status: show more info than "currently not on any branch"

When a remote ref or a tag is checked out, HEAD is automatically
detached. There is no user-friendly way to find out what ref is
checked out in this case. This patch digs in reflog for this
information and shows "HEAD detached from origin/master" or "HEAD
detached at v1.8.0" instead of "currently not on any branch".

When it cannot figure out the original ref, it shows an abbreviated
SHA-1. "Currently not on any branch" would never display (unless
reflog is pruned to near empty that the last checkout entry is lost).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 125 insertions, 26 deletions

diff --git a/t/t7406-submodule-update.sh b/t/t7406-submodule-update.sh
index 4975ec07ce..50ac02028d 100755
--- a/t/t7406-submodule-update.sh
+++ b/t/t7406-submodule-update.sh
@@ -664,8 +664,10 @@ test_expect_success 'submodule add properly re-creates deeper level submodules'
 
 test_expect_success 'submodule update properly revives a moved submodule' '
 	(cd super &&
+	 H=$(git rev-parse --short HEAD) &&
 	 git commit -am "pre move" &&
-	 git status >expect&&
+	 H2=$(git rev-parse --short HEAD) &&
+	 git status | sed "s/$H/XXX/" >expect &&
 	 H=$(cd submodule2; git rev-parse HEAD) &&
 	 git rm --cached submodule2 &&
 	 rm -rf submodule2 &&
@@ -674,7 +676,7 @@ test_expect_success 'submodule update properly revives a moved submodule' '
 	 git config -f .gitmodules submodule.submodule2.path "moved/sub module"
 	 git commit -am "post move" &&
 	 git submodule update &&
-	 git status >actual &&
+	 git status | sed "s/$H2/XXX/" >actual &&
 	 test_cmp expect actual
 	)
 '
diff --git a/t/t7512-status-help.sh b/t/t7512-status-help.sh
index 51ab89411f..5adba4f5e4 100755
--- a/t/t7512-status-help.sh
+++ b/t/t7512-status-help.sh
@@ -76,7 +76,7 @@ test_expect_success 'status when rebase in progress before resolving conflicts'
 	ONTO=$(git rev-parse --short HEAD^^) &&
 	test_must_fail git rebase HEAD^ --onto HEAD^^ &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached at $ONTO
 	# You are currently rebasing branch '\''rebase_conflicts'\'' on '\''$ONTO'\''.
 	#   (fix conflicts and then run "git rebase --continue")
 	#   (use "git rebase --skip" to skip this patch)
@@ -103,7 +103,7 @@ test_expect_success 'status when rebase in progress before rebase --continue' '
 	echo three >main.txt &&
 	git add main.txt &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached at $ONTO
 	# You are currently rebasing branch '\''rebase_conflicts'\'' on '\''$ONTO'\''.
 	#   (all conflicts fixed: run "git rebase --continue")
 	#
@@ -135,7 +135,7 @@ test_expect_success 'status during rebase -i when conflicts unresolved' '
 	ONTO=$(git rev-parse --short rebase_i_conflicts) &&
 	test_must_fail git rebase -i rebase_i_conflicts &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached at $ONTO
 	# You are currently rebasing branch '\''rebase_i_conflicts_second'\'' on '\''$ONTO'\''.
 	#   (fix conflicts and then run "git rebase --continue")
 	#   (use "git rebase --skip" to skip this patch)
@@ -161,7 +161,7 @@ test_expect_success 'status during rebase -i after resolving conflicts' '
 	test_must_fail git rebase -i rebase_i_conflicts &&
 	git add main.txt &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached at $ONTO
 	# You are currently rebasing branch '\''rebase_i_conflicts_second'\'' on '\''$ONTO'\''.
 	#   (all conflicts fixed: run "git rebase --continue")
 	#
@@ -187,9 +187,10 @@ test_expect_success 'status when rebasing -i in edit mode' '
 	export FAKE_LINES &&
 	test_when_finished "git rebase --abort" &&
 	ONTO=$(git rev-parse --short HEAD~2) &&
+	TGT=$(git rev-parse --short two_rebase_i) &&
 	git rebase -i HEAD~2 &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $TGT
 	# You are currently editing a commit while rebasing branch '\''rebase_i_edit'\'' on '\''$ONTO'\''.
 	#   (use "git commit --amend" to amend the current commit)
 	#   (use "git rebase --continue" once you are satisfied with your changes)
@@ -214,8 +215,9 @@ test_expect_success 'status when splitting a commit' '
 	ONTO=$(git rev-parse --short HEAD~3) &&
 	git rebase -i HEAD~3 &&
 	git reset HEAD^ &&
+	TGT=$(git rev-parse --short HEAD) &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached at $TGT
 	# You are currently splitting a commit while rebasing branch '\''split_commit'\'' on '\''$ONTO'\''.
 	#   (Once your working directory is clean, run "git rebase --continue")
 	#
@@ -243,10 +245,11 @@ test_expect_success 'status after editing the last commit with --amend during a
 	export FAKE_LINES &&
 	test_when_finished "git rebase --abort" &&
 	ONTO=$(git rev-parse --short HEAD~3) &&
+	TGT=$(git rev-parse --short three_amend) &&
