[PATCH v10 7/8] worktree add: extend DWIM to infer --orphan

[Jacob Abel <jacobabel@nullpo.dev>]

Extend DWIM to try to infer `--orphan` when in an empty repository. i.e.
a repository with an invalid/unborn HEAD, no local branches, and if
`--guess-remote` is used then no remote branches.

This behavior is equivalent to `git switch -c` or `git checkout -b` in
an empty repository.

Also warn the user (overriden with `-f`/`--force`) when they likely
intend to checkout a remote branch to the worktree but have not yet
fetched from the remote. i.e. when using `--guess-remote` and there is a
remote but no local or remote refs.

Current Behavior:
% git --no-pager branch --list --remotes
% git remote
origin
% git workree add ../main
hint: If you meant to create a worktree containing a new orphan branch
[...]
hint: Disable this message with "git config advice.worktreeAddOrphan false"
fatal: invalid reference: HEAD
% git workree add --guess-remote ../main
hint: If you meant to create a worktree containing a new orphan branch
[...]
hint: Disable this message with "git config advice.worktreeAddOrphan false"
fatal: invalid reference: HEAD
% git fetch --quiet
% git --no-pager branch --list --remotes
origin/HEAD -> origin/main
origin/main
% git workree add --guess-remote ../main
Preparing worktree (new branch 'main')
branch 'main' set up to track 'origin/main'.
HEAD is now at dadc8e6dac commit message
%

New Behavior:
% git --no-pager branch --list --remotes
% git remote
origin
% git workree add ../main
No possible source branch, inferring '--orphan'
Preparing worktree (new branch 'main')
% git worktree remove ../main
% git workree add --guess-remote ../main
fatal: No local or remote refs exist despite at least one remote
present, stopping; use 'add -f' to overide or fetch a remote first
% git workree add --guess-remote -f ../main
No possible source branch, inferring '--orphan'
Preparing worktree (new branch 'main')
% git worktree remove ../main
% git fetch --quiet
% git --no-pager branch --list --remotes
origin/HEAD -> origin/main
origin/main
% git workree add --guess-remote ../main
Preparing worktree (new branch 'main')
branch 'main' set up to track 'origin/main'.
HEAD is now at dadc8e6dac commit message
%

Signed-off-by: Jacob Abel <jacobabel@nullpo.dev>
---
 Documentation/git-worktree.txt |  10 +
 builtin/worktree.c             | 114 +++++++++++-
 t/t2400-worktree-add.sh        | 326 +++++++++++++++++++++++++++++++++
 3 files changed, 449 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-worktree.txt b/Documentation/git-worktree.txt
index 485d865eb2..a4fbf5e838 100644
--- a/Documentation/git-worktree.txt
+++ b/Documentation/git-worktree.txt
@@ -95,6 +95,16 @@ exist, a new branch based on `HEAD` is automatically created as if
 `-b <branch>` was given.  If `<branch>` does exist, it will be checked out
 in the new worktree, if it's not checked out anywhere else, otherwise the
 command will refuse to create the worktree (unless `--force` is used).
++
+If `<commit-ish>` is omitted, neither `--detach`, or `--orphan` is
+used, and there are no valid local branches (or remote branches if
+`--guess-remote` is specified) then, as a convenience, the new worktree is
+associated with a new orphan branch named `<branch>` (after
+`$(basename <path>)` if neither `-b` or `-B` is used) as if `--orphan` was
+passed to the command. In the event the repository has a remote and
+`--guess-remote` is used, but no remote or local branches exist, then the
+command fails with a warning reminding the user to fetch from their remote
+first (or override by using `-f/--force`).

 list::

diff --git a/builtin/worktree.c b/builtin/worktree.c
index 15bdb380c7..093b2cb032 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -12,6 +12,7 @@
 #include "strvec.h"
 #include "branch.h"
 #include "refs.h"
+#include "remote.h"
 #include "run-command.h"
 #include "hook.h"
 #include "sigchain.h"
@@ -40,6 +41,9 @@
 #define BUILTIN_WORKTREE_UNLOCK_USAGE \
 	N_("git worktree unlock <worktree>")

