[PATCH v2] checkout: teach --worktree

[Denton Liu <liu.denton@gmail.com>]

A complaint that has come up frequently in the past is that it is not
possible to checkout files directly into a worktree without modifying
the index. Even though this could be worked around by redirecting the
output of `git show` to overwrite files, this was not feasible if one
wanted to use patch mode.

Since `git restore` was implemented, this has since been possible using
the `--worktree` option. However, some long-time users of Git still
prefer to use `git checkout` over `git restore` and would like to see
the functionality ported over.

Teach `git checkout --worktree`, allowing users to checkout files
directly into the worktree without affecting the index.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
---
I realised that making `git checkout --worktree` without a pathspec to
check out the current directory doesn't really make sense so I didn't
make that change.

Range-diff against v1:
1:  d10cb03dd8 ! 1:  2a434d3284 checkout: teach --worktree
    @@ Documentation/git-checkout.txt: When switching branches with `--merge`, staged c
     +-W::
     +--worktree::
     +	When writing contents, only modify files in the worktree. Do not
    -+	modify the index. This option is essentially a no-op when used
    -+	without a `<tree-ish>`.
    ++	modify the index.  This option does not make sense without a
    ++	`<pathspec>`.
     +
      -p::
      --patch::
    @@ builtin/checkout.c: static int checkout_main(int argc, const char **argv, const
     +
     +	opts->checkout_index = 0;
     +	opts->checkout_worktree = 1;
    ++	opts->empty_pathspec_ok = 0;
     +
     +	return 0;
     +}
    @@ builtin/checkout.c: int cmd_checkout(int argc, const char **argv, const char *pr
      		OPT_STRING('B', NULL, &opts.new_branch_force, N_("branch"),
      			   N_("create/reset and checkout a branch")),
     +		OPT_CALLBACK_F('W', "worktree", &opts, NULL,
    -+			   N_("restore the working tree (default)"),
    ++			   N_("restore the working tree"),
     +			   PARSE_OPT_NOARG | PARSE_OPT_NONEG, handle_worktree_opt),
      		OPT_BOOL('l', NULL, &opts.new_branch_log, N_("create reflog for new branch")),
      		OPT_BOOL(0, "guess", &opts.dwim_new_local_branch,
    @@ t/t2028-checkout-worktree.sh (new)
     +	test_cmp_rev HEAD tip
     +'
     +
    ++test_expect_success 'checkout --worktree without pathspec fails' '
    ++	test_must_fail git checkout --worktree
    ++'
    ++
     +test_expect_success 'checkout --no-worktree fails' '
     +	test_must_fail git checkout --no-worktree
     +'

 Documentation/git-checkout.txt | 22 +++++++++-----
 builtin/checkout.c             | 18 +++++++++++
 t/t2028-checkout-worktree.sh   | 55 ++++++++++++++++++++++++++++++++++
 t/t9902-completion.sh          |  1 +
 4 files changed, 89 insertions(+), 7 deletions(-)
 create mode 100755 t/t2028-checkout-worktree.sh

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 5b697eee1b..c303839920 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -12,9 +12,9 @@ SYNOPSIS
 'git checkout' [-q] [-f] [-m] --detach [<branch>]
 'git checkout' [-q] [-f] [-m] [--detach] <commit>
 'git checkout' [-q] [-f] [-m] [[-b|-B|--orphan] <new_branch>] [<start_point>]
-'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] [--] <pathspec>...
-'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] --pathspec-from-file=<file> [--pathspec-file-nul]
-'git checkout' (-p|--patch) [<tree-ish>] [--] [<pathspec>...]
+'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [--worktree] [<tree-ish>] [--] <pathspec>...
+'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [--worktree] [<tree-ish>] --pathspec-from-file=<file> [--pathspec-file-nul]
+'git checkout' (-p|--patch) [--worktree] [<tree-ish>] [--] [<pathspec>...]
 
 DESCRIPTION
 -----------
@@ -79,14 +79,16 @@ be used to detach `HEAD` at the tip of the branch (`git checkout
 +
 Omitting `<branch>` detaches `HEAD` at the tip of the current branch.
 
-'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] [--] <pathspec>...::
-'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] --pathspec-from-file=<file> [--pathspec-file-nul]::
+'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [--worktree] [<tree-ish>] [--] <pathspec>...::
+'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [--worktree] [<tree-ish>] --pathspec-from-file=<file> [--pathspec-file-nul]::
 
 	Overwrite the contents of the files that match the pathspec.
 	When the `<tree-ish>` (most often a commit) is not given,
 	overwrite working tree with the contents in the index.
 	When the `<tree-ish>` is given, overwrite both the index and
-	the working tree with the contents at the `<tree-ish>`.
+	the working tree with the contents at the `<tree-ish>` unless
+	`--worktree` is given in which case _only_ the working tree is
+	overwritten.
 +
 The index may contain unmerged entries because of a previous failed merge.
 By default, if you try to check out such an entry from the index, the
@@ -96,7 +98,7 @@ specific side of the merge can be checked out of the index by
 using `--ours` or `--theirs`.  With `-m`, changes made to the working tree
 file can be discarded to re-create the original conflicted merge result.
 
