// Merge already sorted `MergeInput`s.
pub fn merge_with_file_limit<
    'a,
    M: MergeInput + 'static,
    F: ExactSizeIterator<Item = UResult<M>>,
    Tmp: WriteableTmpFile + 'static,
>(
    files: F,
    settings: &'a GlobalSettings,
    tmp_dir: &mut TmpDirWrapper,
) -> UResult<FileMerger<'a>> {
    if files.len() > settings.merge_batch_size {
        let mut remaining_files = files.len();
        let batches = files.chunks(settings.merge_batch_size);
        let mut batches = batches.into_iter();
        let mut temporary_files = vec![];
        while remaining_files != 0 {
            // Work around the fact that `Chunks` is not an `ExactSizeIterator`.
            remaining_files = remaining_files.saturating_sub(settings.merge_batch_size);

// ***ISSUE IS ON THE LINE BELOW*** The call to batches.next() advances the underlying iterator, opening the file too soon.
            let merger = merge_without_limit(batches.next().unwrap(), settings)?;         
            let mut tmp_file =
                Tmp::create(tmp_dir.next_file()?, settings.compress_prog.as_deref())?;
            merger.write_all_to(settings, tmp_file.as_write())?;
            temporary_files.push(tmp_file.finished_writing()?);
        }