+#define WORKTREE_ADD_DWIM_ORPHAN_INFER_TEXT \
+	_("No possible source branch, inferring '--orphan'")
+
 #define WORKTREE_ADD_ORPHAN_WITH_DASH_B_HINT_TEXT \
 	_("If you meant to create a worktree containing a new orphan branch\n" \
 	"(branch with no commits) for this repository, you can do so\n" \
@@ -613,6 +617,107 @@ static void print_preparing_worktree_line(int detach,
 	}
 }

+/**
+ * Callback to short circuit iteration over refs on the first reference
+ * corresponding to a valid oid.
+ *
+ * Returns 0 on failure and non-zero on success.
+ */
+static int first_valid_ref(const char *refname,
+			   const struct object_id *oid,
+			   int flags,
+			   void *cb_data)
+{
+	return 1;
+}
+
+/**
+ * Verifies HEAD and determines whether there exist any valid local references.
+ *
+ * - Checks whether HEAD points to a valid reference.
+ *
+ * - Checks whether any valid local branches exist.
+ *
+ * Returns 1 if any of the previous checks are true, otherwise returns 0.
+ */
+static int can_use_local_refs(const struct add_opts *opts)
+{
+	if (head_ref(first_valid_ref, NULL)) {
+		return 1;
+	} else if (for_each_branch_ref(first_valid_ref, NULL)) {
+		return 1;
+	}
+	return 0;
+}
+
+/**
+ * Reports whether the necessary flags were set and whether the repository has
+ * remote references to attempt DWIM tracking of upstream branches.
+ *
+ * 1. Checks that `--guess-remote` was used or `worktree.guessRemote = true`.
+ *
+ * 2. Checks whether any valid remote branches exist.
+ *
+ * 3. Checks that there exists at least one remote and emits a warning/error
+ *    if both checks 1. and 2. are false (can be bypassed with `--force`).
+ *
+ * Returns 1 if checks 1. and 2. are true, otherwise 0.
+ */
+static int can_use_remote_refs(const struct add_opts *opts)
+{
+	if (!guess_remote) {
+		if (!opts->quiet)
+			fprintf_ln(stderr, WORKTREE_ADD_DWIM_ORPHAN_INFER_TEXT);
+		return 0;
+	} else if (for_each_remote_ref(first_valid_ref, NULL)) {
+		return 1;
+	} else if (!opts->force && remote_get(NULL)) {
+		die(_("No local or remote refs exist despite at least one remote\n"
+		      "present, stopping; use 'add -f' to overide or fetch a remote first"));
+	} else if (!opts->quiet) {
+		fprintf_ln(stderr, WORKTREE_ADD_DWIM_ORPHAN_INFER_TEXT);
+	}
+	return 0;
+}
+
+/**
+ * Determines whether `--orphan` should be inferred in the evaluation of
+ * `worktree add path/` or `worktree add -b branch path/` and emits an error
+ * if the supplied arguments would produce an illegal combination when the
+ * `--orphan` flag is included.
+ *
+ * `opts` and `opt_track` contain the other options & flags supplied to the
+ * command.
+ *
+ * remote determines whether to check `can_use_remote_refs()` or not. This
+ * is primarily to differentiate between the basic `add` DWIM and `add -b`.
+ *
+ * Returns 1 when inferring `--orphan`, 0 otherwise, and emits an error when
+ * `--orphan` is inferred but doing so produces an illegal combination of
+ * options and flags. Additionally produces an error when remote refs are
+ * checked and the repo is in a state that looks like the user added a remote
+ * but forgot to fetch (and did not override the warning with -f).
+ */
+static int dwim_orphan(const struct add_opts *opts, int opt_track, int remote)
+{
+	if (can_use_local_refs(opts)) {
+		return 0;
+	} else if (remote && can_use_remote_refs(opts)) {
+		return 0;
+	} else if (!opts->quiet) {
+		fprintf_ln(stderr, WORKTREE_ADD_DWIM_ORPHAN_INFER_TEXT);
+	}
+
+	if (opt_track) {
+		die(_("'%s' and '%s' cannot be used together"), "--orphan",
+		    "--track");
+	} else if (!opts->checkout) {
+		die(_("'%s' and '%s' cannot be used together"), "--orphan",
+		    "--no-checkout");
+	}
+	return 1;
+}
+
 static const char *dwim_branch(const char *path, const char **new_branch)
 {
 	int n;
@@ -723,12 +828,19 @@ static int add(int ac, const char **av, const char *prefix)
 		int n;
 		const char *s = worktree_basename(path, &n);
 		new_branch = xstrndup(s, n);
-	} else if (new_branch || opts.detach || opts.orphan) {
+	} else if (opts.orphan || opts.detach) {
 		// No-op
+	} else if (ac < 2 && new_branch) {
+		// DWIM: Infer --orphan when repo has no refs.
+		opts.orphan = dwim_orphan(&opts, !!opt_track, 0);
 	} else if (ac < 2) {
+		// DWIM: Guess branch name from path.
 		const char *s = dwim_branch(path, &new_branch);
 		if (s)
 			branch = s;
+
+		// DWIM: Infer --orphan when repo has no refs.
+		opts.orphan = (!s) && dwim_orphan(&opts, !!opt_track, 1);
 	} else if (ac == 2) {
 		struct object_id oid;
 		struct commit *commit;
diff --git a/t/t2400-worktree-add.sh b/t/t2400-worktree-add.sh
index 46eef26179..c7ca8df586 100755
--- a/t/t2400-worktree-add.sh
+++ b/t/t2400-worktree-add.sh
@@ -705,6 +705,332 @@ test_expect_success 'git worktree --no-guess-remote option overrides config' '
 	)
 '

