avoid circular inheritance when using blocks to set sprites' parents

HISTORY.md

@@ -12,6 +12,7 @@
     * when creating a costume from pen trails (raster or vector) make its rotation center the position of the sprite
 * **Notable Fixes:**
     * support null-serialization in list-csv conversions
+    * avoid circular inheritance when using blocks to set sprites' parents
 * **Translation Updates:**
     * NEW Slovak translation, thanks, Peter Lukacovic
     * German
@@ -19,6 +20,7 @@
 ### 2019-12-18
 * gui, api: rearranged Snap! API into its own file
 * added API documentation
+* threads: avoid circular inheritance when using blocks to set sprites' parents
 
 ### 2019-12-16
 * gui, objects: added ability to add general message listeners for "any" message

snap.html

@@ -7,7 +7,7 @@
         <script type="text/javascript" src="src/morphic.js?version=2019-11-12"></script>
         <script type="text/javascript" src="src/widgets.js?version=2019-10-16"></script>
         <script type="text/javascript" src="src/blocks.js?version=2019-12-13"></script>
-        <script type="text/javascript" src="src/threads.js?version=2019-12-15"></script>
+        <script type="text/javascript" src="src/threads.js?version=2019-12-18"></script>
         <script type="text/javascript" src="src/objects.js?version=2019-12-16"></script>
         <script type="text/javascript" src="src/gui.js?version=2019-12-18"></script>
         <script type="text/javascript" src="src/paint.js?version=2019-06-27"></script>

src/threads.js

@@ -61,7 +61,7 @@ StageMorph, SpriteMorph, StagePrompterMorph, Note, modules, isString, copy,
 isNil, WatcherMorph, List, ListWatcherMorph, alert, console, TableMorph, Color,
 TableFrameMorph, ColorSlotMorph, isSnapObject, newCanvas, Symbol, SVG_Costume*/
 
-modules.threads = '2019-December-16';
+modules.threads = '2019-December-18';
 
 var ThreadManager;
 var Process;
@@ -4554,7 +4554,13 @@ Process.prototype.doSet = function (attribute, value) {
     case 'parent':
         this.assertType(rcvr, 'sprite');
         value = value instanceof SpriteMorph ? value : null;
-        // needed: circularity avoidance
+        // avoid circularity
+        if (value instanceof SpriteMorph &&
+                contains(value.allExemplars(), rcvr)) {
+            throw new Error(
+                localize('unable to inherit\n(disabled or circular?)')
+            );
+        }
         rcvr.setExemplar(value);
         break;
     case 'temporary?':