Re: [PATCH v2 7/7] odb/transaction: add transaction interface to write packfiles

> In git-receive-pack(1), the incoming packfile is written to the ODB via
> `unpack()`, which spawns git-index-pack(1) or git-unpack-objects(1)
> directly. With pluggable object databases, an alternative backend may
> need to handle writing packfile data differently though.
>
> Introduce `odb_transaction_write_pack()` as a generic interface to
> handle writing a packfile to a transaction and use the logic from
> `unpack()` as the "files" backend implementation. Note that a packfile
> written via git-index-pack(1) is kept in place by a ".keep" lockfile
> that must be retained until references are updated. To faciliate this in
> an ODB backend agnostic manner, the "files" transaction backend takes
> ownership of these lockfiles and removes them post-commit through its
> release callback.
> ...
> +static int odb_transaction_files_write_pack(struct odb_transaction *base,
> +					    int pack_fd, struct strbuf *err_msg,
> +					    const struct odb_transaction_write_pack_opts *opts)
> +{
> + ...
> +		odb_source_prepare(repo->objects->sources,
> +				   ODB_PREPARE_FLUSH_CACHES);
> +	}