Never throttle creation of compound files. (#12070)

`ConcurrentMergeScheduler` uses the rate at which a merge writes bytes as a
proxy for CPU usage, in order to prevent merging from disrupting searches too
much. However creating compound files are lightweight CPU-wise and do not need
throttling.

Closes #12068

lucene/CHANGES.txt

@@ -199,6 +199,9 @@ Improvements
 
 * GITHUB#12034: Remove null check in IndexReaderContext#leaves() usages (Erik Pellizzon)
 
+* GITHUB#12070: Compound file creation is no longer subject to merge throttling.
+  (Adrien Grand)
+
 Bug Fixes
 ---------------------
 * GITHUB#11726: Indexing term vectors on large documents could fail due to

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

@@ -5210,7 +5210,10 @@ public class IndexWriter
         success = false;
 
         Collection<String> filesToRemove = merge.info.files();
-        TrackingDirectoryWrapper trackingCFSDir = new TrackingDirectoryWrapper(mergeDirectory);
+        // NOTE: Creation of the CFS file must be performed with the original
+        // directory rather than with the merging directory, so that it is not
+        // subject to merge throttling.
+        TrackingDirectoryWrapper trackingCFSDir = new TrackingDirectoryWrapper(directory);
         try {
           createCompoundFile(
               infoStream, trackingCFSDir, merge.info.info, context, this::deleteNewFiles);