refs: allow @{n} to work with n-sized reflog

This sequence works

	$ git checkout -b newbranch
	$ git commit --allow-empty -m one
	$ git show -s newbranch@{1}

and shows the state that was immediately after the newbranch was
created.

But then if you do

	$ git reflog expire --expire=now refs/heads/newbranch
	$ git commit --allow=empty -m two
	$ git show -s newbranch@{1}

you'd be scolded with

	fatal: log for 'newbranch' only has 1 entries

While it is true that it has only 1 entry, we have enough
information in that single entry that records the transition between
the state in which the tip of the branch was pointing at commit
'one' to the new commit 'two' built on it, so we should be able to
answer "what object newbranch was pointing at?". But we refuse to
do so.

Make @{0} the special case where we use the new side to look up that
entry. Otherwise, look up @{n} using the old side of the (n-1)th entry
of the reflog.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 32 insertions, 14 deletions

diff --git a/refs.c b/refs.c
index bfdd04aefd..03968ad787 100644
--- a/refs.c
+++ b/refs.c
@@ -900,40 +900,53 @@ static int read_ref_at_ent(struct object_id *ooid, struct object_id *noid,
 		const char *message, void *cb_data)
 {
 	struct read_ref_at_cb *cb = cb_data;
+	int reached_count;
 
 	cb->tz = tz;
 	cb->date = timestamp;
 
-	if (timestamp <= cb->at_time || cb->cnt == 0) {
+	/*
+	 * It is not possible for cb->cnt == 0 on the first iteration because
+	 * that special case is handled in read_ref_at().
+	 */
+	if (cb->cnt > 0)
+		cb->cnt--;
+	reached_count = cb->cnt == 0 && !is_null_oid(ooid);
+	if (timestamp <= cb->at_time || reached_count) {
 		set_read_ref_cutoffs(cb, timestamp, tz, message);
 		/*
 		 * we have not yet updated cb->[n|o]oid so they still
 		 * hold the values for the previous record.
 		 */
-		if (!is_null_oid(&cb->ooid)) {
-			oidcpy(cb->oid, noid);
-			if (!oideq(&cb->ooid, noid))
-				warning(_("log for ref %s has gap after %s"),
+		if (!is_null_oid(&cb->ooid) && !oideq(&cb->ooid, noid))
+			warning(_("log for ref %s has gap after %s"),
 					cb->refname, show_date(cb->date, cb->tz, DATE_MODE(RFC2822)));
-		}
-		else if (cb->date == cb->at_time)
+		if (reached_count)
+			oidcpy(cb->oid, ooid);
+		else if (!is_null_oid(&cb->ooid) || cb->date == cb->at_time)
 			oidcpy(cb->oid, noid);
 		else if (!oideq(noid, cb->oid))
 			warning(_("log for ref %s unexpectedly ended on %s"),
 				cb->refname, show_date(cb->date, cb->tz,
 						       DATE_MODE(RFC2822)));
-		cb->reccnt++;
-		oidcpy(&cb->ooid, ooid);
-		oidcpy(&cb->noid, noid);
 		cb->found_it = 1;
-		return 1;
 	}
 	cb->reccnt++;
 	oidcpy(&cb->ooid, ooid);
 	oidcpy(&cb->noid, noid);
-	if (cb->cnt > 0)
-		cb->cnt--;
-	return 0;
+	return cb->found_it;
+}
+
+static int read_ref_at_ent_newest(struct object_id *ooid, struct object_id *noid,
+				  const char *email, timestamp_t timestamp,
+				  int tz, const char *message, void *cb_data)
+{
+	struct read_ref_at_cb *cb = cb_data;
+
+	set_read_ref_cutoffs(cb, timestamp, tz, message);
+	oidcpy(cb->oid, noid);
+	/* We just want the first entry */
+	return 1;
 }
 
 static int read_ref_at_ent_oldest(struct object_id *ooid, struct object_id *noid,
@@ -967,6 +980,11 @@ int read_ref_at(struct ref_store *refs, const char *refname,
 	cb.cutoff_cnt = cutoff_cnt;
 	cb.oid = oid;
 
+	if (cb.cnt == 0) {
+		refs_for_each_reflog_ent_reverse(refs, refname, read_ref_at_ent_newest, &cb);
+		return 0;
+	}
+
 	refs_for_each_reflog_ent_reverse(refs, refname, read_ref_at_ent, &cb);
 
 	if (!cb.reccnt) {