v2writable: newest epochs go first in alternates

New epochs are the most likely to have loose objects.  git won't
be able to take advantage of pack indices and needs to scan
every alternate for the loose object via open/openat syscalls.
Those syscalls will add up some day when we've got hundreds or
thousands of epochs.

1 files changed, 24 insertions, 0 deletions

diff --git a/t/v2writable.t b/t/v2writable.t
index 63f27f96..7ceaa6e4 100644
--- a/t/v2writable.t
+++ b/t/v2writable.t
@@ -284,4 +284,28 @@ eval {
 };
 ok($@, 'V2Writable fails on non-existent dir');
 
+{
+        my $v2w = PublicInbox::V2Writable->new($tmp, 1);
+        ok($v2w, 'creat flag works');
+        $v2w->{parallel} = 0;
+        $v2w->init_inbox(0);
+        my $alt = "$tmp->{inboxdir}/all.git/objects/info/alternates";
+        open my $fh, '>>', $alt or die $!;
+        print $fh "$inboxdir/all.git/objects\n" or die $!;
+        chmod(0664, $fh) or die "fchmod: $!";
+        close $fh or die $!;
+        open $fh, '<', $alt or die $!;
+        my $before = do { local $/; <$fh> };
+
+        ok($v2w->git_init(3), 'init a new epoch');
+        open $fh, '<', $alt or die $!;
+        my $after = do { local $/; <$fh> };
+        ok(index($after, $before) > 0,
+                'old contents preserved after adding epoch');
+        like($after, qr!\A[^\n]+?/3\.git/objects\n!s,
+                'first line is newest epoch');
+        my $mode = (stat($alt))[2] & 07777;
+        is($mode, 0664, sprintf('0%03o', $mode).' is 0664');
+}
+
 done_testing();