Re: [PATCH v3 2/8] branch: consider refs under 'update-refs'

> From: Derrick Stolee <derrickstolee@github.com>
> 
> The branch_checked_out() helper helps commands like 'git branch' and
> 'git fetch' from overwriting refs that are currently checked out in
> other worktrees.
> 
> A future update to 'git rebase' will introduce a new '--update-refs'
> option which will update the local refs that point to commits that are
> being rebased. To avoid collisions as the rebase completes, we want to
> make the future data store for these refs to be considered by
> branch_checked_out().
> 
> The data store is a plaintext file inside the 'rebase-merge' directory
> for that worktree. The file alternates refnames and OIDs. The OIDs will
> be used to store the to-be-written values as the rebase progresses, but
> can be ignored at the moment.
> 
> Create a new sequencer_get_update_refs_state() method that parses this
> file and populates a struct string_list with the ref-OID pairs. We can
> then use this list to add to the current_checked_out_branches strmap
> used by branch_checked_out().
> 
> To properly navigate to the rebase directory for a given worktree,
> extract the static strbuf_worktree_gitdir() method to a public API
> method.
> 
> We can test that this works without having Git write this file by
> artificially creating one in our test script.

> Signed-off-by: Derrick Stolee <derrickstolee@github.com>
> ---
>   branch.c                  | 13 ++++++++++++
>   sequencer.c               | 42 +++++++++++++++++++++++++++++++++++++++
>   sequencer.h               |  9 +++++++++
>   t/t2407-worktree-heads.sh | 22 ++++++++++++++++----
>   4 files changed, 82 insertions(+), 4 deletions(-)
> 
> diff --git a/branch.c b/branch.c
> index 526e8237673..f252c4eef6c 100644
> --- a/branch.c
> +++ b/branch.c
> @@ -365,6 +365,7 @@ static void prepare_checked_out_branches(void)
>   		char *old;
>   		struct wt_status_state state = { 0 };
>   		struct worktree *wt = worktrees[i++];
> +		struct string_list update_refs = STRING_LIST_INIT_DUP;
>   
>   		if (wt->is_bare)
>   			continue;
> @@ -400,6 +401,18 @@ static void prepare_checked_out_branches(void)
>   			strbuf_release(&ref);
>   		}
>   		wt_status_state_free_buffers(&state);
> +
> +		if (!sequencer_get_update_refs_state(get_worktree_git_dir(wt),
> +						     &update_refs)) {
> +			struct string_list_item *item;
> +			for_each_string_list_item(item, &update_refs) {
> +				old = strmap_put(&current_checked_out_branches,
> +						 item->string,
> +						 xstrdup(wt->path));
> +				free(old);
> +			}
> +			string_list_clear(&update_refs, 1);
> +		}
>   	}
>   
>   	free_worktrees(worktrees);
> diff --git a/sequencer.c b/sequencer.c
> index 8c3ed3532ac..1094e146b99 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -5901,3 +5901,45 @@ int sequencer_determine_whence(struct repository *r, enum commit_whence *whence)
>   
>   	return 0;
>   }
> +
> +int sequencer_get_update_refs_state(const char *wt_dir,
> +				    struct string_list *refs)
> +{
> +	int result = 0;
> +	struct stat st;
> +	FILE *fp = NULL;
> +	struct strbuf ref = STRBUF_INIT;
> +	struct strbuf hash = STRBUF_INIT;
> +	char *path = xstrfmt("%s/rebase-merge/update-refs", wt_dir);