Re: [PATCH v2 0/4] loose: fix migrating "loose-object-idx" with batch fsync

[Junio C Hamano <gitster@pobox.com>]

Justin Tobler <jltobler@gmail.com> writes:

> Greetings,
>
> While working on a separate patch series to convert the "files" ODB
> transaction to be eagerly created instead of lazily, I encountered this
> bug that causes an ODB transaction to fail on commit when a
> compatibility hash is also configured. This occurs because the temporary
> directory migration code does not know how to handle the loose object
> index file.
>
> This patch addresses updating the migration logic to explicitly merge
> mapping entries in the transaction and atomically write a new
> "loose-object-idx" file.
>
> Some other approaches I considered:
>   - Storing the uncommitted object mapping entries in the transaction
>     without writing a file and only writing on transaction commit. I
>     chose to keep writing the temp index into the transaction so that
>     child processes that write to the tempdir could still append entries.
>   - Also writing mapping entires to the main ODB loose object index.
>     This also didn't seem like a good apprach because it write entries
>     that may not ultimately get committed.
>
> Changes since V1:
>   - The loose object index write after the merging maps is now atomic.
>   - Added a fixed timeout to all loose object index writers.
>   - Duplicate entries are now dropped when writing the merged mapping file.
>
> Thanks for the review,
> -Justin

Thanks.

Merging this topic to 'seen' had a funny interaction with other
topics in flight.  repo_write_loose_object_map() function, from
which loose_object_map_write() helper function was extracted out of,
loses all callers, and will disappear in the merge.  But the new
helper, which gained a caller loose_object_idx_merge() in this
series, remains.