Wrote the code to check for parent cycles. I'm not going to actually hook it up yet; I want to resolve the one TODO about limiting the scope of the checkProgrammerError() calls first.

common/controls.c

@@ -190,6 +190,35 @@ void uiControlFree(uiControl *c)
 	uiprivFree(c);
 }
 
+static bool parentHasCycle(uiControl *c, uiControl *parent)
+{
+	uiprivArray parents;
+	size_t i;
+
+	if (parent == NULL)
+		return false;
+	if (parent == c)			// easy case
+		return true;
+
+	uiprivArrayInit(parents, uiControl *, 16, "uiControl parent list");
+	// add these now, as they are counted as part of any cycles
+	*((uiControl *) uiprivArrayAppend(parents, 1)) = c;
+	*((uiControl *) uiprivArrayAppend(parents, 1)) = parent;
+	for (c = parent->parent; c != NULL; c = c->parent) {
+		// TODO this doesn't need to be sequential, but I don't imagine this list will ever be long enough to make it matter... yet
+		for (i = 0; i < parents.len; i++)
+			if (c == uiprivArrayAt(parents, uiControl *, i)) {
+				uiprivArrayFree(parents);
+				return true;
+			}
+		// new parent; mark it as visited
+		*((uiControl *) uiprivArrayAppend(parents, 1)) = c;
+	}
+
+	uiprivArrayFree(parents);
+	return false;
+}
+
 void uiControlSetParent(uiControl *c, uiControl *parent)
 {
 	if (!uiprivCheckInitializedAndThread())