widgets/msglist: fix MessageList.store race

This field could be written to in the middle of a Draw call, which reads it
multiple times. Use an atomic variable instead.

widgets/msglist.go

@@ -2,6 +2,7 @@ package widgets
 
 import (
 	"log"
+	"sync/atomic"
 
 	"github.com/gdamore/tcell"
 
@@ -19,7 +20,7 @@ type MessageList struct {
 	scroll   int
 	selected int
 	spinner  *Spinner
-	store    *lib.MessageStore
+	store    atomic.Value // *lib.MessageStore
 }
 
 // TODO: fish in config
@@ -29,6 +30,7 @@ func NewMessageList(logger *log.Logger) *MessageList {
 		selected: 0,
 		spinner:  NewSpinner(),
 	}
+	ml.store.Store((*lib.MessageStore)(nil))
 	ml.spinner.OnInvalidate(func(_ ui.Drawable) {
 		ml.Invalidate()
 	})
@@ -45,7 +47,8 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 	ml.height = ctx.Height()
 	ctx.Fill(0, 0, ctx.Width(), ctx.Height(), ' ', tcell.StyleDefault)
 
-	if ml.store == nil {
+	store := ml.Store()
+	if store == nil {
 		ml.spinner.Draw(ctx)
 		return
 	}
@@ -55,9 +58,9 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 		row          int = 0
 	)
 
-	for i := len(ml.store.Uids) - 1 - ml.scroll; i >= 0; i-- {
-		uid := ml.store.Uids[i]
-		msg := ml.store.Messages[uid]
+	for i := len(store.Uids) - 1 - ml.scroll; i >= 0; i-- {
+		uid := store.Uids[i]
+		msg := store.Messages[uid]
 
 		if row >= ctx.Height() {
 			break
@@ -74,7 +77,7 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 		if row == ml.selected-ml.scroll {
 			style = style.Reverse(true)
 		}
-		if _, ok := ml.store.Deleted[msg.Uid]; ok {
+		if _, ok := store.Deleted[msg.Uid]; ok {
 			style = style.Foreground(tcell.ColorGray)
 		}
 		ctx.Fill(0, row, ctx.Width(), 1, ' ', style)
@@ -83,14 +86,14 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 		row += 1
 	}
 
-	if len(ml.store.Uids) == 0 {
+	if len(store.Uids) == 0 {
 		msg := "(no messages)"
 		ctx.Printf((ctx.Width()/2)-(len(msg)/2), 0,
 			tcell.StyleDefault, "%s", msg)
 	}
 
 	if len(needsHeaders) != 0 {
-		ml.store.FetchHeaders(needsHeaders, nil)
+		store.FetchHeaders(needsHeaders, nil)
 		ml.spinner.Start()
 	} else {
 		ml.spinner.Stop()
@@ -102,26 +105,28 @@ func (ml *MessageList) Height() int {
 }
 
 func (ml *MessageList) storeUpdate(store *lib.MessageStore) {
-	if ml.store != store {
+	if ml.Store() != store {
 		return
 	}
-	if len(ml.store.Uids) > 0 {
-		for ml.selected >= len(ml.store.Uids) {
+
+	if len(store.Uids) > 0 {
+		for ml.selected >= len(store.Uids) {
 			ml.Prev()
 		}
 	}
+
 	ml.Invalidate()
 }
 
 func (ml *MessageList) SetStore(store *lib.MessageStore) {
-	if ml.store == store {
+	if ml.Store() == store {
 		ml.scroll = 0
 		ml.selected = 0
 	}
-	ml.store = store
+	ml.store.Store(store)
 	if store != nil {
 		ml.spinner.Stop()
-		ml.store.OnUpdate(ml.storeUpdate)
+		store.OnUpdate(ml.storeUpdate)
 	} else {
 		ml.spinner.Start()
 	}
@@ -129,23 +134,26 @@ func (ml *MessageList) SetStore(store *lib.MessageStore) {
 }
 
 func (ml *MessageList) Store() *lib.MessageStore {
-	return ml.store
+	return ml.store.Load().(*lib.MessageStore)
 }
 
 func (ml *MessageList) Empty() bool {
-	return ml.store == nil || len(ml.store.Uids) == 0
+	store := ml.Store()
+	return store == nil || len(store.Uids) == 0
 }
 
 func (ml *MessageList) Selected() *types.MessageInfo {
-	return ml.store.Messages[ml.store.Uids[len(ml.store.Uids)-ml.selected-1]]
+	store := ml.Store()
+	return store.Messages[store.Uids[len(store.Uids)-ml.selected-1]]
 }
 
 func (ml *MessageList) Select(index int) {
+	store := ml.Store()
 	ml.selected = index
-	for ; ml.selected < 0; ml.selected = len(ml.store.Uids) + ml.selected {
+	for ; ml.selected < 0; ml.selected = len(store.Uids) + ml.selected {
 	}
-	if ml.selected > len(ml.store.Uids) {
-		ml.selected = len(ml.store.Uids)
+	if ml.selected > len(store.Uids) {
+		ml.selected = len(store.Uids)
 	}
 	// I'm too lazy to do the math right now
 	for ml.selected-ml.scroll >= ml.Height() {
@@ -157,15 +165,16 @@ func (ml *MessageList) Select(index int) {
 }
 
 func (ml *MessageList) nextPrev(delta int) {
-	if ml.store == nil || len(ml.store.Uids) == 0 {
+	store := ml.Store()
+	if store == nil || len(store.Uids) == 0 {
 		return
 	}
 	ml.selected += delta
 	if ml.selected < 0 {
 		ml.selected = 0
 	}
-	if ml.selected >= len(ml.store.Uids) {
-		ml.selected = len(ml.store.Uids) - 1
+	if ml.selected >= len(store.Uids) {
+		ml.selected = len(store.Uids) - 1
 	}
 	if ml.Height() != 0 {
 		if ml.selected-ml.scroll >= ml.Height() {