Fix potential NPE for KnnVectorQuery (#15334)

* Fix potential NPE for KnnVectorQuery

* adding test

* removing unused test code

Author: benwtrent

lucene/CHANGES.txt

@@ -214,6 +214,9 @@ Bug Fixes
 
 * GITHUB#15319: Fix potential hang in initialisation of TermsEnum and BaseTermsEnum (Chris Hegarty)
 
+* GITHUB#15334: Fix NPE for MemoryIndex when using nearest neighbor search. MemoryIndex does not support
+  nearest neighbor search, but it should not throw an NPE (Ben Trent)
+
 Other
 ---------------------
 * GITHUB#15237: Fix SmartChinese to only deserialize dictionary data from classpath with a native array

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

@@ -211,8 +211,15 @@ private TopDocs getLeafResults(
     final int cost = acceptDocs.cost();
     QueryTimeout queryTimeout = timeLimitingKnnCollectorManager.getQueryTimeout();
 
-    float leafProportion = ctx.reader().maxDoc() / (float) ctx.parent.reader().maxDoc();
-    int perLeafTopK = perLeafTopKCalculation(k, leafProportion);
+    final int perLeafTopK;
+    // ctx.parent could be null if this is a MemoryIndex
+    if (ctx.parent != null) {
+      float leafProportion = ctx.reader().maxDoc() / (float) ctx.parent.reader().maxDoc();
+      perLeafTopK = perLeafTopKCalculation(k, leafProportion);
+      // We don't have a good way to estimate perLeafTopK here, so just do approximate search
+    } else {
+      perLeafTopK = k;
+    }
 
     if (cost <= perLeafTopK) {
       // If there are <= perLeafTopK possible matches, short-circuit and perform exact search, since
@@ -255,7 +262,8 @@ public KnnCollector newCollector(
         int visitedLimit, KnnSearchStrategy searchStrategy, LeafReaderContext context)
         throws IOException {
       // The delegate supports optimistic collection
-      if (delegate.isOptimistic()) {
+      // ctx.parent could be null if this is a MemoryIndex
+      if (delegate.isOptimistic() && context.parent != null) {
         @SuppressWarnings("resource")
         float leafProportion = context.reader().maxDoc() / (float) context.parent.reader().maxDoc();
         int perLeafTopK = perLeafTopKCalculation(k, leafProportion);

lucene/memory/src/test/org/apache/lucene/index/memory/TestMemoryIndex.java

@@ -77,11 +77,14 @@
 import org.apache.lucene.search.CollectionStatistics;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.IndexSearcher;
+import org.apache.lucene.search.KnnByteVectorQuery;
+import org.apache.lucene.search.KnnFloatVectorQuery;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.PhraseQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.search.TermStatistics;
+import org.apache.lucene.search.TopDocs;
 import org.apache.lucene.search.VectorScorer;
 import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.ClassicSimilarity;
@@ -793,6 +796,11 @@ public void testKnnFloatVectors() throws IOException {
             .get(0)
             .reader()
             .getByteVectorValues("knnByteVectorValue"));
+    TopDocs docs =
+        mi.createSearcher()
+            .search(new KnnFloatVectorQuery("knnFloatA", new float[] {1.0f, 1.0f}, 1), 10);
+    // we don't really do knn search right now for MemoryIndex
+    assertEquals(0, docs.totalHits.value());
   }
 
   public void testKnnByteVectorOnlyOneVectorAllowed() throws IOException {
@@ -840,6 +848,10 @@ public void testKnnByteVectors() throws IOException {
             .get(0)
             .reader()
             .getFloatVectorValues("knnFloatVectorValue"));
+    TopDocs docs =
+        mi.createSearcher().search(new KnnByteVectorQuery("knnByteA", new byte[] {1, 1}, 1), 10);
+    // we don't really do knn search right now for MemoryIndex
+    assertEquals(0, docs.totalHits.value());
   }
 
   private static void assertFloatVectorValue(MemoryIndex mi, String fieldName, float[] expected)