Prevent parent field from added to existing index

This change prevents users from adding a parent field to an existing index.
Parent field must be added before any documents are added to the index to
prevent documents without the parent field from being indexed and later
to be treated as child documents upon merge.

Relates to apache#12829

lucene/core/src/java/org/apache/lucene/index/IndexWriter.java

@@ -1121,6 +1121,12 @@ public IndexWriter(Directory d, IndexWriterConfig conf) throws IOException {
       // NOTE: this is correct even for an NRT reader because we'll pull FieldInfos even for the
       // un-committed segments:
       globalFieldNumberMap = getFieldNumberMap();
+      if (create == false
+          && conf.getParentField() != null
+          && globalFieldNumberMap.getFieldNames().contains(conf.getParentField()) == false) {
+        throw new IllegalArgumentException(
+            "can't add a parent field to an already existing index without a parent field");
+      }
 
       validateIndexSort();
 
@@ -1270,7 +1276,6 @@ private FieldNumbers getFieldNumberMap() throws IOException {
         map.addOrGet(fi);
       }
     }
-
     return map;
   }
 

lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java

@@ -4843,6 +4843,48 @@ public void testParentAndSoftDeletesAreTheSame() throws IOException {
     }
   }
 
+  public void testParentFieldExistingIndex() throws IOException {
+    try (Directory dir = newDirectory()) {
+      IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));
+      try (IndexWriter writer = new IndexWriter(dir, iwc)) {
+        writer.addDocument(new Document());
+      }
+      IllegalArgumentException iae =
+          expectThrows(
+              IllegalArgumentException.class,
+              () ->
+                  new IndexWriter(
+                      dir,
+                      new IndexWriterConfig(new MockAnalyzer(random()))
+                          .setOpenMode(OpenMode.APPEND)
+                          .setParentField("foo")));
+      assertEquals(
+          "can't add a parent field to an already existing index without a parent field",
+          iae.getMessage());
+      iae =
+          expectThrows(
+              IllegalArgumentException.class,
+              () ->
+                  new IndexWriter(
+                      dir,
+                      new IndexWriterConfig(new MockAnalyzer(random()))
+                          .setOpenMode(OpenMode.CREATE_OR_APPEND)
+                          .setParentField("foo")));
+      assertEquals(
+          "can't add a parent field to an already existing index without a parent field",
+          iae.getMessage());
+
+      try (IndexWriter writer =
+          new IndexWriter(
+              dir,
+              new IndexWriterConfig(new MockAnalyzer(random()))
+                  .setOpenMode(OpenMode.CREATE)
+                  .setParentField("foo"))) {
+        writer.addDocument(new Document());
+      }
+    }
+  }
+
   public void testIndexWithParentFieldIsCongruent() throws IOException {
     try (Directory dir = newDirectory()) {
       IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));