the applying flag is atomic; its test waits instead of polling

The race detector found my own new flag: Apply writes it on a worker
and Close reads it from the main loop. The test was polling a plain
field too, where the real window hears about the apply on the main
loop, which orders the writes.

1 files changed, 6 insertions, 5 deletions

diff --git a/gui/internal/model/history.go b/gui/internal/model/history.go
index 87ab6e7..d86984b 100644
--- a/gui/internal/model/history.go
+++ b/gui/internal/model/history.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"sort"
 	"strings"
+	"sync/atomic"
 	"time"
 
 	"git.labunix.xyz/krino/internal/engine"
@@ -116,8 +117,8 @@ type UndoTab struct {
 	locks []*lock.Lock
 
 	// applying is set while the reversal is in flight; Close refuses then,
-	// for the reason PlanTab.applying gives.
-	applying bool
+	// for the reason PlanTab.applying gives. Atomic for the same reason.
+	applying atomic.Bool
 }
 
 // PlanUndo builds the reversal of runID and locks every directory it
@@ -212,8 +213,8 @@ func (t *UndoTab) SelectedCount() int {
 // said no to (spec §9). Refused files ride along unchanged, as they do on
 // the command line. The locks are released afterwards: the plan is history.
 func (t *UndoTab) Apply(ctx context.Context) (*engine.ApplyResult, error) {
-	t.applying = true
-	defer func() { t.applying = false }()
+	t.applying.Store(true)
+	defer t.applying.Store(false)
 	toApply := &engine.UndoPlan{Run: t.up.Run, Cleanup: t.up.Cleanup}
 	for i, f := range t.up.Files {
 		if f.Refused == "" && !t.Rows[i].Selected {
@@ -272,7 +273,7 @@ func undoOutcome(row UndoRow, fr engine.FileResult) string {
 // does. One lock's failure never stops the rest from being released, or the
 // session from being closed. Closing twice is not an error.
 func (t *UndoTab) Close() error {
-	if t.applying {
+	if t.applying.Load() {
 		return ErrApplying
 	}
 	var first error