Re: [PATCH] builtin/fetch: iterate symrefs instead of all when checking dangling refs

> From: Liu Zhongbo <liuzhongbo.6666@bytedance.com>
>
> refs_warn_dangling_symref() traverse all references to check if there are
> any dangling symbolic references. The complexity is
> O(number of deleted references * total number of references).
> It will take a lot of time if there are tens of thousands of branches in
> monorepo.
>
> So I first identified all the symbolic references, and then only traverse
> in these references. The complexity is
> O (number of deleted references * number of symbolic references).
>
> Due to the infrequent use of symbolic references, there will be significant
> performance improvements here. In my case, the prune_refs() time has been
> reduced from 20 seconds to 4 seconds.
>
> Signed-off-by: Liu Zhongbo <liuzhongbo.6666@bytedance.com>
> ---
>     builtin/fetch: iterate symrefs instead of all refs
>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1812%2Flzb6666%2Fspeed_up_prune_refs-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1812/lzb6666/speed_up_prune_refs-v1
> Pull-Request: https://github.com/git/git/pull/1812
>
>  builtin/fetch.c |  7 +++++--
>  refs.c          | 35 ++++++++++++++++++++++++++---------
>  refs.h          |  4 +++-
>  3 files changed, 34 insertions(+), 12 deletions(-)
>
> diff --git a/builtin/fetch.c b/builtin/fetch.c
> index 80a64d0d269..ec4be60cfeb 100644
> --- a/builtin/fetch.c
> +++ b/builtin/fetch.c
> @@ -1412,15 +1412,18 @@ static int prune_refs(struct display_state *display_state,
>
>  	if (verbosity >= 0) {
>  		int summary_width = transport_summary_width(stale_refs);
> +	    struct string_list symrefs = STRING_LIST_INIT_NODUP;
> +	    refs_get_symrefs(get_main_ref_store(the_repository), &symrefs);