Re: [PATCH v4] gc: add `--expire-to` option

> From: ZheNing Hu <adlternative@gmail.com>
>
> This commit extends the functionality of `git gc`
> by adding a new option, `--expire-to=<dir>`. Previously,
> this feature was implemented in 91badeba32 (builtin/repack.c:
> implement `--expire-to` for storing pruned objects, 2022-10-24),
> which allowing users to specify a directory where unreachable
> and expired cruft packs are stored during garbage collection.
> However, users had to run `git repack --cruft --expire-to=<dir>`
> followed by `git prune` to achieve similar results within `git gc`.
>
> By introducing `--expire-to=<dir>` directly into `git gc`,
> we simplify the process for users who wish to manage their
> repository's cleanup more efficiently. This change involves
> passing the `--expire-to=<dir>` parameter through to `git repack`,
> making it easier for users to set up a backup location for cruft
> packs that will be pruned.
>
> Due to the original `git gc --prune=now` deleting all unreachable
> objects by passing the `-a` parameter to git repack. With the
> addition of the `--cruft` and `--expire-to` options, it is necessary
> to modify this default behavior: instead of deleting these
> unreachable objects, they should be merged into a cruft pack and
> collected in a specified directory. Therefore, we do not pass `-a`
> to the repack command but instead pass `--cruft`, `--expire-to`,
> and `--cruft-expiration=now` to repack.
>
> Signed-off-by: ZheNing Hu <adlternative@gmail.com>
> ---

>  Documentation/git-gc.txt |  7 +++++++
>  builtin/gc.c             |  9 +++++++--
>  t/t6500-gc.sh            | 33 +++++++++++++++++++++++++++++++++
>  3 files changed, 47 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/git-gc.txt b/Documentation/git-gc.txt
> index 370e22faaeb..0eac8e85f08 100644
> --- a/Documentation/git-gc.txt
> +++ b/Documentation/git-gc.txt
> @@ -69,6 +69,13 @@ be performed as well.
>  	the `--max-cruft-size` option of linkgit:git-repack[1] for
>  	more.
>  
> +--expire-to=<dir>::
> +	When packing unreachable objects into a cruft pack, write a cruft
> +	pack containing pruned objects (if any) to the directory `<dir>`.
> +	This option only has an effect when used together with `--cruft`.
> +	See the `--expire-to` option of linkgit:git-repack[1] for
> +	more information.
> +
>  --prune=<date>::
>  	Prune loose objects older than date (default is 2 weeks ago,
>  	overridable by the config variable `gc.pruneExpire`).
> diff --git a/builtin/gc.c b/builtin/gc.c
> index d52735354c9..8656e1caff0 100644
> --- a/builtin/gc.c
> +++ b/builtin/gc.c
> @@ -136,6 +136,7 @@ struct gc_config {
>  	char *prune_worktrees_expire;
>  	char *repack_filter;
>  	char *repack_filter_to;
> +	char *repack_expire_to;
>  	unsigned long big_pack_threshold;
>  	unsigned long max_delta_cache_size;
>  };
> @@ -432,7 +433,8 @@ static int keep_one_pack(struct string_list_item *item, void *data UNUSED)
>  static void add_repack_all_option(struct gc_config *cfg,
>  				  struct string_list *keep_pack)
>  {
> -	if (cfg->prune_expire && !strcmp(cfg->prune_expire, "now"))
> +	if (cfg->prune_expire && !strcmp(cfg->prune_expire, "now")
