R-1253 (P2): an export that stops its app refuses while another operation (a restore) holds the app-stop marker — its Undo is never replaced. Commit only.

controller/cmd/controller/main.go

@@ -3601,9 +3601,12 @@ func (a *exportAdapter) GetStacksBaseDir() string {
 type exportStopGuard struct{ g *backup.AppStopGuard }
 
 func (a exportStopGuard) Begin(opID string, stacks []string) error {
-	return a.g.Begin(opID, backup.ReasonAppExport, stacks)
+	// R-1253: never replace the marker another operation (a restore) holds — its Undo is the only crash record.
+	return a.g.BeginExclusive(opID, backup.ReasonAppExport, stacks)
 }
 
+func (a exportStopGuard) BusyWith() string { return a.g.BusyWith() }
+
 func (a exportStopGuard) End() { a.g.End() }
 
 func (a exportStopGuard) ReleaseFailed(stacks ...string) { a.g.ReleaseFailed(stacks...) }