[PATCH 4/5] status: show the ref that is checked out, even if it's detached

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

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 "Detached from origin/master" or "Detached from
v1.8.0" instead of "Currently not on any branch".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 wt-status.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
 wt-status.h |  3 ++-
 2 files changed, 70 insertions(+), 5 deletions(-)

diff --git a/wt-status.c b/wt-status.c
index 6a3566b..4b6421a 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -1001,7 +1001,60 @@ static void read_and_strip_branch(struct strbuf *sb,
 		*branch = xstrdup(*branch);
 }
 
-void wt_status_get_state(struct wt_status_state *state)
+struct grab_1st_switch_cbdata {
+	struct strbuf buf;
+	unsigned char sha1[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;
+
+	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->sha1, nsha1);
+	if (!prefixcmp(target, "refs/")) {
+		const char *end = target;
+		while (*end && *end != '\n')
+			end++;
+		strbuf_add(&cb->buf, target, end - target);
+	}
+	return 0;
+}
+
+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];
+
+	strbuf_init(&cb.buf, 0);
+	for_each_recent_reflog_ent("HEAD", grab_1st_switch, 40960, &cb);
+	if (cb.buf.len &&
+	    !read_ref(cb.buf.buf, sha1) &&
+	    (commit = lookup_commit_reference_gently(sha1, 1)) != NULL &&
+	    !hashcmp(cb.sha1, commit->object.sha1)) {
+		int ofs = 0;
+		if (!prefixcmp(cb.buf.buf, "refs/tags/"))
+			ofs = strlen("refs/tags/");
+		else if (!prefixcmp(cb.buf.buf, "refs/remotes/"))
+			ofs = strlen("refs/remotes/");
+		state->detached_from = xstrdup(cb.buf.buf + ofs);
+	}
+	strbuf_release(&cb.buf);
+}
+
+void wt_status_get_state(struct wt_status_state *state,
+			 int get_detached_from)
 {
 	struct strbuf branch = STRBUF_INIT;
 	struct strbuf onto = STRBUF_INIT;
@@ -1040,6 +1093,10 @@ void wt_status_get_state(struct wt_status_state *state)
 		read_and_strip_branch(&branch, &state->branch,
 				      "BISECT_START");
 	}
+
+	if (get_detached_from)
+		wt_status_get_detached_from(state);
+
 	strbuf_release(&branch);
 	strbuf_release(&onto);
 }
@@ -1066,7 +1123,8 @@ void wt_status_print(struct wt_status *s)
 	const char *branch_status_color = color(WT_STATUS_HEADER, s);
 	struct wt_status_state 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 ");
@@ -1074,9 +1132,14 @@ 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) {
+				branch_name = state.detached_from;
+				on_what = _("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);
@@ -1088,6 +1151,7 @@ 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 5ddcbf6..74c9055 100644
--- a/wt-status.h
+++ b/wt-status.h
@@ -81,12 +81,13 @@ struct wt_status_state {
 	int bisect_in_progress;
 	char *branch;
 	char *onto;
+	char *detached_from;
 };
 
 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);
-- 
1.8.1.2.536.gf441e6d