+test_dwim_orphan () {
+	local info_text="No possible source branch, inferring '--orphan'" &&
+	local fetch_error_text="fatal: No local or remote refs exist despite at least one remote" &&
+	local orphan_hint="hint: If you meant to create a worktree containing a new orphan branch" &&
+	local invalid_ref_regex="^fatal: invalid reference:\s\+.*" &&
+	local bad_combo_regex="^fatal: '[a-z-]\+' and '[a-z-]\+' cannot be used together" &&
+
+	local git_ns="repo" &&
+	local dashc_args="-C $git_ns" &&
+	local use_cd=0 &&
+
+	local bad_head=0 &&
+	local empty_repo=1 &&
+	local local_ref=0 &&
+	local use_quiet=0 &&
+	local remote=0 &&
+	local remote_ref=0 &&
+	local use_new_branch=0 &&
+
+	local outcome="$1" &&
+	local outcome_text &&
+	local success &&
+	shift &&
+	local args="" &&
+	local context="" &&
+	case "$outcome" in
+	"infer")
+		success=1 &&
+		outcome_text='"add" DWIM infer --orphan'
+		;;
+	"no_infer")
+		success=1 &&
+		outcome_text='"add" DWIM doesnt infer --orphan'
+		;;
+	"fetch_error")
+		success=0 &&
+		outcome_text='"add" error need fetch'
+		;;
+	"fatal_orphan_bad_combo")
+		success=0 &&
+		outcome_text='"add" error inferred "--orphan" gives illegal opts combo'
+		;;
+	*)
+		echo "test_dwim_orphan(): invalid outcome: '$outcome'" >&2 &&
+		return 1
+		;;
+	esac &&
+	while [ $# -gt 0 ]
+	do
+		case "$1" in
+		# How and from where to create the worktree
+		"-C_repo")
+			use_cd=0 &&
+			git_ns="repo" &&
+			dashc_args="-C $git_ns" &&
+			context="$context, 'git -C repo'"
+			;;
+		"-C_wt")
+			use_cd=0 &&
+			git_ns="wt" &&
+			dashc_args="-C $git_ns" &&
+			context="$context, 'git -C wt'"
+			;;
+		"cd_repo")
+			use_cd=1 &&
+			git_ns="repo" &&
+			dashc_args="" &&
+			context="$context, 'cd repo && git'"
+			;;
+		"cd_wt")
+			use_cd=1 &&
+			git_ns="wt" &&
+			dashc_args="" &&
+			context="$context, 'cd wt && git'"
+			;;
+
+		# Bypass the "pull first" warning
+		"force")
+			args="$args --force" &&
+			context="$context, --force"
+			;;
+
+		# Try to use remote refs when DWIM
+		"guess_remote")
+			args="$args --guess-remote" &&
+			context="$context, --guess-remote"
+			;;
+		"no_guess_remote")
+			args="$args --no-guess-remote" &&
+			context="$context, --no-guess-remote"
+			;;
+
+		# Whether there is at least one local branch present
+		"local_ref")
+			empty_repo=0 &&
+			local_ref=1 &&
+			context="$context, >=1 local branches"
+			;;
+		"no_local_ref")
+			empty_repo=0 &&
+			context="$context, 0 local branches"
+			;;
+
+		# Whether the HEAD points at a valid ref (skip this opt when no refs)
+		"good_head")
+			# requires: local_ref
+			context="$context, valid HEAD"
+			;;
+		"bad_head")
+			bad_head=1 &&
+			context="$context, invalid (or orphan) HEAD"
+			;;
+
+		# Whether the code path is tested with the base add command or -b
+		"no_-b")
+			use_new_branch=0 &&
+			context="$context, no --branch"
+			;;
+		"-b")
+			use_new_branch=1 &&
+			context="$context, --branch"
+			;;
+
+		# Whether to check that all output is suppressed (except errors)
+		# or that the output is as expected
