Openning IndexWriter with an existing commit incorrectly marks a index as changed

[Shibi-bala]

Description

Hey,

I found what looks to be a bug here: https://github.com/apache/lucene/blob/releases/lucene/9.9.0/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java#L1102-L1105

if (commit != null) {
  // Swap out all segments, but, keep metadata in
  // SegmentInfos, like version & generation, to
  // preserve write-once.  This is important if
  // readers are open against the future commit
  // points.
  if (commit.getDirectory() != directoryOrig) {
    throw new IllegalArgumentException(
        "IndexCommit's directory doesn't match my directory, expected="
            + directoryOrig
            + ", got="
            + commit.getDirectory());
  }

  SegmentInfos oldInfos =
      SegmentInfos.readCommit(directoryOrig, commit.getSegmentsFileName());
  segmentInfos.replace(oldInfos);
  changed();

  if (infoStream.isEnabled("IW")) {
    infoStream.message(
        "IW", "init: loaded commit \"" + commit.getSegmentsFileName() + "\"");
  }
}

When we called segmentsInfos.replace point, there is no guarantee that the information replaced actually changed. In the cases where information actually isn't changed, we still call changed(). This means the next commit we attempt will actually write a new commit rather than being a no-op.

I have some ideas on how to fix, like add a check to .replace() to see if information actually changed, but want to hear other thoughts.

Version and environment details

No response