> +		&& !(cfg->cruft_packs && cfg->repack_expire_to))
>  		strvec_push(&repack, "-a");
>  	else if (cfg->cruft_packs) {
>  		strvec_push(&repack, "--cruft");
> @@ -441,6 +443,8 @@ static void add_repack_all_option(struct gc_config *cfg,
>  		if (cfg->max_cruft_size)
>  			strvec_pushf(&repack, "--max-cruft-size=%lu",
>  				     cfg->max_cruft_size);
> +		if (cfg->repack_expire_to)
> +			strvec_pushf(&repack, "--expire-to=%s", cfg->repack_expire_to);
>  	} else {
>  		strvec_push(&repack, "-A");
>  		if (cfg->prune_expire)
> @@ -675,7 +679,6 @@ struct repository *repo UNUSED)
>  	const char *prune_expire_sentinel = "sentinel";
>  	const char *prune_expire_arg = prune_expire_sentinel;
>  	int ret;
> -
>  	struct option builtin_gc_options[] = {
>  		OPT__QUIET(&quiet, N_("suppress progress reporting")),
>  		{ OPTION_STRING, 0, "prune", &prune_expire_arg, N_("date"),
> @@ -694,6 +697,8 @@ struct repository *repo UNUSED)
>  			   PARSE_OPT_NOCOMPLETE),
>  		OPT_BOOL(0, "keep-largest-pack", &keep_largest_pack,
>  			 N_("repack all other packs except the largest pack")),
> +		OPT_STRING(0, "expire-to", &cfg.repack_expire_to, N_("dir"),
> +			   N_("pack prefix to store a pack containing pruned objects")),
>  		OPT_END()
>  	};
>  
> diff --git a/t/t6500-gc.sh b/t/t6500-gc.sh
> index ee074b99b70..74f7bd09046 100755
> --- a/t/t6500-gc.sh
> +++ b/t/t6500-gc.sh
> @@ -339,6 +339,39 @@ test_expect_success 'gc.maxCruftSize sets appropriate repack options' '
>  	test_subcommand $cruft_max_size_opts --max-cruft-size=3145728 <trace2.txt
>  '
>  
> +test_expect_success '--expire-to sets repack --expire-to' '
> +	rm -rf expired &&
> +	mkdir expired &&
> +	expire_to="$(pwd)/expired/pack" &&
> +	GIT_TRACE2_EVENT=$(pwd)/trace2.txt git -C cruft--max-size gc --cruft --expire-to="$expire_to" &&
> +	test_subcommand $cruft_max_size_opts --expire-to="$expire_to" <trace2.txt
> +'
> +
> +test_expect_success '--expire-to with --prune=now sets repack --expire-to' '
> +	rm -rf expired &&
> +	mkdir expired &&
> +	expire_to="$(pwd)/expired/pack" &&
> +	GIT_TRACE2_EVENT=$(pwd)/trace2.txt git -C cruft--max-size gc --cruft --prune=now --expire-to="$expire_to" &&
> +	test_subcommand git repack -d -l --cruft --cruft-expiration=now --expire-to="$expire_to" <trace2.txt
> +'
> +
> +
> +test_expect_success '--expire-to with --no-cruft sets repack -A' '
> +	rm -rf expired &&
> +	mkdir expired &&
> +	expire_to="$(pwd)/expired/pack" &&
> +	GIT_TRACE2_EVENT=$(pwd)/trace2.txt git -C cruft--max-size gc --no-cruft --expire-to="$expire_to" &&
> +	test_subcommand git repack -d -l -A --unpack-unreachable=2.weeks.ago <trace2.txt
> +'
> +
> +test_expect_success '--expire-to with --no-cruft sets repack -a' '
> +	rm -rf expired &&
> +	mkdir expired &&
> +	expire_to="$(pwd)/expired/pack" &&
> +	GIT_TRACE2_EVENT=$(pwd)/trace2.txt git -C cruft--max-size gc --no-cruft --prune=now --expire-to="$expire_to" &&
> +	test_subcommand git repack -d -l -a <trace2.txt
> +'
> +
>  run_and_wait_for_gc () {
>  	# We read stdout from gc for the side effect of waiting until the
>  	# background gc process exits, closing its fd 9.  Furthermore, the
>
> base-commit: 92999a42db1c5f43f330e4f2bca4026b5b81576f