+		"quiet")
+			use_quiet=1 &&
+			args="$args --quiet" &&
+			context="$context, --quiet"
+			;;
+		"no_quiet")
+			use_quiet=0 &&
+			context="$context, no --quiet (expect output)"
+			;;
+
+		# Whether there is at least one remote attached to the repo
+		"remote")
+			empty_repo=0 &&
+			remote=1 &&
+			context="$context, >=1 remotes"
+			;;
+		"no_remote")
+			empty_repo=0 &&
+			remote=0 &&
+			context="$context, 0 remotes"
+			;;
+
+		# Whether there is at least one valid remote ref
+		"remote_ref")
+			# requires: remote
+			empty_repo=0 &&
+			remote_ref=1 &&
+			context="$context, >=1 fetched remote branches"
+			;;
+		"no_remote_ref")
+			empty_repo=0 &&
+			remote_ref=0 &&
+			context="$context, 0 fetched remote branches"
+			;;
+
+		# Options or flags that become illegal when --orphan is inferred
+		"no_checkout")
+			args="$args --no-checkout" &&
+			context="$context, --no-checkout"
+			;;
+		"track")
+			args="$args --track" &&
+			context="$context, --track"
+			;;
+
+		# All other options are illegal
+		*)
+			echo "test_dwim_orphan(): invalid arg: '$1'" >&2 &&
+			return 1
+			;;
+		esac &&
+		shift
+	done &&
+	context="${context#', '}" &&
+	if [ $use_new_branch -eq 1 ]
+	then
+		args="$args -b foo"
+	else
+		context="DWIM (no --branch), $context"
+	fi &&
+	if [ $empty_repo -eq 1 ]
+	then
+		context="empty repo, $context"
+	fi &&
+	args="$args ../foo" &&
+	context="${context%', '}" &&
+	test_expect_success "$outcome_text w/ $context" '
+		test_when_finished "rm -rf repo" &&
+		git init repo &&
+		if [ $local_ref -eq 1 ] && [ "$git_ns" = "repo" ]
+		then
+			(cd repo && test_commit commit) &&
+			if [ $bad_head -eq 1 ]
+			then
+				git -C repo symbolic-ref HEAD refs/heads/badbranch
+			fi
+		elif [ $local_ref -eq 1 ] && [ "$git_ns" = "wt" ]
+		then
+			test_when_finished "git -C repo worktree remove -f ../wt" &&
+			git -C repo worktree add --orphan -b main ../wt &&
+			(cd wt && test_commit commit) &&
+			if [ $bad_head -eq 1 ]
+			then
+				git -C wt symbolic-ref HEAD refs/heads/badbranch
+			fi
+		elif [ $local_ref -eq 0 ] && [ "$git_ns" = "wt" ]
+		then
+			test_when_finished "git -C repo worktree remove -f ../wt" &&
+			git -C repo worktree add --orphan -b orphanbranch ../wt
+		fi &&
+
+		if [ $remote -eq 1 ]
+		then
+			test_when_finished "rm -rf upstream" &&
+			git init upstream &&
+			(cd upstream && test_commit commit) &&
+			git -C upstream switch -c foo &&
+			git -C repo remote add upstream ../upstream
+		fi &&
+
+		if [ $remote_ref -eq 1 ]
+		then
+			git -C repo fetch
+		fi &&
+		if [ $success -eq 1 ]
+		then
+			test_when_finished git -C repo worktree remove ../foo
+		fi &&
+		(
+			if [ $use_cd -eq 1 ]
+			then
+				cd $git_ns
+			fi &&
+			if [ "$outcome" = "infer" ]
+			then
+				git $dashc_args worktree add $args 2>actual &&
+				if [ $use_quiet -eq 1 ]
+				then
+					test_must_be_empty actual
+				else
+					grep "$info_text" actual
+				fi
+			elif [ "$outcome" = "no_infer" ]
+			then
+				git $dashc_args worktree add $args 2>actual &&
