[PATCH 6/6] reflog-walk: stop using fake parents

[Jeff King <peff@peff.net>]

The reflog-walk system works by putting a ref's tip into the
pending queue, and then "traversing" the reflog by
pretending that the parent of each commit is the previous
reflog entry.

This causes a number of user-visible oddities, as documented
in t1414 (and the commit message which introduced it). We
can fix all of them in one go by replacing the fake-reflog
system with a much simpler one: just keeping a list of
reflogs to show, and walking through them entry by entry.

The implementation is fairly straight-forward, but there are
a few items to note:

  1. We obviously must skip calling add_parents_to_list()
     when we are traversing reflogs, since we do not want to
     walk the original parents at all.

     As a result, we must call try_to_simplify_commit()
     ourselves and skip any TREESAME commits.

     There are other parts of add_parents_to_list() we skip,
     as well, but none of them should matter for a reflog
     traversal:

       -  We do not allow UNINTERESTING commits, nor
          symmetric ranges (and we bail when these are used
          with "-g").

       - Using --source makes no sense, since we aren't
         traversing. The reflog selector shows the same
         information with more detail.

       - Using --first-parent is still sensible, since you
         may want to see the first-parent diff for each
         entry. But since we're not traversing, we don't
         need to cull the parent list here.

  2. Since we now just walk the reflog entries themselves,
     rather than starting with the ref tip, we now look at
     the "new" field of each entry rather than the "old"
     (i.e., we are showing entries, not faking parents).
     This removes all of the tricky logic around skipping
     past root commits.

     But note that we have no way to show an entry with the
     null sha1 in its "new" field (because such a commit
     obviously does not exist). Normally this would not
     happen, since we delete reflogs along with refs, but
     there is one special case. When we rename the currently
     checked out branch, we write two reflog entries into
     the HEAD log: one where the commit goes away, and
     another where it comes back.

     Prior to this commit, we show both entries with
     identical reflog messages. After this commit, we show
     only the "comes back" entry. See the update in t3200
     which demonstrates this.

     Arguably either is fine, as the whole double-entry
     thing is a bit hacky in the first place. And until a
     recent fix, we truncated the traversal in such a case
     anyway, which was _definitely_ wrong.

Signed-off-by: Jeff King <peff@peff.net>
---
 reflog-walk.c          | 116 +++++++++++++------------------------------------
 reflog-walk.h          |   4 +-
 revision.c             |  30 ++++++++-----
 t/t1414-reflog-walk.sh |  12 ++---
 t/t3200-branch.sh      |   3 +-
 5 files changed, 57 insertions(+), 108 deletions(-)

diff --git a/reflog-walk.c b/reflog-walk.c
index 89e719c459..a7644d944e 100644
--- a/reflog-walk.c
+++ b/reflog-walk.c
@@ -78,45 +78,6 @@ static int get_reflog_recno_by_time(struct complete_reflogs *array,
 	return -1;
 }
 
-struct commit_info_lifo {
-	struct commit_info {
-		struct commit *commit;
-		void *util;
-	} *items;
-	int nr, alloc;
-};
-
-static struct commit_info *get_commit_info(struct commit *commit,
-		struct commit_info_lifo *lifo, int pop)
-{
-	int i;
-	for (i = 0; i < lifo->nr; i++)
-		if (lifo->items[i].commit == commit) {
-			struct commit_info *result = &lifo->items[i];
-			if (pop) {
-				if (i + 1 < lifo->nr)
-					memmove(lifo->items + i,
-						lifo->items + i + 1,
-						(lifo->nr - i) *
-						sizeof(struct commit_info));
-				lifo->nr--;
-			}
-			return result;
-		}
-	return NULL;
-}
-
-static void add_commit_info(struct commit *commit, void *util,
-		struct commit_info_lifo *lifo)
-{
-	struct commit_info *info;
-	ALLOC_GROW(lifo->items, lifo->nr + 1, lifo->alloc);
-	info = lifo->items + lifo->nr;
-	info->commit = commit;
-	info->util = util;
-	lifo->nr++;
-}
-
 struct commit_reflog {
 	int recno;
 	enum selector_type {
@@ -128,7 +89,8 @@ struct commit_reflog {
 };
 
 struct reflog_walk_info {
-	struct commit_info_lifo reflogs;
+	struct commit_reflog **logs;
+	size_t nr, alloc, cur;
 	struct string_list complete_reflogs;
 	struct commit_reflog *last_commit_reflog;
 };
@@ -226,52 +188,10 @@ int add_reflog_for_walk(struct reflog_walk_info *info,
 	commit_reflog->selector = selector;
 	commit_reflog->reflogs = reflogs;
 
