Re: [PATCH v5 3/4] refs: allow reference location in refstorage config

> The 'extensions.refStorage' config is used to specify the reference
> backend for a given repository. Both the 'files' and 'reftable' backends
> utilize the $GIT_DIR as the reference folder by default in
> `get_main_ref_store()`.
> 
> Since the reference backends are pluggable, this means that they could
> work with out-of-tree reference directories too. Extend the 'refStorage'
> config to also support taking an URI input, where users can specify the
> reference backend and the location.
> 
> Add the required changes to obtain and propagate this value to the
> individual backends also add the necessary documentation and tests.

> diff --git a/builtin/worktree.c b/builtin/worktree.c
> index fbdaf2eb2e..94480be5c4 100644
> --- a/builtin/worktree.c
> +++ b/builtin/worktree.c
> @@ -425,6 +425,39 @@ static int make_worktree_orphan(const char * ref, const struct add_opts *opts,
>  	return run_command(&cp);
>  }
>  
> +/*
> + * References for worktress are generally stored in '$GIT_DIR/worktrees/<wt_id>'.

> diff --git a/t/t1423-ref-backend.sh b/t/t1423-ref-backend.sh
> new file mode 100755
> index 0000000000..9c777b79f3
> --- /dev/null
> +++ b/t/t1423-ref-backend.sh
> @@ -0,0 +1,159 @@
> +#!/bin/sh
> +
> +test_description='Test reference backend URIs'
> +
> +. ./test-lib.sh
> +
> +# Run a git command with the provided reference storage. Reset the backend
> +# post running the command.
> +# Usage: run_with_uri <repo> <backend> <uri> <cmd>
> +#   <repo> is the relative path to the repo to run the command in.
> +#   <backend> is the original ref storage of the repo.
> +#   <uri> is the new URI to be set for the ref storage.
> +#   <cmd> is the git subcommand to be run in the repository.
> +run_with_uri() {
> +	repo=$1 &&
> +	backend=$2 &&
> +	uri=$3 &&
> +	cmd=$4 &&
> +
> +	git -C "$repo" config set core.repositoryformatversion 1
> +	git -C "$repo" config set extensions.refStorage "$uri" &&
> +	git -C "$repo" $cmd &&
> +	git -C "$repo" config set extensions.refStorage "$backend"
> +}
> +
> +# Test a repository with a given reference storage by running and comparing
> +# 'git refs list' before and after setting the new reference backend. If
> +# err_msg is set, expect the command to fail and grep for the provided err_msg.
> +# Usage: run_with_uri <repo> <backend> <uri> <cmd>
> +#   <repo> is the relative path to the repo to run the command in.
> +#   <backend> is the original ref storage of the repo.
> +#   <uri> is the new URI to be set for the ref storage.
> +#   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
> +test_refs_backend() {
> +	repo=$1 &&
> +	backend=$2 &&
> +	uri=$3 &&
> +	err_msg=$4 &&
> +
> +	git -C "$repo" config set core.repositoryformatversion 1 &&
> +	if test -n "$err_msg";
> +	then
> +		git -C "$repo" config set extensions.refStorage "$uri" &&
> +		test_must_fail git -C "$repo" refs list 2>err &&
> +		test_grep "$err_msg" err
> +	else
> +		git -C "$repo" refs list >expect &&
> +		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
> +		test_cmp expect actual
> +	fi
> +}
> +
> +test_expect_success 'URI is invalid' '
> +	test_when_finished "rm -rf repo" &&
> +	git init repo &&
> +	test_refs_backend repo files "reftable@/home/reftable" \
> +		"invalid value for ${SQ}extensions.refstorage${SQ}"
> +'
> +
> +test_expect_success 'URI ends with colon' '
> +	test_when_finished "rm -rf repo" &&
> +	git init repo &&
> +	test_refs_backend repo files "reftable:" \
> +		"invalid value for ${SQ}extensions.refstorage${SQ}"
> +'
> +
> +test_expect_success 'unknown reference backend' '
> +	test_when_finished "rm -rf repo" &&
> +	git init repo &&
> +	test_refs_backend repo files "db://.git" \
> +		"invalid value for ${SQ}extensions.refstorage${SQ}"
> +'
> +
> +ref_formats="files reftable"
> +for from_format in $ref_formats
> +do
> +
> +for to_format in $ref_formats
> +do
> +	if test "$from_format" = "$to_format"
> +	then
> +		continue
> +	fi
> +
> +
> +	for dir in "$(pwd)/repo/.git" "./"

> +	do
> +
> +		test_expect_success "$read from $to_format backend, $dir dir" '
> +			test_when_finished "rm -rf repo" &&
> +			git init --ref-format=$from_format repo &&
> +			(
> +				cd repo &&
> +				test_commit 1 &&
> +				test_commit 2 &&
> +				test_commit 3 &&
> +
> +				git refs migrate --dry-run --ref-format=$to_format >out &&