+				if [ $use_quiet -eq 1 ]
+				then
+					test_must_be_empty actual
+				else
+					! grep "$info_text" actual
+				fi
+			elif [ "$outcome" = "fetch_error" ]
+			then
+				test_must_fail git $dashc_args worktree add $args 2>actual &&
+				grep "$fetch_error_text" actual
+			elif [ "$outcome" = "fatal_orphan_bad_combo" ]
+			then
+				test_must_fail git $dashc_args worktree add $args 2>actual &&
+				if [ $use_quiet -eq 1 ]
+				then
+					! grep "$info_text" actual
+				else
+					grep "$info_text" actual
+				fi &&
+				grep "$bad_combo_regex" actual
+			elif [ "$outcome" = "warn_bad_head" ]
+			then
+				test_must_fail git $dashc_args worktree add $args 2>actual &&
+				if [ $use_quiet -eq 1 ]
+				then
+					grep "$invalid_ref_regex" actual &&
+					! grep "$orphan_hint" actual
+				else
+					headpath=$(git $dashc_args rev-parse --sq --path-format=absolute --git-path HEAD) &&
+					headcontents=$(cat "$headpath") &&
+					grep "HEAD points to an invalid (or orphaned) reference" actual &&
+					grep "HEAD path:\s*.$headpath." actual &&
+					grep "HEAD contents:\s*.$headcontents." actual &&
+					grep "$orphan_hint" actual &&
+					! grep "$info_text" actual
+				fi &&
+				grep "$invalid_ref_regex" actual
+			else
+				# Unreachable
+				false
+			fi
+		) &&
+		if [ $success -ne 1 ]
+		then
+			test_path_is_missing foo
+		fi
+	'
+}
+
+for quiet_mode in "no_quiet" "quiet"
+do
+	for changedir_type in "cd_repo" "cd_wt" "-C_repo" "-C_wt"
+	do
+		dwim_test_args="$quiet_mode $changedir_type"
+		test_dwim_orphan 'infer' $dwim_test_args no_-b
+		test_dwim_orphan 'no_infer' $dwim_test_args no_-b local_ref good_head
+		test_dwim_orphan 'infer' $dwim_test_args no_-b no_local_ref no_remote no_remote_ref no_guess_remote
+		test_dwim_orphan 'infer' $dwim_test_args no_-b no_local_ref remote no_remote_ref no_guess_remote
+		test_dwim_orphan 'fetch_error' $dwim_test_args no_-b no_local_ref remote no_remote_ref guess_remote
+		test_dwim_orphan 'infer' $dwim_test_args no_-b no_local_ref remote no_remote_ref guess_remote force
+		test_dwim_orphan 'no_infer' $dwim_test_args no_-b no_local_ref remote remote_ref guess_remote
+
+		test_dwim_orphan 'infer' $dwim_test_args -b
+		test_dwim_orphan 'no_infer' $dwim_test_args -b local_ref good_head
+		test_dwim_orphan 'infer' $dwim_test_args -b no_local_ref no_remote no_remote_ref no_guess_remote
+		test_dwim_orphan 'infer' $dwim_test_args -b no_local_ref remote no_remote_ref no_guess_remote
+		test_dwim_orphan 'infer' $dwim_test_args -b no_local_ref remote no_remote_ref guess_remote
+		test_dwim_orphan 'infer' $dwim_test_args -b no_local_ref remote remote_ref guess_remote
+	done
+
+	test_dwim_orphan 'fatal_orphan_bad_combo' $quiet_mode no_-b no_checkout
+	test_dwim_orphan 'fatal_orphan_bad_combo' $quiet_mode no_-b track
+	test_dwim_orphan 'fatal_orphan_bad_combo' $quiet_mode -b no_checkout
+	test_dwim_orphan 'fatal_orphan_bad_combo' $quiet_mode -b track
+done
+
 post_checkout_hook () {
 	test_when_finished "rm -rf .git/hooks" &&
 	mkdir .git/hooks &&
--
2.39.3