reflogs: clear flags properly in corner case

The reflog-walking mechanism is based on the regular
revision traversal. We just rewrite the parents of each
commit in fake_reflog_parent to point to the commit in the
next reflog entry instead of the real parents.

However, the regular revision traversal tries not to show
the same commit twice, and so sets the SHOWN flag on each
commit it shows. In a reflog, however, we may want to see
the same commit more than once if it appears in the reflog
multiple times (which easily happens, for example, if you do
a reset to a prior state).

The fake_reflog_parent function takes care of this by
clearing flags, including SHOWN. Unfortunately, it does so
at the very end of the function, and it is possible to
return early from the function if there is no fake parent to
set up (e.g., because we are at the very first reflog entry
on the branch). In such a case the flag is not cleared, and
the entry is skipped by the revision traversal machinery as
already shown.

You can see this by walking the log of a ref which is set to
its very first commit more than once (the test below shows
such a situation). In this case the reflog walk will fail to
show the entry for the initial creation of the ref.

We don't want to simply move the flag-clearing to the top of
the function; we want to make sure flags set during the
fake-parent installation are also cleared. Instead, let's
hoist the flag-clearing out of the fake_reflog_parent
function entirely. It's not really about fake parents
anyway, and the only caller is the get_revision machinery.
Reported-by: NMartin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: NJeff King <peff@peff.net>
Acked-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

reflog-walk.c

@@ -239,7 +239,6 @@ void fake_reflog_parent(struct reflog_walk_info *info, struct commit *commit)
 
 	commit->parents = xcalloc(sizeof(struct commit_list), 1);
 	commit->parents->item = commit_info->commit;
-	commit->object.flags &= ~(ADDED | SEEN | SHOWN);
 }
 
 void get_reflog_selector(struct strbuf *sb,

revision.c

@@ -1896,8 +1896,10 @@ static struct commit *get_revision_1(struct rev_info *revs)
 		revs->commits = entry->next;
 		free(entry);
 
-		if (revs->reflog_info)
+		if (revs->reflog_info) {
 			fake_reflog_parent(revs->reflog_info, commit);
+			commit->object.flags &= ~(ADDED | SEEN | SHOWN);
+		}
 
 		/*
 		 * If we haven't done the list limiting, we need to look at

t/t1412-reflog-loop.sh

+#!/bin/sh
+
+test_description='reflog walk shows repeated commits again'
+. ./test-lib.sh
+
+test_expect_success 'setup commits' '
+	test_tick &&
+	echo content >file && git add file && git commit -m one &&
+	git tag one &&
+	echo content >>file && git add file && git commit -m two &&
+	git tag two
+'
+
+test_expect_success 'setup reflog with alternating commits' '
+	git checkout -b topic &&
+	git reset one &&
+	git reset two &&
+	git reset one &&
+	git reset two
+'
+
+test_expect_success 'reflog shows all entries' '
+	cat >expect <<-\EOF
+		topic@{0} two: updating HEAD
+		topic@{1} one: updating HEAD
+		topic@{2} two: updating HEAD
+		topic@{3} one: updating HEAD
+		topic@{4} branch: Created from HEAD
+	EOF
+	git log -g --format="%gd %gs" topic >actual &&
+	test_cmp expect actual
+'
+
+test_done