Make MAX_DIMENSIONS configurable via a system property.

lucene/core/build.gradle

@@ -23,3 +23,8 @@ dependencies {
   moduleTestImplementation project(':lucene:codecs')
   moduleTestImplementation project(':lucene:test-framework')
 }
+
+test {
+  // test higher dimensions than the default limit
+  systemProperty 'org.apache.lucene.hnsw.maxDimensions', '2048'
+}

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

@@ -17,8 +17,12 @@
 package org.apache.lucene.index;
 
 import java.io.IOException;
+import java.security.AccessController;
+import java.security.PrivilegedAction;
+import java.util.logging.Logger;
 import org.apache.lucene.document.KnnFloatVectorField;
 import org.apache.lucene.search.DocIdSetIterator;
+import org.apache.lucene.util.SuppressForbidden;
 
 /**
  * This class provides access to per-document floating point vector values indexed as {@link
@@ -27,9 +31,40 @@
  * @lucene.experimental
  */
 public abstract class FloatVectorValues extends DocIdSetIterator {
+  private static final Logger LOG = Logger.getLogger(FloatVectorValues.class.getName());
 
-  /** The maximum length of a vector */
-  public static final int MAX_DIMENSIONS = 1024;
+  /**
+   * The maximum length of a vector. Can be overridden via a system property
+   * "org.apache.lucene.hnsw.maxDimensions".
+   *
+   * @deprecated Expected to move to a codec specific limit.
+   */
+  @Deprecated public static final int MAX_DIMENSIONS = doPrivileged(FloatVectorValues::initMaxDim);
+
+  // Extracted to a method to be able to apply the SuppressForbidden annotation
+  @SuppressWarnings("removal")
+  @SuppressForbidden(reason = "security manager")
+  private static <T> T doPrivileged(PrivilegedAction<T> action) {
+    return AccessController.doPrivileged(action);
+  }
+
+  private static int initMaxDim() {
+    final var PROP_NAME = "org.apache.lucene.hnsw.maxDimensions";
+    final int DEFAULT_MAX = 1024;
+    try {
+      int d = Integer.getInteger(PROP_NAME, DEFAULT_MAX);
+      if (d > 2048) {
+        LOG.warning("Lucene HNSW only supports up to 2048 dimensions.");
+      }
+      return d;
+    } catch (
+        @SuppressWarnings("unused")
+        Exception ignored) { // e.g. SecurityException
+      LOG.warning(
+          "Cannot read sysprop " + PROP_NAME + ", so the dimension limit will be unchanged.");
+      return DEFAULT_MAX;
+    }
+  }
 
   /** Sole constructor */
   protected FloatVectorValues() {}

lucene/core/src/test/org/apache/lucene/codecs/lucene95/TestLucene95HnswVectorsFormat.java

@@ -18,6 +18,7 @@
 
 import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.KnnVectorsFormat;
+import org.apache.lucene.index.FloatVectorValues;
 import org.apache.lucene.tests.index.BaseKnnVectorsFormatTestCase;
 import org.apache.lucene.tests.util.TestUtil;
 
@@ -48,4 +49,8 @@ public void testLimits() {
     expectThrows(IllegalArgumentException.class, () -> new Lucene95HnswVectorsFormat(512 + 1, 20));
     expectThrows(IllegalArgumentException.class, () -> new Lucene95HnswVectorsFormat(20, 3201));
   }
+
+  public void testTestedWithHighDimensionLimit() {
+    assertEquals(2048, FloatVectorValues.MAX_DIMENSIONS);
+  }
 }