LUCENE-8804: Forbid calls to putAttribute on frozen FieldType instances.

lucene/CHANGES.txt

@@ -41,6 +41,9 @@ Bug Fixes
   This causes assertion errors and potentially broken field attributes in the IndexWriter when
   IndexWriter#deleteAll is called while actively indexing. (Simon Willnauer)
 
+* LUCENE-8804: Forbid calls to putAttribute on frozen FieldType instances.
+  (Vamshi Vijay Nakkirtha via Adrien Grand)
+
 Improvements
 
 * LUCENE-7840: Non-scoring BooleanQuery now removes SHOULD clauses before building the scorer supplier

lucene/core/src/java/org/apache/lucene/document/FieldType.java

@@ -361,6 +361,7 @@ public class FieldType implements IndexableFieldType  {
    * @lucene.experimental
    */
   public String putAttribute(String key, String value) {
+    checkIfFrozen();
     if (attributes == null) {
       attributes = new HashMap<>();
     }

lucene/core/src/test/org/apache/lucene/document/TestFieldType.java

@@ -72,6 +72,27 @@ public class TestFieldType extends LuceneTestCase {
     assertEquals("pointDataDimensionCount=1,pointIndexDimensionCount=1,pointNumBytes=4", ft.toString());
   }
 
+  /**
+   * FieldType's attribute map should not be modifiable/add after freeze
+   */
+  public void testAttributeMapFrozen() {
+    FieldType ft = new FieldType();
+    ft.putAttribute("dummy", "d");
+    ft.freeze();
+    expectThrows(IllegalStateException.class, () -> ft.putAttribute("dummy", "a"));
+  }
+
+  /**
+   * FieldType's attribute map can be changed if not frozen
+   */
+  public void testAttributeMapNotFrozen() {
+    FieldType ft = new FieldType();
+    ft.putAttribute("dummy", "d");
+    ft.putAttribute("dummy", "a");
+    assertEquals(ft.getAttributes().size(), 1);
+    assertEquals(ft.getAttributes().get("dummy"), "a");
+  }
+
   private static Object randomValue(Class<?> clazz) {
     if (clazz.isEnum()) {
       return RandomPicks.randomFrom(random(), clazz.getEnumConstants());