IndexWriter InfoStream improvements: log elapsed seconds waiting for merges to abort; don't log merge-on-commit when there are no merges to run (#15103)

Author: mikemccand

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

@@ -872,7 +872,7 @@ final void writeSomeDocValuesUpdates() throws IOException {
                       count,
                       readerPool.ramBytesUsed() / 1024. / 1024.,
                       ramBufferSizeMB,
-                      ((System.nanoTime() - startNS) / (double) TimeUnit.SECONDS.toNanos(1))));
+                      (System.nanoTime() - startNS) / (double) TimeUnit.SECONDS.toNanos(1)));
             }
           }
         }
@@ -2697,6 +2697,7 @@ public long deleteAll() throws IOException {
    * you lose a lot of work that must later be redone.
    */
   private synchronized void abortMerges() throws IOException {
+    long startNS = System.nanoTime();
     merges.disable();
     // Abort all pending & running merges:
     IOUtils.applyToAll(
@@ -2739,7 +2740,10 @@ private synchronized void abortMerges() throws IOException {
 
     notifyAll();
     if (infoStream.isEnabled("IW")) {
-      infoStream.message("IW", "all running merges have aborted");
+      double elapsedSec = (System.nanoTime() - startNS) / (double) TimeUnit.SECONDS.toNanos(1);
+      infoStream.message(
+          "IW",
+          String.format(Locale.ROOT, "all running merges have aborted [%.3f seconds]", elapsedSec));
     }
   }
 
@@ -3730,7 +3734,7 @@ private long prepareCommitInternal() throws IOException {
         maybeCloseOnTragicEvent();
       }
 
-      if (pointInTimeMerges != null) {
+      if (pointInTimeMerges != null && pointInTimeMerges.merges.size() != 0) {
         if (infoStream.isEnabled("IW")) {
           infoStream.message(
               "IW", "now run merges during commit: " + pointInTimeMerges.segString(directory));