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>

1 files changed, 85 insertions, 4 deletions

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), "");