-'git checkout' (-p|--patch) [<tree-ish>] [--] [<pathspec>...]::
+'git checkout' (-p|--patch) [--worktree] [<tree-ish>] [--] [<pathspec>...]::
 	This is similar to the previous mode, but lets you use the
 	interactive interface to show the "diff" output and choose which
 	hunks to use in the result.  See below for the description of
@@ -264,6 +266,12 @@ When switching branches with `--merge`, staged changes may be lost.
 	"merge" (default) and "diff3" (in addition to what is shown by
 	"merge" style, shows the original contents).
 
+-W::
+--worktree::
+	When writing contents, only modify files in the worktree. Do not
+	modify the index.  This option does not make sense without a
+	`<pathspec>`.
+
 -p::
 --patch::
 	Interactively select hunks in the difference between the
diff --git a/builtin/checkout.c b/builtin/checkout.c
index af849c644f..8c533a305b 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -1741,6 +1741,21 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 		return checkout_branch(opts, &new_branch_info);
 }
 
+static int handle_worktree_opt(const struct option *opt, const char *arg, int unset)
+{
+	struct checkout_opts *opts = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+	BUG_ON_OPT_ARG(arg);
+
+	opts->checkout_index = 0;
+	opts->checkout_worktree = 1;
+	opts->empty_pathspec_ok = 0;
+
+	return 0;
+}
+
+
 int cmd_checkout(int argc, const char **argv, const char *prefix)
 {
 	struct checkout_opts opts;
@@ -1750,6 +1765,9 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 			   N_("create and checkout a new branch")),
 		OPT_STRING('B', NULL, &opts.new_branch_force, N_("branch"),
 			   N_("create/reset and checkout a branch")),
+		OPT_CALLBACK_F('W', "worktree", &opts, NULL,
+			   N_("restore the working tree"),
+			   PARSE_OPT_NOARG | PARSE_OPT_NONEG, handle_worktree_opt),
 		OPT_BOOL('l', NULL, &opts.new_branch_log, N_("create reflog for new branch")),
 		OPT_BOOL(0, "guess", &opts.dwim_new_local_branch,
 			 N_("second guess 'git checkout <no-such-branch>' (default)")),
diff --git a/t/t2028-checkout-worktree.sh b/t/t2028-checkout-worktree.sh
new file mode 100755
index 0000000000..b5b5e287c1
--- /dev/null
+++ b/t/t2028-checkout-worktree.sh
@@ -0,0 +1,55 @@
+#!/bin/sh
+
+test_description='checkout --worktree'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	echo first >file1 &&
+	echo file2 >file2 &&
+	git add file1 file2 &&
+	git commit -m first &&
+
+	echo second >file1 &&
+	git commit -am second &&
+	git tag tip
+'
+
+test_expect_success 'checkout --worktree on a commit' '
+	test_when_finished "git reset --hard tip" &&
+	git diff HEAD HEAD~ >expect &&
+	git checkout --worktree HEAD~ file1 &&
+	git diff >actual &&
+	test_cmp expect actual &&
+	git diff --cached --exit-code &&
+	test_cmp_rev HEAD tip
+'
+
+test_expect_success 'checkout --worktree with no commit' '
+	test_when_finished "git reset --hard tip" &&
+	echo worktree >file1 &&
+	git checkout --worktree file1 &&
+	git diff --exit-code &&
+	test_cmp_rev HEAD tip
+'
+
+test_expect_success 'checkout --worktree without pathspec fails' '
+	test_must_fail git checkout --worktree
+'
+
+test_expect_success 'checkout --no-worktree fails' '
+	test_must_fail git checkout --no-worktree
+'
+
+test_expect_success PERL 'git checkout -p --worktree' '
+	test_when_finished "git reset --hard tip" &&
+	echo changed >file2 &&
+	git diff -R --src-prefix=b/ --dst-prefix=a/ >expect &&
+	git commit -am file12 &&
+	test_write_lines n y | git checkout --worktree -p HEAD~2 &&
+	git diff >actual &&
+	test_cmp expect actual &&
+	git diff --cached --exit-code
+'
+
+test_done
diff --git a/t/t9902-completion.sh b/t/t9902-completion.sh
index 3c44af6940..1db0bb3a31 100755
--- a/t/t9902-completion.sh
+++ b/t/t9902-completion.sh
@@ -1481,6 +1481,7 @@ test_expect_success 'double dash "git checkout"' '
 	--quiet Z
 	--detach Z
 	--track Z
+	--worktree Z
 	--orphan=Z
 	--ours Z
 	--theirs Z
-- 
2.27.0.132.g321788e831