 	git rebase -i HEAD~3 &&
 	git commit --amend -m "foo" &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $TGT
 	# You are currently editing a commit while rebasing branch '\''amend_last'\'' on '\''$ONTO'\''.
 	#   (use "git commit --amend" to amend the current commit)
 	#   (use "git rebase --continue" once you are satisfied with your changes)
@@ -276,7 +279,7 @@ test_expect_success 'status: (continue first edit) second edit' '
 	git rebase -i HEAD~3 &&
 	git rebase --continue &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently editing a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (use "git commit --amend" to amend the current commit)
 	#   (use "git rebase --continue" once you are satisfied with your changes)
@@ -298,7 +301,7 @@ test_expect_success 'status: (continue first edit) second edit and split' '
 	git rebase --continue &&
 	git reset HEAD^ &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently splitting a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (Once your working directory is clean, run "git rebase --continue")
 	#
@@ -325,7 +328,7 @@ test_expect_success 'status: (continue first edit) second edit and amend' '
 	git rebase --continue &&
 	git commit --amend -m "foo" &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently editing a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (use "git commit --amend" to amend the current commit)
 	#   (use "git rebase --continue" once you are satisfied with your changes)
@@ -347,7 +350,7 @@ test_expect_success 'status: (amend first edit) second edit' '
 	git commit --amend -m "a" &&
 	git rebase --continue &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently editing a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (use "git commit --amend" to amend the current commit)
 	#   (use "git rebase --continue" once you are satisfied with your changes)
@@ -370,7 +373,7 @@ test_expect_success 'status: (amend first edit) second edit and split' '
 	git rebase --continue &&
 	git reset HEAD^ &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently splitting a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (Once your working directory is clean, run "git rebase --continue")
 	#
@@ -398,7 +401,7 @@ test_expect_success 'status: (amend first edit) second edit and amend' '
 	git rebase --continue &&
 	git commit --amend -m "d" &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently editing a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (use "git commit --amend" to amend the current commit)
 	#   (use "git rebase --continue" once you are satisfied with your changes)
@@ -422,7 +425,7 @@ test_expect_success 'status: (split first edit) second edit' '
 	git commit -m "e" &&
 	git rebase --continue &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently editing a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (use "git commit --amend" to amend the current commit)
 	#   (use "git rebase --continue" once you are satisfied with your changes)
@@ -447,7 +450,7 @@ test_expect_success 'status: (split first edit) second edit and split' '
 	git rebase --continue &&
 	git reset HEAD^ &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently splitting a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (Once your working directory is clean, run "git rebase --continue")
 	#
@@ -477,7 +480,7 @@ test_expect_success 'status: (split first edit) second edit and amend' '
 	git rebase --continue &&
 	git commit --amend -m "h" &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached from $ONTO
 	# You are currently editing a commit while rebasing branch '\''several_edits'\'' on '\''$ONTO'\''.
 	#   (use "git commit --amend" to amend the current commit)
 	#   (use "git rebase --continue" once you are satisfied with your changes)
@@ -572,8 +575,9 @@ test_expect_success 'status when bisecting' '
 	git bisect start &&
 	git bisect bad &&
 	git bisect good one_bisect &&
-	cat >expected <<-\EOF &&
-	# Not currently on any branch.
+	TGT=$(git rev-parse --short two_bisect) &&
+	cat >expected <<-EOF &&
+	# HEAD detached at $TGT
 	# You are currently bisecting branch '\''bisect'\''.
 	#   (use "git bisect reset" to get back to the original branch)
 	#
@@ -596,7 +600,7 @@ test_expect_success 'status when rebase conflicts with statushints disabled' '
 	ONTO=$(git rev-parse --short HEAD^^) &&
 	test_must_fail git rebase HEAD^ --onto HEAD^^ &&
 	cat >expected <<-EOF &&
-	# Not currently on any branch.
+	# HEAD detached at $ONTO
 	# You are currently rebasing branch '\''statushints_disabled'\'' on '\''$ONTO'\''.
 	#
 	# Unmerged paths:
@@ -662,5 +666,15 @@ test_expect_success 'status when cherry-picking after resolving conflicts' '
 	test_i18ncmp expected actual
 '
 
