sklad: prevent creating container cycles

cmd/sklad/container.tmpl

@@ -12,6 +12,8 @@
 <p>Chyba: Řadu u neprázdných obalů nelze měnit.
 {{ else if .ErrorCannotChangeNumber }}
 <p>Chyba: Číslo obalu v řadě nelze měnit.
+{{ else if .ErrorWouldContainItself }}
+<p>Chyba: Obal by obsahoval sám sebe.
 {{ else if .ErrorContainerInUse }}
 <p>Chyba: Obal se používá.
 {{ else if .Error }}

cmd/sklad/db.go

@@ -163,6 +163,7 @@ var errNoSuchContainer = errors.New("no such container")
 var errCannotChangeSeriesNotEmpty = errors.New(
 	"cannot change the series of a non-empty container")
 var errCannotChangeNumber = errors.New("cannot change the number")
+var errWouldContainItself = errors.New("container would contain itself")
 var errContainerInUse = errors.New("container is in use")
 
 // Find and filter out the container in O(n).
@@ -204,19 +205,27 @@ func dbContainerCreate(c *Container) error {
 }
 
 func dbContainerUpdate(c *Container, updated Container) error {
-	newId := updated.Id()
+	newID := updated.Id()
 	if updated.Series != c.Series && len(c.Children()) > 0 {
 		return errCannotChangeSeriesNotEmpty
 	}
 	if updated.Number != c.Number {
 		return errCannotChangeNumber
 	}
-	if _, ok := indexContainer[newId]; ok && newId != c.Id() {
+	if _, ok := indexContainer[newID]; ok && newID != c.Id() {
 		return errContainerAlreadyExists
 	}
 	if updated.Parent != c.Parent {
+		// Relying on the invariant that we can't change the ID
+		// of a non-empty container.
+		for pv := &updated; pv.Parent != ""; pv = indexContainer[pv.Parent] {
+			if pv.Parent == updated.Id() {
+				return errWouldContainItself
+			}
+		}
+
 		indexChildren[c.Parent] = filterContainer(indexChildren[c.Parent], c)
-		indexChildren[newId] = append(indexChildren[newId], c)
+		indexChildren[newID] = append(indexChildren[newID], c)
 	}
 	*c = updated
 	return dbCommit()

cmd/sklad/main.go

@@ -130,6 +130,7 @@ func handleContainer(w http.ResponseWriter, r *http.Request) {
 		ErrorNoSuchContainer            bool
 		ErrorCannotChangeSeriesNotEmpty bool
 		ErrorCannotChangeNumber         bool
+		ErrorWouldContainItself         bool
 		ErrorContainerInUse             bool
 		Container                       *Container
 		Children                        []*Container
@@ -141,6 +142,7 @@ func handleContainer(w http.ResponseWriter, r *http.Request) {
 		ErrorNoSuchContainer:            err == errNoSuchContainer,
 		ErrorCannotChangeSeriesNotEmpty: err == errCannotChangeSeriesNotEmpty,
 		ErrorCannotChangeNumber:         err == errCannotChangeNumber,
+		ErrorWouldContainItself:         err == errWouldContainItself,
 		ErrorContainerInUse:             err == errContainerInUse,
 		Container:                       container,
 		Children:                        children,