-	add_commit_info(commit, commit_reflog, &info->reflogs);
-	return 0;
-}
-
-void fake_reflog_parent(struct reflog_walk_info *info, struct commit *commit)
-{
-	struct commit_info *commit_info =
-		get_commit_info(commit, &info->reflogs, 0);
-	struct commit_reflog *commit_reflog;
-	struct object *logobj;
-	struct reflog_info *reflog;
-
-	info->last_commit_reflog = NULL;
-	if (!commit_info)
-		return;
+	ALLOC_GROW(info->logs, info->nr + 1, info->alloc);
+	info->logs[info->nr++] = commit_reflog;
 
-	commit_reflog = commit_info->util;
-	if (commit_reflog->recno < 0) {
-		commit->parents = NULL;
-		return;
-	}
-	info->last_commit_reflog = commit_reflog;
-
-	do {
-		reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
-		commit_reflog->recno--;
-		logobj = parse_object(&reflog->ooid);
-	} while (commit_reflog->recno && (logobj && logobj->type != OBJ_COMMIT));
-
-	if (!logobj && commit_reflog->recno >= 0 && is_null_oid(&reflog->ooid)) {
-		/* a root commit, but there are still more entries to show */
-		reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
-		logobj = parse_object(&reflog->noid);
-		if (!logobj)
-			logobj = parse_object(&reflog->ooid);
-	}
-
-	if (!logobj || logobj->type != OBJ_COMMIT) {
-		commit_info->commit = NULL;
-		commit->parents = NULL;
-		return;
-	}
-	commit_info->commit = (struct commit *)logobj;
-
-	commit->parents = xcalloc(1, sizeof(struct commit_list));
-	commit->parents->item = commit_info->commit;
+	return 0;
 }
 
 void get_reflog_selector(struct strbuf *sb,
@@ -361,5 +281,29 @@ void show_reflog_message(struct reflog_walk_info *reflog_info, int oneline,
 
 int reflog_walk_empty(struct reflog_walk_info *info)
 {
-	return !info || !info->reflogs.nr;
+	return !info || !info->nr;
+}
+
+struct commit *next_entry_in_log(struct commit_reflog *log)
+{
+	while (log->recno >= 0) {
+		struct reflog_info *entry = &log->reflogs->items[log->recno--];
+		struct object *obj = parse_object(&entry->noid);
+
+		if (obj && obj->type == OBJ_COMMIT)
+			return (struct commit *)obj;
+	}
+	return NULL;
+}
+
+struct commit *next_reflog_entry(struct reflog_walk_info *walk)
+{
+	for (; walk->cur < walk->nr; walk->cur++) {
+		struct commit *ret = next_entry_in_log(walk->logs[walk->cur]);
+		if (ret) {
+			walk->last_commit_reflog = walk->logs[walk->cur];
+			return ret;
+		}
+	}
+	return NULL;
 }
diff --git a/reflog-walk.h b/reflog-walk.h
index af32361072..373388cd14 100644
--- a/reflog-walk.h
+++ b/reflog-walk.h
@@ -8,8 +8,6 @@ struct reflog_walk_info;
 extern void init_reflog_walk(struct reflog_walk_info **info);
 extern int add_reflog_for_walk(struct reflog_walk_info *info,
 		struct commit *commit, const char *name);
-extern void fake_reflog_parent(struct reflog_walk_info *info,
-		struct commit *commit);
 extern void show_reflog_message(struct reflog_walk_info *info, int,
 				const struct date_mode *, int force_date);
 extern void get_reflog_message(struct strbuf *sb,
@@ -22,4 +20,6 @@ extern void get_reflog_selector(struct strbuf *sb,
 
 extern int reflog_walk_empty(struct reflog_walk_info *walk);
 
+struct commit *next_reflog_entry(struct reflog_walk_info *reflog_info);
+
 #endif
diff --git a/revision.c b/revision.c
index 4dc7c63654..5fc01f2d26 100644
--- a/revision.c
+++ b/revision.c
@@ -148,16 +148,14 @@ static void add_pending_object_with_path(struct rev_info *revs,
 	if (revs->reflog_info && obj->type == OBJ_COMMIT) {
 		struct strbuf buf = STRBUF_INIT;
 		int len = interpret_branch_name(name, 0, &buf, 0);
-		int st;
 
 		if (0 < len && name[len] && buf.len)
 			strbuf_addstr(&buf, name + len);
-		st = add_reflog_for_walk(revs->reflog_info,
-					 (struct commit *)obj,
-					 buf.buf[0] ? buf.buf: name);
+		add_reflog_for_walk(revs->reflog_info,
+				    (struct commit *)obj,
+				    buf.buf[0] ? buf.buf: name);
 		strbuf_release(&buf);
-		if (st)
-			return;
+		return; /* do not add the commit itself */
 	}
 	add_object_array_with_path(obj, name, &revs->pending, mode, path);
 }
@@ -3110,16 +3108,18 @@ static void track_linear(struct rev_info *revs, struct commit *commit)
 static struct commit *get_revision_1(struct rev_info *revs)
 {
 	while (1) {
-		struct commit *commit = pop_commit(&revs->commits);
+		struct commit *commit;
+
+		if (revs->reflog_info)
+			commit = next_reflog_entry(revs->reflog_info);
+		else
+			commit = pop_commit(&revs->commits);
 
 		if (!commit)
 			return NULL;
 
