Guard FieldExistsQuery against null pointers (#11794)

FieldExistsQuery checks if there are points for a certain field, and then retrieves the
corresponding point values. When all documents that had points for a certain field have
been deleted from a certain segments, as well as merged away, field info may report
that there are points yet the corresponding point values are null.

With this change we add a null check in FieldExistsQuery. Long term, we will likely want
to prevent this situation from happening.

Relates #11393

lucene/CHANGES.txt

@@ -169,6 +169,8 @@ Bug Fixes
 
 * LUCENE-10674: Ensure BitSetConjDISI returns NO_MORE_DOCS when sub-iterator exhausts. (Jack Mazanec)
 
+* GITHUB#11794: Guard FieldExistsQuery against null pointers (Luca Cavanna)
+
 Build
 ---------------------
 

lucene/core/src/java/org/apache/lucene/search/FieldExistsQuery.java

@@ -230,9 +230,11 @@ public class FieldExistsQuery extends Query {
             != DocValuesType.NONE) { // the field indexes doc values
           if (reader.hasDeletions() == false) {
             if (fieldInfo.getPointDimensionCount() > 0) {
-              return reader.getPointValues(field).getDocCount();
+              PointValues pointValues = reader.getPointValues(field);
+              return pointValues == null ? 0 : pointValues.getDocCount();
             } else if (fieldInfo.getIndexOptions() != IndexOptions.NONE) {
-              return reader.terms(field).getDocCount();
+              Terms terms = reader.terms(field);
+              return terms == null ? 0 : terms.getDocCount();
             }
           }
 

lucene/core/src/test/org/apache/lucene/search/TestFieldExistsQuery.java

@@ -702,6 +702,64 @@ public class TestFieldExistsQuery extends LuceneTestCase {
     return v;
   }
 
+  public void testDeleteAllPointDocs() throws Exception {
+    try (Directory dir = newDirectory();
+        RandomIndexWriter iw = new RandomIndexWriter(random(), dir)) {
+
+      Document doc = new Document();
+      doc.add(new StringField("id", "0", Field.Store.NO));
+      doc.add(new LongPoint("long", 17));
+      doc.add(new NumericDocValuesField("long", 17));
+      iw.addDocument(doc);
+      // add another document before the flush, otherwise the segment only has the document that
+      // we are going to delete and the merge simply ignores the segment without carrying over its
+      // field infos
+      iw.addDocument(new Document());
+      // make sure there are two segments or force merge will be a no-op
+      iw.flush();
+      iw.addDocument(new Document());
+      iw.commit();
+
+      iw.deleteDocuments(new Term("id", "0"));
+      iw.forceMerge(1);
+
+      try (IndexReader reader = iw.getReader()) {
+        assertTrue(reader.leaves().size() == 1 && reader.hasDeletions() == false);
+        IndexSearcher searcher = newSearcher(reader);
+        assertEquals(0, searcher.count(new FieldExistsQuery("long")));
+      }
+    }
+  }
+
+  public void testDeleteAllTermDocs() throws Exception {
+    try (Directory dir = newDirectory();
+        RandomIndexWriter iw = new RandomIndexWriter(random(), dir)) {
+
+      Document doc = new Document();
+      doc.add(new StringField("id", "0", Field.Store.NO));
+      doc.add(new StringField("str", "foo", Store.NO));
+      doc.add(new SortedDocValuesField("str", new BytesRef("foo")));
+      iw.addDocument(doc);
+      // add another document before the flush, otherwise the segment only has the document that
+      // we are going to delete and the merge simply ignores the segment without carrying over its
+      // field infos
+      iw.addDocument(new Document());
+      // make sure there are two segments or force merge will be a no-op
+      iw.flush();
+      iw.addDocument(new Document());
+      iw.commit();
+
+      iw.deleteDocuments(new Term("id", "0"));
+      iw.forceMerge(1);
+
+      try (IndexReader reader = iw.getReader()) {
+        assertTrue(reader.leaves().size() == 1 && reader.hasDeletions() == false);
+        IndexSearcher searcher = newSearcher(reader);
+        assertEquals(0, searcher.count(new FieldExistsQuery("str")));
+      }
+    }
+  }
+
   private void assertSameMatches(IndexSearcher searcher, Query q1, Query q2, boolean scores)
       throws IOException {
     final int maxDoc = searcher.getIndexReader().maxDoc();