better concurrency safety in Clear() and Done()

internal/cache/timeline/preload.go

@@ -60,7 +60,7 @@ func (p *preloader) Check() bool {
 // CheckPreload will safely check the preload state,
 // and if needed call the provided function. if a
 // preload is in progress, it will wait until complete.
-func (p *preloader) CheckPreload(preload func()) {
+func (p *preloader) CheckPreload(preload func(*any)) {
 	for {
 		// Get state ptr.
 		ptr := p.p.Load()
@@ -93,7 +93,7 @@ func (p *preloader) CheckPreload(preload func()) {
 
 // start attempts to start the given preload function, by
 // performing a CAS operation with 'old'. return is success.
-func (p *preloader) start(old *any, preload func()) bool {
+func (p *preloader) start(old *any, preload func(*any)) bool {
 
 	// Optimistically setup a
 	// new waitgroup to set as
@@ -105,29 +105,24 @@ func (p *preloader) start(old *any, preload func()) bool {
 	// Wrap waitgroup in
 	// 'any' for pointer.
 	new := any(&wg)
+	ptr := &new
 
 	// Attempt CAS operation to claim start.
-	started := p.p.CompareAndSwap(old, &new)
+	started := p.p.CompareAndSwap(old, ptr)
 	if !started {
 		return false
 	}
 
 	// Start.
-	preload()
+	preload(ptr)
 	return true
 }
 
 // done marks state as preloaded,
 // i.e. no more preload required.
-func (p *preloader) Done() {
-	old := p.p.Swap(new(any))
-	if old == nil { // was brand-new
-		return
-	}
-	switch t := (*old).(type) {
-	case *sync.WaitGroup: // was preloading
-	default:
-		log.Errorf(nil, "BUG: invalid preloader state: %#v", t)
+func (p *preloader) Done(ptr *any) {
+	if !p.p.CompareAndSwap(ptr, new(any)) {
+		log.Errorf(nil, "BUG: invalid preloader state: %#v", (*p.p.Load()))
 	}
 }
 
@@ -137,17 +132,21 @@ func (p *preloader) Clear() {
 	b := false
 	a := any(b)
 	for {
-		old := p.p.Swap(&a)
-		if old == nil { // was brand-new
-			return
+		// Load current ptr.
+		ptr := p.p.Load()
+		if ptr == nil {
+			return // was brand-new
 		}
-		switch t := (*old).(type) {
-		case nil: // was preloaded
+
+		// Check for a preload currently in progress.
+		if wg, _ := (*ptr).(*sync.WaitGroup); wg != nil {
+			wg.Wait()
+			continue
+		}
+
+		// Try mark as needing preload.
+		if p.p.CompareAndSwap(ptr, &a) {
 			return
-		case bool: // was cleared
-			return
-		case *sync.WaitGroup: // was preloading
-			t.Wait()
 		}
 	}
 }

internal/cache/timeline/status.go

@@ -196,14 +196,14 @@ func (t *StatusTimeline) Preload(
 	n int,
 	err error,
 ) {
-	t.preloader.CheckPreload(func() {
+	t.preloader.CheckPreload(func(ptr *any) {
 		n, err = t.preload(loadPage, filter)
 		if err != nil {
 			return
 		}
 
-		// Mark preloaded.
-		t.preloader.Done()
+		// Mark as preloaded.
+		t.preloader.Done(ptr)
 	})
 	return
 }