Bytes fastfield codec mismatch

[PSeitz]

Bytes fast field consists of two indices, the raw bytes data and a fast field index to find the bytes for a doc.

When writing a bytes fast field for the first time, the index is always written with the bitpack codec

tantivy/src/fastfield/bytes/writer.rs

Lines 112 to 113 in 505e6a4

	let mut doc_index_serializer =
	serializer.new_u64_fast_field_with_idx(self.field, 0, self.vals.len() as u64, 0)?;

When merging indices there is generic code used, which has an auto-detection for the best codec

tantivy/src/indexer/merger.rs

Lines 506 to 510 in 505e6a4

	fn write_1_n_fast_field_idx_generic<T: MultiValueLength>(
	field: Field,
	fast_field_serializer: &mut CompositeFastFieldSerializer,
	doc_id_mapping: &SegmentDocIdMapping,
	reader_and_field_accessors: &[(&SegmentReader, T)],

The index part of the bytes field is always read as Bitpacked serialized index, which can lead to following error after a merge:

thread 'merge_thread0' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`: Tried to open fast field as bitpacked encoded (id=1), but got serializer with different id', C:\Users\ChillFish8\.cargo\git\checkouts\tantivy-f70b7ea03dadae9a\13a4473\src\fastfield\reader.rs:160:9