-		if (revs->reflog_info) {
-			save_parents(revs, commit);
-			fake_reflog_parent(revs->reflog_info, commit);
+		if (revs->reflog_info)
 			commit->object.flags &= ~(ADDED | SEEN | SHOWN);
-		}
 
 		/*
 		 * If we haven't done the list limiting, we need to look at
@@ -3130,7 +3130,8 @@ static struct commit *get_revision_1(struct rev_info *revs)
 			if (revs->max_age != -1 &&
 			    (commit->date < revs->max_age))
 				continue;
-			if (add_parents_to_list(revs, commit, &revs->commits, NULL) < 0) {
+			if (!revs->reflog_info &&
+			    add_parents_to_list(revs, commit, &revs->commits, NULL) < 0) {
 				if (!revs->ignore_missing_links)
 					die("Failed to traverse parents of commit %s",
 						oid_to_hex(&commit->object.oid));
@@ -3146,6 +3147,11 @@ static struct commit *get_revision_1(struct rev_info *revs)
 		default:
 			if (revs->track_linear)
 				track_linear(revs, commit);
+			if (revs->reflog_info) {
+				try_to_simplify_commit(revs, commit);
+				if (commit->object.flags & TREESAME)
+					continue;
+			}
 			return commit;
 		}
 	}
diff --git a/t/t1414-reflog-walk.sh b/t/t1414-reflog-walk.sh
index fba6788e94..7cc52d946d 100755
--- a/t/t1414-reflog-walk.sh
+++ b/t/t1414-reflog-walk.sh
@@ -34,32 +34,32 @@ test_expect_success 'reflog walk shows expected logs' '
 	test_cmp expect.all actual
 '
 
-test_expect_failure 'reflog can limit with --no-merges' '
+test_expect_success 'reflog can limit with --no-merges' '
 	grep -v merge expect.all >expect &&
 	do_walk --no-merges >actual &&
 	test_cmp expect actual
 '
 
-test_expect_failure 'reflog can limit with pathspecs' '
+test_expect_success 'reflog can limit with pathspecs' '
 	grep two expect.all >expect &&
 	do_walk -- two.t >actual &&
 	test_cmp expect actual
 '
 
-test_expect_failure 'pathspec limiting handles merges' '
+test_expect_success 'pathspec limiting handles merges' '
 	sed -n "1p;3p;5p" expect.all >expect &&
 	do_walk -- one.t >actual &&
 	test_cmp expect actual
 '
 
-test_expect_failure '--parents shows true parents' '
+test_expect_success '--parents shows true parents' '
 	# convert newlines to spaces
 	echo $(git rev-parse HEAD HEAD^1 HEAD^2) >expect &&
 	git rev-list -g --parents -1 HEAD >actual &&
 	test_cmp expect actual
 '
 
-test_expect_failure 'walking multiple reflogs shows both' '
+test_expect_success 'walking multiple reflogs shows both' '
 	{
 		do_walk HEAD &&
 		do_walk side
@@ -68,7 +68,7 @@ test_expect_failure 'walking multiple reflogs shows both' '
 	test_cmp expect actual
 '
 
-test_expect_failure 'walk prefers reflog to ref tip' '
+test_expect_success 'walk prefers reflog to ref tip' '
 	head=$(git rev-parse HEAD) &&
 	one=$(git rev-parse one) &&
 	ident="$GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE" &&
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
index dd37ac47c5..9d707d2a40 100755
--- a/t/t3200-branch.sh
+++ b/t/t3200-branch.sh
@@ -166,10 +166,9 @@ test_expect_success 'resulting reflog can be shown by log -g' '
 	oid=$(git rev-parse HEAD) &&
 	cat >expect <<-EOF &&
 	HEAD@{0} $oid $msg
-	HEAD@{1} $oid $msg
 	HEAD@{2} $oid checkout: moving from foo to baz
 	EOF
-	git log -g --format="%gd %H %gs" -3 HEAD >actual &&
+	git log -g --format="%gd %H %gs" -2 HEAD >actual &&
 	test_cmp expect actual
 '
 
-- 
2.13.2.892.g25f9b59978