Made this a complete no-op if IndexWriterRAMManager is not specified

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

@@ -57,7 +57,7 @@ && flushOnDocCount()
   @Override
   public void flushRamManager(IndexWriter writer) throws IOException {
     IndexWriterRAMManager ramManager = writer.getConfig().indexWriterRAMManager;
-    if (ramManager.getRamBufferSizeMB() != IndexWriterConfig.DISABLE_AUTO_FLUSH) {
+    if (ramManager.getRamBufferSizeMB() != IndexWriterConfig.DISABLE_AUTO_FLUSH && ramManager.getWriterCount() > 1) {
       long totalBytes = ramManager.updateAndGetCurrentBytesUsed(writer.ramManagerId);
       if (totalBytes > ramManager.getRamBufferSizeMB() * 1024 * 1024) {
         ramManager.flushRoundRobin();

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

@@ -37,7 +37,7 @@ public class IndexWriterRAMManager {
    * @param ramBufferSizeMB the RAM buffer size to use between all registered {@link IndexWriter}
    *     instances
    */
-  IndexWriterRAMManager(double ramBufferSizeMB) {
+  public IndexWriterRAMManager(double ramBufferSizeMB) {
     if (ramBufferSizeMB != IndexWriterConfig.DISABLE_AUTO_FLUSH && ramBufferSizeMB <= 0.0) {
       throw new IllegalArgumentException("ramBufferSize should be > 0.0 MB when enabled");
     }
@@ -63,6 +63,13 @@ public int flushRoundRobin() throws IOException {
     return idToWriter.flushRoundRobin();
   }
 
+  /**
+   * Gets the number of writers registered with this ram manager
+   */
+  public int getWriterCount() {
+    return idToWriter.size();
+  }
+
   /** Registers a writer can returns the associated ID */
   protected int registerWriter(IndexWriter writer) {
     int id = idGenerator.incrementAndGet();
@@ -170,6 +177,12 @@ long getTotalRamTracker(int id) {
       }
     }
 
+    int size() {
+      synchronized (lock) {
+        return idToWriterNode.size();
+      }
+    }
+
     private static class IndexWriterNode {
       IndexWriter writer;
       int id;