Ensure that the lock cleanup handler is run after the global one

cleanup handlers run in the order in which they are added. As Go calls
init() functions in lexical order, the cleanup handler from global.go
was registered before that from lock.go, which is the correct order.

Make this order explicit to ensure that this won't break accidentally.

cmd/restic/global.go

@@ -98,6 +98,8 @@ func init() {
 	var cancel context.CancelFunc
 	globalOptions.ctx, cancel = context.WithCancel(context.Background())
 	AddCleanupHandler(func() error {
+		// Must be called before the unlock cleanup handler to ensure that the latter is
+		// not blocked due to limited number of backend connections, see #1434
 		cancel()
 		return nil
 	})

cmd/restic/lock.go

@@ -16,6 +16,7 @@ var globalLocks struct {
 	cancelRefresh chan struct{}
 	refreshWG     sync.WaitGroup
 	sync.Mutex
+	sync.Once
 }
 
 func lockRepo(ctx context.Context, repo *repository.Repository) (*restic.Lock, error) {
@@ -27,6 +28,12 @@ func lockRepoExclusive(ctx context.Context, repo *repository.Repository) (*resti
 }
 
 func lockRepository(ctx context.Context, repo *repository.Repository, exclusive bool) (*restic.Lock, error) {
+	// make sure that a repository is unlocked properly and after cancel() was
+	// called by the cleanup handler in global.go
+	globalLocks.Do(func() {
+		AddCleanupHandler(unlockAll)
+	})
+
 	lockFn := restic.NewLock
 	if exclusive {
 		lockFn = restic.NewExclusiveLock
@@ -128,7 +135,3 @@ func unlockAll() error {
 
 	return nil
 }
-
-func init() {
-	AddCleanupHandler(unlockAll)
-}