Fix 9.12.0 backcompat break (Lucene 9.12.0 cannot read 9.11.x indices written with quantized HNSW, `Lucene99HnswScalarQuantizedVectorsFormat`) (#13874)

* carefully regenerate the int8_hnsw bwc indices so that they do in fact use Lucene99ScalarQuantizedVectorsFormat ... when running TestInt8HnswBackwardsCompatibility it now fails (as expected) on 9.11.0 and 9.11.1 bwc indices, but not on 9.10.0

* rename int8 -> int7 bwc tests since we are actually testing 7 bit quantization

* actually fix the bwc bug: only allow compress=true when bits is 7 or 8 in HNSW scalar quantization

* tidy

* Revert "rename int8 -> int7 bwc tests since we are actually testing 7 bit quantization"

This reverts commit eeb3f8a668.

* Reapply "rename int8 -> int7 bwc tests since we are actually testing 7 bit quantization"

This reverts commit 3487c4210b.

* #13880: add test to verify the int7 quantized indices are in fact using quantized vectors not float32

* bump 9.12.x version to 9.12.1 and add bwc indices for 9.12.0

* remove duplicate 9.12.0 Version constant

* revert changes to index.9.12.0-cfs.zip, index.9.12.0-nocfs.zip, sorted.9.12.0.zip

* remove unused bwc index

Closes #13867
Closes #13880

lucene/backward-codecs/src/test/org/apache/lucene/backward_index/BackwardsCompatibilityTestBase.java

@@ -106,8 +106,8 @@ public abstract class BackwardsCompatibilityTestBase extends LuceneTestCase {
    * This is a base constructor for parameterized BWC tests. The constructor arguments are provided
    * by {@link com.carrotsearch.randomizedtesting.RandomizedRunner} during test execution. A {@link
    * com.carrotsearch.randomizedtesting.annotations.ParametersFactory} specified in a subclass
-   * provides a list lists of arguments for the tests and RandomizedRunner will execute the test for
-   * each of the argument list.
+   * provides a list of arguments for the tests and RandomizedRunner will execute the test for each
+   * of the argument list.
    *
    * @param version the version this test should run for
    * @param indexPattern an index pattern in order to open an index of see {@link

lucene/backward-codecs/src/test/org/apache/lucene/backward_index/TestGenerateBwcIndices.java

@@ -39,7 +39,7 @@ public class TestGenerateBwcIndices extends LuceneTestCase {
   // To generate backcompat indexes with the current default codec, run the following gradle
   // command:
   //  gradlew test -Ptests.bwcdir=/path/to/store/indexes -Ptests.codec=default
-  //               -Ptests.useSecurityManager=false --tests TestGenerateBwcIndices
+  //               -Ptests.useSecurityManager=false --tests TestGenerateBwcIndices --max-workers=1
   //
   // Also add testmethod with one of the index creation methods below, for example:
   //    -Ptestmethod=testCreateCFS
@@ -82,14 +82,14 @@ public class TestGenerateBwcIndices extends LuceneTestCase {
     sortedTest.createBWCIndex();
   }
 
-  public void testCreateInt8HNSWIndices() throws IOException {
-    TestInt8HnswBackwardsCompatibility int8HnswBackwardsCompatibility =
-        new TestInt8HnswBackwardsCompatibility(
+  public void testCreateInt7HNSWIndices() throws IOException {
+    TestInt7HnswBackwardsCompatibility int7HnswBackwardsCompatibility =
+        new TestInt7HnswBackwardsCompatibility(
             Version.LATEST,
             createPattern(
-                TestInt8HnswBackwardsCompatibility.INDEX_NAME,
-                TestInt8HnswBackwardsCompatibility.SUFFIX));
-    int8HnswBackwardsCompatibility.createBWCIndex();
+                TestInt7HnswBackwardsCompatibility.INDEX_NAME,
+                TestInt7HnswBackwardsCompatibility.SUFFIX));
+    int7HnswBackwardsCompatibility.createBWCIndex();
   }
 
   private boolean isInitialMajorVersionRelease() {

lucene/backward-codecs/src/test/org/apache/lucene/backward_index/TestInt7HnswBackwardsCompatibility.java

@@ -23,17 +23,22 @@ import java.io.IOException;
 import org.apache.lucene.backward_codecs.lucene99.Lucene99Codec;
 import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.KnnVectorsFormat;
+import org.apache.lucene.codecs.KnnVectorsReader;
 import org.apache.lucene.codecs.lucene99.Lucene99HnswScalarQuantizedVectorsFormat;
 import org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsFormat;
+import org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsReader;
+import org.apache.lucene.codecs.perfield.PerFieldKnnVectorsFormat;
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.Field;
 import org.apache.lucene.document.FieldType;
 import org.apache.lucene.document.KnnFloatVectorField;
 import org.apache.lucene.document.StringField;
+import org.apache.lucene.index.CodecReader;
 import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.IndexWriterConfig;
+import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.index.NoMergePolicy;
 import org.apache.lucene.index.VectorSimilarityFunction;
 import org.apache.lucene.search.IndexSearcher;
@@ -41,23 +46,23 @@ import org.apache.lucene.store.Directory;
 import org.apache.lucene.tests.analysis.MockAnalyzer;
 import org.apache.lucene.tests.util.TestUtil;
 import org.apache.lucene.util.Version;
+import org.apache.lucene.util.quantization.QuantizedByteVectorValues;
 
