widgets/terminal: fix damage race condition

Terminal.damage is accessed when drawing and when invalidating the widget. For
this reason we need to protect it with a mutex.

This seems to fix various damage issues I've been experiencing (where some
regions of the terminal weren't correctly repainted).

Race detector trace:

    Read at 0x00c0000c6670 by main goroutine:
      git.sr.ht/~sircmpwn/aerc/widgets.(*Terminal).Draw()
          /home/simon/src/aerc/widgets/terminal.go:292 +0x191
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*Grid).Draw()
          /home/simon/src/aerc/lib/ui/grid.go:117 +0x575
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*Grid).Draw()
          /home/simon/src/aerc/lib/ui/grid.go:117 +0x575
      git.sr.ht/~sircmpwn/aerc/widgets.(*MessageViewer).Draw()
          /home/simon/src/aerc/widgets/msgviewer.go:231 +0x253
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*TabContent).Draw()
          /home/simon/src/aerc/lib/ui/tab.go:124 +0x12e
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*Grid).Draw()
          /home/simon/src/aerc/lib/ui/grid.go:117 +0x575
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).Draw()
          /home/simon/src/aerc/widgets/aerc.go:95 +0x5a
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*UI).Tick()
          /home/simon/src/aerc/lib/ui/ui.go:93 +0x1dd
      main.main()
          /home/simon/src/aerc/aerc.go:105 +0x539

    Previous write at 0x00c0000c6670 by goroutine 37:
      git.sr.ht/~sircmpwn/aerc/widgets.(*Terminal).onDamage-fm()
          /home/simon/src/aerc/widgets/terminal.go:429 +0x131
      git.sr.ht/~sircmpwn/go-libvterm._go_handle_damage()
          /home/simon/go/pkg/mod/git.sr.ht/~sircmpwn/go-libvterm@v0.0.0-20190421201021-3184f6f13687/vterm.go:481 +0xf9
      git.sr.ht/~sircmpwn/go-libvterm._cgoexpwrap_5e22200b58b7__go_handle_damage()
          _cgo_gotypes.go:731 +0x58
      runtime.call32()
          /usr/lib/go/src/runtime/asm_amd64.s:519 +0x3a
      git.sr.ht/~sircmpwn/go-libvterm.(*VTerm).Write.func1()
          /home/simon/go/pkg/mod/git.sr.ht/~sircmpwn/go-libvterm@v0.0.0-20190421201021-3184f6f13687/vterm.go:329 +0x9d
      git.sr.ht/~sircmpwn/go-libvterm.(*VTerm).Write()
          /home/simon/go/pkg/mod/git.sr.ht/~sircmpwn/go-libvterm@v0.0.0-20190421201021-3184f6f13687/vterm.go:329 +0x7f
      git.sr.ht/~sircmpwn/aerc/widgets.NewTerminal.func1()
          /home/simon/src/aerc/widgets/terminal.go:131 +0x18c

    Goroutine 37 (running) created at:
      git.sr.ht/~sircmpwn/aerc/widgets.NewTerminal()
          /home/simon/src/aerc/widgets/terminal.go:121 +0x23f
      git.sr.ht/~sircmpwn/aerc/widgets.NewMessageViewer()
          /home/simon/src/aerc/widgets/msgviewer.go:147 +0xfbe
      git.sr.ht/~sircmpwn/aerc/commands/account.ViewMessage()
          /home/simon/src/aerc/commands/account/view-message.go:26 +0x4a4
      git.sr.ht/~sircmpwn/aerc/commands.(*Commands).ExecuteCommand()
          /home/simon/src/aerc/commands/commands.go:47 +0x1f0
      main.main.func1()
          /home/simon/src/aerc/aerc.go:76 +0x205
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).BeginExCommand.func1()
          /home/simon/src/aerc/widgets/aerc.go:262 +0x89
      git.sr.ht/~sircmpwn/aerc/widgets.(*ExLine).Event()
          /home/simon/src/aerc/widgets/exline.go:47 +0x222
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).Event()
          /home/simon/src/aerc/widgets/aerc.go:133 +0x83c
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).simulate()
          /home/simon/src/aerc/widgets/aerc.go:126 +0x12a
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).Event()
          /home/simon/src/aerc/widgets/aerc.go:148 +0x766
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*UI).Tick()
          /home/simon/src/aerc/lib/ui/ui.go:86 +0x11b
      main.main()
          /home/simon/src/aerc/aerc.go:105 +0x539

1 files changed, 12 insertions, 1 deletions

diff --git a/widgets/terminal.go b/widgets/terminal.go
index a3615a5e..99e99ae8 100644
--- a/widgets/terminal.go
+++ b/widgets/terminal.go
@@ -4,6 +4,7 @@ import (
 	gocolor "image/color"
 	"os"
 	"os/exec"
+	"sync"
 
 	"git.sr.ht/~sircmpwn/aerc/lib/ui"
 
@@ -95,7 +96,6 @@ type Terminal struct {
 	ctx         *ui.Context
 	cursorPos   vterm.Pos
 	cursorShown bool
-	damage      []vterm.Rect
 	destroyed   bool
 	err         error
 	focus       bool
@@ -103,6 +103,9 @@ type Terminal struct {
 	start       chan interface{}
 	vterm       *vterm.VTerm
 
+	damage []vterm.Rect // protected by mutex
+	mutex  sync.Mutex
+
 	OnClose func(err error)
 	OnEvent func(event tcell.Event) bool
 	OnStart func()
@@ -230,7 +233,9 @@ func (term *Terminal) Invalidate() {
 	if term.vterm != nil {
 		width, height := term.vterm.Size()
 		rect := vterm.NewRect(0, width, 0, height)
+		term.mutex.Lock()
 		term.damage = append(term.damage, *rect)
+		term.mutex.Unlock()
 	}
 	term.invalidate()
 }
@@ -274,7 +279,9 @@ func (term *Terminal) Draw(ctx *ui.Context) {
 			pty.Setsize(term.pty, &winsize)
 			term.vterm.SetSize(ctx.Height(), ctx.Width())
 			rect := vterm.NewRect(0, ctx.Width(), 0, ctx.Height())
+			term.mutex.Lock()
 			term.damage = append(term.damage, *rect)
+			term.mutex.Unlock()
 			return
 		}
 	}
@@ -289,6 +296,7 @@ func (term *Terminal) Draw(ctx *ui.Context) {
 	// naive optimization
 	visited := make(map[coords]interface{})
 
+	term.mutex.Lock()
 	for _, rect := range term.damage {
 		for x := rect.StartCol(); x < rect.EndCol() && x < ctx.Width(); x += 1 {
 
@@ -311,6 +319,7 @@ func (term *Terminal) Draw(ctx *ui.Context) {
 	}
 
 	term.damage = nil
+	term.mutex.Unlock()
 
 	if term.focus && !term.closed {
 		if !term.cursorShown {
@@ -426,7 +435,9 @@ func (term *Terminal) styleFromCell(cell *vterm.ScreenCell) tcell.Style {
 }
 
 func (term *Terminal) onDamage(rect *vterm.Rect) int {
+	term.mutex.Lock()
 	term.damage = append(term.damage, *rect)
+	term.mutex.Unlock()
 	term.invalidate()
 	return 1
 }