Terminate pointer-only descriptor traversal

internal/engine/schema.go

@@ -334,8 +334,21 @@ func prependIndex(index int, nested []int) []int {
 
 type schemaPath struct {
 	descriptor *typeDescriptor
-	field      *fieldDescriptor
-	dynamic    bool
+}
+
+// terminalDescriptor follows a pointer-only descriptor chain without entering
+// any value. Recursive pointer aliases can refer back to an earlier descriptor,
+// so callers must handle a repeated descriptor as an unaddressable chain.
+func terminalDescriptor(descriptor *typeDescriptor) (*typeDescriptor, bool) {
+	visited := make(map[*typeDescriptor]struct{})
+	for descriptor != nil && descriptor.kind == descriptorPointer {
+		if _, exists := visited[descriptor]; exists {
+			return nil, false
+		}
+		visited[descriptor] = struct{}{}
+		descriptor = descriptor.element
+	}
+	return descriptor, descriptor != nil
 }
 
 func resolveSchemaPath(schema *schemaDescriptor, path Path) (schemaPath, bool) {
@@ -343,10 +356,11 @@ func resolveSchemaPath(schema *schemaDescriptor, path Path) (schemaPath, bool) {
 		return schemaPath{}, false
 	}
 	current := schema.root
-	var field *fieldDescriptor
 	for _, segment := range path.segments {
-		for current.kind == descriptorPointer {
-			current = current.element
+		var ok bool
+		current, ok = terminalDescriptor(current)
+		if !ok {
+			return schemaPath{}, false
 		}
 		switch current.kind {
 		case descriptorStruct:
@@ -355,12 +369,10 @@ func resolveSchemaPath(schema *schemaDescriptor, path Path) (schemaPath, bool) {
 			}
 			position, ok := current.fieldByName[segment.key]
 			if ok {
-				field = &current.fields[position]
-				current = field.descriptor
+				current = current.fields[position].descriptor
 				continue
 			}
 			if current.inlineMap != nil {
-				field = current.inlineMap
 				current = current.inlineMap.descriptor.element
 				continue
 			}
@@ -369,26 +381,20 @@ func resolveSchemaPath(schema *schemaDescriptor, path Path) (schemaPath, bool) {
 			if segment.kind == pathSegmentIndex {
 				return schemaPath{}, false
 			}
-			field = nil
 			current = current.element
 		case descriptorSlice, descriptorArray:
 			index, ok := schemaIndex(segment)
 			if !ok || (current.kind == descriptorArray && index >= current.length) {
 				return schemaPath{}, false
 			}
-			field = nil
 			current = current.element
 		case descriptorDynamic:
-			return schemaPath{descriptor: current, field: field, dynamic: true}, true
+			return schemaPath{descriptor: current}, true
 		default:
 			return schemaPath{}, false
 		}
 	}
-	dynamic := current
-	for dynamic.kind == descriptorPointer {
-		dynamic = dynamic.element
-	}
-	return schemaPath{descriptor: current, field: field, dynamic: dynamic.kind == descriptorDynamic}, true
+	return schemaPath{descriptor: current}, true
 }
 
 func schemaIndex(segment pathSegment) (int, bool) {