-public class TestInt8HnswBackwardsCompatibility extends BackwardsCompatibilityTestBase {
+public class TestInt7HnswBackwardsCompatibility extends BackwardsCompatibilityTestBase {
 
-  static final String INDEX_NAME = "int8_hnsw";
+  static final String INDEX_NAME = "int7_hnsw";
   static final String SUFFIX = "";
-  private static final Version FIRST_INT8_HNSW_VERSION = Version.LUCENE_9_10_0;
+  private static final Version FIRST_INT7_HNSW_VERSION = Version.LUCENE_9_10_0;
   private static final String KNN_VECTOR_FIELD = "knn_field";
   private static final int DOC_COUNT = 30;
   private static final FieldType KNN_VECTOR_FIELD_TYPE =
       KnnFloatVectorField.createFieldType(3, VectorSimilarityFunction.COSINE);
   private static final float[] KNN_VECTOR = {0.2f, -0.1f, 0.1f};
 
-  public TestInt8HnswBackwardsCompatibility(Version version, String pattern) {
+  public TestInt7HnswBackwardsCompatibility(Version version, String pattern) {
     super(version, pattern);
   }
 
-  /** Provides all sorted versions to the test-framework */
   @ParametersFactory(argumentFormatting = "Lucene-Version:%1$s; Pattern: %2$s")
   public static Iterable<Object[]> testVersionsFactory() throws IllegalAccessException {
     return allVersion(INDEX_NAME, SUFFIX);
@@ -76,7 +81,7 @@ public class TestInt8HnswBackwardsCompatibility extends BackwardsCompatibilityTe
 
   @Override
   protected boolean supportsVersion(Version version) {
-    return version.onOrAfter(FIRST_INT8_HNSW_VERSION);
+    return version.onOrAfter(FIRST_INT7_HNSW_VERSION);
   }
 
   @Override
@@ -84,7 +89,7 @@ public class TestInt8HnswBackwardsCompatibility extends BackwardsCompatibilityTe
     // We don't use the default codec
   }
 
-  public void testInt8HnswIndexAndSearch() throws Exception {
+  public void testInt7HnswIndexAndSearch() throws Exception {
     IndexWriterConfig indexWriterConfig =
         newIndexWriterConfig(new MockAnalyzer(random()))
             .setOpenMode(IndexWriterConfig.OpenMode.APPEND)
@@ -108,7 +113,6 @@ public class TestInt8HnswBackwardsCompatibility extends BackwardsCompatibilityTe
         assertKNNSearch(searcher, KNN_VECTOR, 10, 10, "0");
       }
     }
-    // This will confirm the docs are really sorted
     TestUtil.checkIndex(directory);
   }
 
@@ -117,7 +121,7 @@ public class TestInt8HnswBackwardsCompatibility extends BackwardsCompatibilityTe
     IndexWriterConfig conf =
         new IndexWriterConfig(new MockAnalyzer(random()))
             .setMaxBufferedDocs(10)
-            .setCodec(TestUtil.getDefaultCodec())
+            .setCodec(getCodec())
             .setMergePolicy(NoMergePolicy.INSTANCE);
     try (IndexWriter writer = new IndexWriter(dir, conf)) {
       for (int i = 0; i < DOC_COUNT; i++) {
@@ -147,4 +151,29 @@ public class TestInt8HnswBackwardsCompatibility extends BackwardsCompatibilityTe
       assertKNNSearch(searcher, KNN_VECTOR, 10, 10, "0");
     }
   }
+
+  // #13880: make sure the BWC index really contains quantized HNSW not float32
+  public void testIndexIsReallyQuantized() throws Exception {
+    try (DirectoryReader reader = DirectoryReader.open(directory)) {
+      for (LeafReaderContext leafContext : reader.leaves()) {
+        KnnVectorsReader knnVectorsReader = ((CodecReader) leafContext.reader()).getVectorReader();
+        assertTrue(
+            "expected PerFieldKnnVectorsFormat.FieldsReader but got: " + knnVectorsReader,
+            knnVectorsReader instanceof PerFieldKnnVectorsFormat.FieldsReader);
+
+        KnnVectorsReader forField =
+            ((PerFieldKnnVectorsFormat.FieldsReader) knnVectorsReader)
+                .getFieldReader(KNN_VECTOR_FIELD);
+
+        assertTrue(forField instanceof Lucene99HnswVectorsReader);
+
+        QuantizedByteVectorValues quantized =
+            ((Lucene99HnswVectorsReader) forField).getQuantizedVectorValues(KNN_VECTOR_FIELD);
+
+        assertNotNull(
+            "KnnVectorsReader should have quantized interface for field " + KNN_VECTOR_FIELD,
+            quantized);
+      }
+    }
+  }
 }

lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99ScalarQuantizedVectorsReader.java

@@ -135,7 +135,7 @@ public final class Lucene99ScalarQuantizedVectorsReader extends FlatVectorsReade
     }
 
     final long quantizedVectorBytes;
-    if (fieldEntry.compress) {
+    if (fieldEntry.bits <= 4 && fieldEntry.compress) {
       // two dimensions -> one byte
       quantizedVectorBytes = ((dimension + 1) >> 1) + Float.BYTES;
     } else {