Update internal state and draw from the same goroutine

This commit introduces a new Aerc.Tick function that should be called to
refresh the internal state. This in turn makes each AccountView process worker
events.

The UI goroutine repeatedly refreshes the internal state before drawing a new
frame. The reason for this is that many worker messages may need to be
processed for a single frame, and drawing the UI is far slower than refreshing
the internal state. This has been confirmed in my testing (calling Aerc.Tick
only once per frame results in a slower display).

Many synchronization code has been removed. We can now write widgets without
having to care so much about races. The remaining sync users are:

- widgets/spinner: the spinner value is updated from inside an internal
  goroutine
- lib/ui/invalidatable: Invalidate may be called from any goroutine
- lib/ui/grid: same
- lib/ui/ui: an internal goroutine needs read access to UI.exit
- worker/types/worker: Worker.callbacks is used for both worker and UI
  callbacks

The exact goroutine requirements for Drawable have been documented.

3 files changed, 22 insertions, 28 deletions

diff --git a/widgets/account.go b/widgets/account.go
index afae1d2..431e7b8 100644
--- a/widgets/account.go
+++ b/widgets/account.go
@@ -65,13 +65,6 @@ func NewAccountView(conf *config.AercConfig, acct *config.AccountConfig,
 	}
 
 	go worker.Backend.Run()
-	go func() {
-		for {
-			msg := <-worker.Messages
-			msg = worker.ProcessMessage(msg)
-			view.onMessage(msg)
-		}
-	}()
 
 	worker.PostAction(&types.Configure{Config: acct}, nil)
 	worker.PostAction(&types.Connect{}, view.connected)
@@ -80,6 +73,17 @@ func NewAccountView(conf *config.AercConfig, acct *config.AccountConfig,
 	return view
 }
 
+func (acct *AccountView) Tick() bool {
+	select {
+	case msg := <-acct.worker.Messages:
+		msg = acct.worker.ProcessMessage(msg)
+		acct.onMessage(msg)
+		return true
+	default:
+		return false
+	}
+}
+
 func (acct *AccountView) AccountConfig() *config.AccountConfig {
 	return acct.acct
 }
diff --git a/widgets/aerc.go b/widgets/aerc.go
index e79e467..187eddb 100644
--- a/widgets/aerc.go
+++ b/widgets/aerc.go
@@ -65,6 +65,14 @@ func NewAerc(conf *config.AercConfig, logger *log.Logger,
 	return aerc
 }
 
+func (aerc *Aerc) Tick() bool {
+	more := false
+	for _, acct := range aerc.accounts {
+		more = acct.Tick() || more
+	}
+	return more
+}
+
 func (aerc *Aerc) Children() []ui.Drawable {
 	return aerc.grid.Children()
 }
diff --git a/widgets/msglist.go b/widgets/msglist.go
index 08f7ea4..ddfb92a 100644
--- a/widgets/msglist.go
+++ b/widgets/msglist.go
@@ -2,7 +2,6 @@ package widgets
 
 import (
 	"log"
-	"sync/atomic"
 
 	"github.com/gdamore/tcell"
 
@@ -20,7 +19,7 @@ type MessageList struct {
 	scroll   int
 	selected int
 	spinner  *Spinner
-	store    atomic.Value // *lib.MessageStore
+	store    *lib.MessageStore
 }
 
 func NewMessageList(conf *config.AercConfig, logger *log.Logger) *MessageList {
@@ -30,7 +29,6 @@ func NewMessageList(conf *config.AercConfig, logger *log.Logger) *MessageList {
 		selected: 0,
 		spinner:  NewSpinner(),
 	}
-	ml.store.Store((*lib.MessageStore)(nil))
 	ml.spinner.OnInvalidate(func(_ ui.Drawable) {
 		ml.Invalidate()
 	})
@@ -53,8 +51,6 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 		return
 	}
 
-	store.Lock()
-
 	var (
 		needsHeaders []uint32
 		row          int = 0
@@ -94,8 +90,6 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 			tcell.StyleDefault, "%s", msg)
 	}
 
-	store.Unlock()
-
 	if len(needsHeaders) != 0 {
 		store.FetchHeaders(needsHeaders, nil)
 		ml.spinner.Start()
@@ -113,13 +107,11 @@ func (ml *MessageList) storeUpdate(store *lib.MessageStore) {
 		return
 	}
 
-	store.Lock()
 	if len(store.Uids) > 0 {
 		for ml.selected >= len(store.Uids) {
 			ml.Prev()
 		}
 	}
-	store.Unlock()
 
 	ml.Invalidate()
 }
@@ -129,7 +121,7 @@ func (ml *MessageList) SetStore(store *lib.MessageStore) {
 		ml.scroll = 0
 		ml.selected = 0
 	}
-	ml.store.Store(store)
+	ml.store = store
 	if store != nil {
 		ml.spinner.Stop()
 		store.OnUpdate(ml.storeUpdate)
@@ -140,29 +132,21 @@ func (ml *MessageList) SetStore(store *lib.MessageStore) {
 }
 
 func (ml *MessageList) Store() *lib.MessageStore {
-	return ml.store.Load().(*lib.MessageStore)
+	return ml.store
 }
 
 func (ml *MessageList) Empty() bool {
 	store := ml.Store()
-	store.Lock()
-	defer store.Unlock()
-
 	return store == nil || len(store.Uids) == 0
 }
 
 func (ml *MessageList) Selected() *types.MessageInfo {
 	store := ml.Store()
-	store.Lock()
-	defer store.Unlock()
-
 	return store.Messages[store.Uids[len(store.Uids)-ml.selected-1]]
 }
 
 func (ml *MessageList) Select(index int) {
 	store := ml.Store()
-	store.Lock()
-	defer store.Unlock()
 
 	ml.selected = index
 	for ; ml.selected < 0; ml.selected = len(store.Uids) + ml.selected {
@@ -181,8 +165,6 @@ func (ml *MessageList) Select(index int) {
 
 func (ml *MessageList) nextPrev(delta int) {
 	store := ml.Store()
-	store.Lock()
-	defer store.Unlock()
 
 	if store == nil || len(store.Uids) == 0 {
 		return