+test_expect_success 'status showing detached from a tag' '
+	test_commit atag tagging &&
+	git checkout atag &&
+	cat >expected <<-\EOF
+	# HEAD detached at atag
+	nothing to commit (use -u to show untracked files)
+	EOF
+	git status --untracked-files=no >actual &&
+	test_i18ncmp expected actual
+'
 
 test_done
diff --git a/wt-status.c b/wt-status.c
index 40dced3ef9..32a51e1323 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -999,7 +999,73 @@ got_nothing:
 	return NULL;
 }
 
-void wt_status_get_state(struct wt_status_state *state)
+struct grab_1st_switch_cbdata {
+	int found;
+	struct strbuf buf;
+	unsigned char nsha1[20];
+};
+
+static int grab_1st_switch(unsigned char *osha1, unsigned char *nsha1,
+			   const char *email, unsigned long timestamp, int tz,
+			   const char *message, void *cb_data)
+{
+	struct grab_1st_switch_cbdata *cb = cb_data;
+	const char *target = NULL, *end;
+
+	if (prefixcmp(message, "checkout: moving from "))
+		return 0;
+	message += strlen("checkout: moving from ");
+	target = strstr(message, " to ");
+	if (!target)
+		return 0;
+	target += strlen(" to ");
+	strbuf_reset(&cb->buf);
+	hashcpy(cb->nsha1, nsha1);
+	for (end = target; *end && *end != '\n'; end++)
+		;
+	strbuf_add(&cb->buf, target, end - target);
+	cb->found = 1;
+	return 1;
+}
+
+static void wt_status_get_detached_from(struct wt_status_state *state)
+{
+	struct grab_1st_switch_cbdata cb;
+	struct commit *commit;
+	unsigned char sha1[20];
+	char *ref = NULL;
+
+	strbuf_init(&cb.buf, 0);
+	if (for_each_reflog_ent_reverse("HEAD", grab_1st_switch, &cb) <= 0) {
+		strbuf_release(&cb.buf);
+		return;
+	}
+
+	if (dwim_ref(cb.buf.buf, cb.buf.len, sha1, &ref) == 1 &&
+	    /* sha1 is a commit? match without further lookup */
+	    (!hashcmp(cb.nsha1, sha1) ||
+	     /* perhaps sha1 is a tag, try to dereference to a commit */
+	     ((commit = lookup_commit_reference_gently(sha1, 1)) != NULL &&
+	      !hashcmp(cb.nsha1, commit->object.sha1)))) {
+		int ofs;
+		if (!prefixcmp(ref, "refs/tags/"))
+			ofs = strlen("refs/tags/");
+		else if (!prefixcmp(ref, "refs/remotes/"))
+			ofs = strlen("refs/remotes/");
+		else
+			ofs = 0;
+		state->detached_from = xstrdup(ref + ofs);
+	} else
+		state->detached_from =
+			xstrdup(find_unique_abbrev(cb.nsha1, DEFAULT_ABBREV));
+	hashcpy(state->detached_sha1, cb.nsha1);
+
+	free(ref);
+	strbuf_release(&cb.buf);
+}
+
+void wt_status_get_state(struct wt_status_state *state,
+			 int get_detached_from)
 {
 	struct stat st;
 
