sequence numbers: remove dead variable, improve comments

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

@@ -77,17 +77,17 @@ final class DocumentsWriterDeleteQueue implements Accountable {
 
   private final DeleteSlice globalSlice;
   private final BufferedUpdates globalBufferedUpdates;
-  private long gen;
   
   // only acquired to update the global deletes, pkg-private for access by tests:
   final ReentrantLock globalBufferLock = new ReentrantLock();
 
   final long generation;
 
+  /** Generates the sequence number that IW returns to callers changing the index, showing the effective serialization of all operations. */
   final AtomicLong seqNo;
   
   DocumentsWriterDeleteQueue() {
-    // seqNo must start at 1 because some APIs negate this to encode a boolean
+    // seqNo must start at 1 because some APIs negate this to also return a boolean
     this(0, 1);
   }
   

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

@@ -481,11 +481,10 @@ final class DocumentsWriterFlushControl implements Accountable {
       // we do another full flush
       //System.out.println("DWFC: fullFLush old seqNo=" + documentsWriter.deleteQueue.seqNo.get() + " activeThreadCount=" + perThreadPool.getActiveThreadStateCount());
 
-      // jump over any possible in flight ops:
-      seqNo = documentsWriter.deleteQueue.seqNo.get() + perThreadPool.getActiveThreadStateCount();
-
       // Insert a gap in seqNo of current active thread count, in the worst case those threads now have one operation in flight.  It's fine
       // if we have some sequence numbers that were never assigned:
+      seqNo = documentsWriter.deleteQueue.seqNo.get() + perThreadPool.getActiveThreadStateCount();
+
       DocumentsWriterDeleteQueue newQueue = new DocumentsWriterDeleteQueue(flushingQueue.generation+1, seqNo+1);
 
       documentsWriter.deleteQueue = newQueue;