www: fix ref cycle from threading w/ extindex

Unlike v1 inboxes (which don't accept duplicate Message-IDs at
all), and v2 inboxes (which generate a new Message-ID for
duplicates), extindex must accept duplicate Message-IDs as-is.

This was fine for storage, but prevented the reference-cycle
mechanism of our message threading display algorithm from working
reliably.  It could no longer delete the ->{parent} field from
clobbered entries in the %id_table.

So we now take into account reused Message-IDs and never clobber
entries in %id_table.  Instead, we mark reused Message-IDs as
"imposters" and special-case them by injecting them as children
after all other threading is complete.

This cycle was noticed using a pre-release of Devel::Mwrap::PSGI:
  https://80x24.org/mwrap-perl.git

1 files changed, 20 insertions, 1 deletions

diff --git a/t/thread-cycle.t b/t/thread-cycle.t
index 4b47c01c..e89b1846 100644
--- a/t/thread-cycle.t
+++ b/t/thread-cycle.t
@@ -96,7 +96,26 @@ if ('sorting by Date') {
         is("\n".$backward, "\n".$forward, 'forward and backward matches');
 }
 
-done_testing();
+SKIP: {
+        require_mods 'Devel::Cycle', 1;
+        Devel::Cycle->import('find_cycle');
+        my @dup = (
+                { mid => 5, references => '<6>' },
+                { mid => 5, references => '<6> <1>' },
+        );
+        open my $fh, '+>', \(my $out = '') or xbail "open: $!";
+        (undef, $smsgs) = $make_objs->(@dup);
+        eval 'package EmptyInbox; sub smsg_by_mid { undef }';
+        my $ctx = { ibx => bless {}, 'EmptyInbox' };
+        my $rootset = PublicInbox::SearchThread::thread($smsgs, sub {
+                [ sort { $a->{mid} cmp $b->{mid} } @{$_[0]} ] }, $ctx);
+        my $oldout = select $fh;
+        find_cycle($rootset);
+        select $oldout;
+        is($out, '', 'nothing from find_cycle');
+} # Devel::Cycle check
+
+done_testing;
 
 sub thread_to_s {
         my ($msgs) = @_;