@@ -1029,6 +1095,9 @@ void wt_status_get_state(struct wt_status_state *state)
 		state->bisect_in_progress = 1;
 		state->branch = read_and_strip_branch("BISECT_START");
 	}
+
+	if (get_detached_from)
+		wt_status_get_detached_from(state);
 }
 
 static void wt_status_print_state(struct wt_status *s,
@@ -1054,7 +1123,8 @@ void wt_status_print(struct wt_status *s)
 	struct wt_status_state state;
 
 	memset(&state, 0, sizeof(state));
-	wt_status_get_state(&state);
+	wt_status_get_state(&state,
+			    s->branch && !strcmp(s->branch, "HEAD"));
 
 	if (s->branch) {
 		const char *on_what = _("On branch ");
@@ -1062,9 +1132,19 @@ void wt_status_print(struct wt_status *s)
 		if (!prefixcmp(branch_name, "refs/heads/"))
 			branch_name += 11;
 		else if (!strcmp(branch_name, "HEAD")) {
-			branch_name = "";
 			branch_status_color = color(WT_STATUS_NOBRANCH, s);
-			on_what = _("Not currently on any branch.");
+			if (state.detached_from) {
+				unsigned char sha1[20];
+				branch_name = state.detached_from;
+				if (!get_sha1("HEAD", sha1) &&
+				    !hashcmp(sha1, state.detached_sha1))
+					on_what = _("HEAD detached at ");
+				else
+					on_what = _("HEAD detached from ");
+			} else {
+				branch_name = "";
+				on_what = _("Not currently on any branch.");
+			}
 		}
 		status_printf(s, color(WT_STATUS_HEADER, s), "");
 		status_printf_more(s, branch_status_color, "%s", on_what);
@@ -1076,6 +1156,7 @@ void wt_status_print(struct wt_status *s)
 	wt_status_print_state(s, &state);
 	free(state.branch);
 	free(state.onto);
+	free(state.detached_from);
 
 	if (s->is_initial) {
 		status_printf_ln(s, color(WT_STATUS_HEADER, s), "");
diff --git a/wt-status.h b/wt-status.h
index 5ddcbf6b17..5cb7df9157 100644
--- a/wt-status.h
+++ b/wt-status.h
@@ -81,12 +81,14 @@ struct wt_status_state {
 	int bisect_in_progress;
 	char *branch;
 	char *onto;
+	char *detached_from;
+	unsigned char detached_sha1[20];
 };
 
 void wt_status_prepare(struct wt_status *s);
 void wt_status_print(struct wt_status *s);
 void wt_status_collect(struct wt_status *s);
-void wt_status_get_state(struct wt_status_state *state);
+void wt_status_get_state(struct wt_status_state *state, int get_detached_from);
 
 void wt_shortstatus_print(struct wt_status *s);
 void wt_porcelain_print(struct wt_status *s);