Use array instead of hash for backend.ID

Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).

This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.

lock.go

@@ -33,7 +33,7 @@ type Lock struct {
 	GID       uint32    `json:"gid,omitempty"`
 
 	repo   *repository.Repository
-	lockID backend.ID
+	lockID *backend.ID
 }
 
 // ErrAlreadyLocked is returned when NewLock or NewExclusiveLock are unable to
@@ -92,11 +92,13 @@ func newLock(repo *repository.Repository, excl bool) (*Lock, error) {
 		return nil, err
 	}
 
-	lock.lockID, err = lock.createLock()
+	lockID, err := lock.createLock()
 	if err != nil {
 		return nil, err
 	}
 
+	lock.lockID = &lockID
+
 	time.Sleep(waitBeforeLockCheck)
 
 	if err = lock.checkForOtherLocks(); err != nil {
@@ -137,7 +139,7 @@ func (l *Lock) fillUserInfo() error {
 // exclusive lock is found.
 func (l *Lock) checkForOtherLocks() error {
 	return eachLock(l.repo, func(id backend.ID, lock *Lock, err error) error {
-		if id.Equal(l.lockID) {
+		if l.lockID != nil && id.Equal(*l.lockID) {
 			return nil
 		}
 
@@ -177,7 +179,7 @@ func eachLock(repo *repository.Repository, f func(backend.ID, *Lock, error) erro
 func (l *Lock) createLock() (backend.ID, error) {
 	id, err := l.repo.SaveJSONUnpacked(backend.Lock, l)
 	if err != nil {
-		return nil, err
+		return backend.ID{}, err
 	}
 
 	return id, nil
@@ -237,7 +239,7 @@ func (l *Lock) Refresh() error {
 	}
 
 	debug.Log("Lock.Refresh", "new lock ID %v", id.Str())
-	l.lockID = id
+	l.lockID = &id
 
 	return nil
 }
@@ -276,7 +278,7 @@ func LoadLock(repo *repository.Repository, id backend.ID) (*Lock, error) {
 	if err := repo.LoadJSONUnpacked(backend.Lock, id, lock); err != nil {
 		return nil, err
 	}
-	lock.lockID = id
+	lock.lockID = &id
 
 	return lock, nil
 }