lib/ui: fix UI.Exit race condition

UI.Exit can be accessed from goroutines drawing, goroutines executing
commands and goroutines waiting for events.

    Write at 0x00c0002b2040 by main goroutine:
      main.main.func1()
          /home/simon/src/aerc2/aerc.go:76 +0x33d
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Aerc).BeginExCommand.func1()
          /home/simon/src/aerc2/widgets/aerc.go:245 +0x89
      git.sr.ht/~sircmpwn/aerc2/widgets.(*ExLine).Event()
          /home/simon/src/aerc2/widgets/exline.go:131 +0x442
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Aerc).Event()
          /home/simon/src/aerc2/widgets/aerc.go:116 +0x83c
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Aerc).simulate()
          /home/simon/src/aerc2/widgets/aerc.go:109 +0x12a
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Aerc).Event()
          /home/simon/src/aerc2/widgets/aerc.go:142 +0x722
      git.sr.ht/~sircmpwn/aerc2/lib/ui.(*UI).Tick()
          /home/simon/src/aerc2/lib/ui/ui.go:75 +0x33f
      main.main()
          /home/simon/src/aerc2/aerc.go:94 +0x497

    Previous read at 0x00c0002b2040 by goroutine 19:
      git.sr.ht/~sircmpwn/aerc2/lib/ui.Initialize.func1()
          /home/simon/src/aerc2/lib/ui/ui.go:45 +0x97

    Goroutine 19 (running) created at:
      git.sr.ht/~sircmpwn/aerc2/lib/ui.Initialize()
          /home/simon/src/aerc2/lib/ui/ui.go:44 +0x372
      main.main()
          /home/simon/src/aerc2/aerc.go:87 +0x3a9

1 files changed, 13 insertions, 2 deletions

diff --git a/lib/ui/ui.go b/lib/ui/ui.go
index ced039f..49e4dcd 100644
--- a/lib/ui/ui.go
+++ b/lib/ui/ui.go
@@ -1,14 +1,16 @@
 package ui
 
 import (
+	"sync/atomic"
+
 	"github.com/gdamore/tcell"
 
 	"git.sr.ht/~sircmpwn/aerc2/config"
 )
 
 type UI struct {
-	Exit    bool
 	Content DrawableInteractive
+	exit    atomic.Value
 	ctx     *Context
 	screen  tcell.Screen
 
@@ -41,8 +43,9 @@ func Initialize(conf *config.AercConfig,
 		tcEvents:      make(chan tcell.Event, 10),
 		invalidations: make(chan interface{}),
 	}
+	state.exit.Store(false)
 	go (func() {
-		for !state.Exit {
+		for !state.ShouldExit() {
 			state.tcEvents <- screen.PollEvent()
 		}
 	})()
@@ -58,6 +61,14 @@ func Initialize(conf *config.AercConfig,
 	return &state, nil
 }
 
+func (state *UI) ShouldExit() bool {
+	return state.exit.Load().(bool)
+}
+
+func (state *UI) Exit() {
+	state.exit.Store(true)
+}
+
 func (state *UI) Close() {
 	state.screen.Fini()
 }