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, 31 insertions, 11 deletions

diff --git a/lib/PublicInbox/V2Writable.pm b/lib/PublicInbox/V2Writable.pm
index 9073d9ef..8ad71b54 100644
--- a/lib/PublicInbox/V2Writable.pm
+++ b/lib/PublicInbox/V2Writable.pm
@@ -663,23 +663,43 @@ sub fill_alternates ($$) {
         unless (-d $all) {
                 PublicInbox::Import::init_bare($all);
         }
-        my $alt = "$all/objects/info/alternates";
-        my %alts;
-        my @add;
+        my $info_dir = "$all/objects/info";
+        my $alt = "$info_dir/alternates";
+        my (%alt, $new);
+        my $mode = 0644;
         if (-e $alt) {
                 open(my $fh, '<', $alt) or die "open < $alt: $!\n";
-                %alts = map { chomp; $_ => 1 } (<$fh>);
+                $mode = (stat($fh))[2] & 07777;
+
+                # we assign a sort score to every alternate and favor
+                # the newest (highest numbered) one when we
+                my $score;
+                my $other = 0; # in case admin adds non-epoch repos
+                %alt = map {;
+                        if (m!\A\Q../../\E([0-9]+)\.git/objects\z!) {
+                                $score = $1 + 0;
+                        } else {
+                                $score = --$other;
+                        }
+                        $_ => $score;
+                } split(/\n+/, do { local $/; <$fh> });
         }
+
         foreach my $i (0..$epoch) {
                 my $dir = "../../git/$i.git/objects";
-                push @add, $dir if !$alts{$dir} && -d "$pfx/$i.git";
-        }
-        return unless @add;
-        open my $fh, '>>', $alt or die "open >> $alt: $!\n";
-        foreach my $dir (@add) {
-                print $fh "$dir\n" or die "print >> $alt: $!\n";
+                if (!exists($alt{$dir}) && -d "$pfx/$i.git") {
+                        $alt{$dir} = $i;
+                        $new = 1;
+                }
         }
-        close $fh or die "close $alt: $!\n";
+        return unless $new;
+
+        my ($fh, $tmp) = tempfile('alt-XXXXXXXX', DIR => $info_dir);
+        print $fh join("\n", sort { $alt{$b} <=> $alt{$a} } keys %alt), "\n"
+                or die "print $tmp: $!\n";
+        chmod($mode, $fh) or die "fchmod $tmp: $!\n";
+        close $fh or die "close $tmp $!\n";
+        rename($tmp, $alt) or die "rename $tmp => $alt: $!\n";
 }
 
 sub git_init {