarkAssistDone
 	// for UserTaskCreate: the UserTaskEnd
-	// for UserSpan: if the start span, the corresponding UserSpan end event
+	// for UsetTaskEnd: the UserTaskCreate
+	// for UserSpan: the corresponding span start or end event
 	Link *Event
 }
 
@@ -809,10 +810,9 @@ func postProcessTrace(ver int, events []*Event) error {
 			}
 			tasks[ev.Args[0]] = ev
 		case EvUserTaskEnd:
-			taskid := ev.Args[0]
-			if taskCreateEv, ok := tasks[taskid]; ok {
-				taskCreateEv.Link = ev
-				delete(tasks, taskid)
+			if prevEv, ok := tasks[ev.Args[0]]; ok {
+				prevEv.Link = ev
+				ev.Link = prevEv
 			}
 		case EvUserSpan:
 			mode := ev.Args[1]
@@ -828,6 +828,7 @@ func postProcessTrace(ver int, events []*Event) error {
 					}
 					// Link span start event with span end event
 					s.Link = ev
+					ev.Link = s
 
 					if n > 1 {
 						activeSpans[ev.G] = spans[:n-1]