lib/PublicInbox/Inbox.pm  |  2 +-
 lib/PublicInbox/Msgmap.pm | 19 +++++++++++++++++++
 t/msgmap.t                |  4 ++++

diff --git a/lib/PublicInbox/Inbox.pm b/lib/PublicInbox/Inbox.pm
index b9cd4c40a6dc7541f799f0da8b520e426af7df4d..f307038399b77437b8e5a94ed163e01559959466 100644
--- a/lib/PublicInbox/Inbox.pm
+++ b/lib/PublicInbox/Inbox.pm
@@ -75,7 +75,7 @@ 	_set_limiter($opts, $pi_config, 'httpbackend');
 	_set_uint($opts, 'feedmax', 25);
 	$opts->{nntpserver} ||= $pi_config->{'publicinbox.nntpserver'};
 	my $dir = $opts->{mainrepo};
-	if (defined $dir && -f "$dir/msgmap.sqlite3") { # XXX DIRTY
+	if (defined $dir && -f "$dir/inbox.lock") {
 		$opts->{version} = 2;
 	}
 	bless $opts, $class;
diff --git a/lib/PublicInbox/Msgmap.pm b/lib/PublicInbox/Msgmap.pm
index 8e81fba08d12b81ba6b1ead7709881f137c38ff4..78922d3693c9647e0b11c3dc7fc300ce1a985147 100644
--- a/lib/PublicInbox/Msgmap.pm
+++ b/lib/PublicInbox/Msgmap.pm
@@ -12,6 +12,7 @@ use strict;
 use warnings;
 use DBI;
 use DBD::SQLite;
+use File::Temp qw(tempfile);
 
 sub new {
 	my ($class, $git_dir, $writable) = @_;
@@ -43,6 +44,18 @@ 		$self->created_at(time) unless $self->created_at;
 		$dbh->commit;
 	}
 	$self;
+}
+
+# used to keep track of used numeric mappings for v2 reindex
+sub tmp_clone {
+	my ($self) = @_;
+	my ($fh, $fn) = tempfile(EXLOCK => 0);
+	$self->{dbh}->sqlite_backup_to_file($fn);
+	my $tmp = ref($self)->new_file($fn, 1);
+	$tmp->{dbh}->do('PRAGMA synchronous = OFF');
+	$tmp->{tmp_name} = $fn; # SQLite won't work if unlinked, apparently
+	$fh = undef;
+	$tmp;
 }
 
 # n.b. invoked directly by scripts/xhdr-num2mid
@@ -187,6 +200,12 @@ 		$self->{dbh}->prepare(
 			'INSERT OR IGNORE INTO msgmap (num,mid) VALUES (?,?)');
 	};
 	$sth->execute($num, $mid);
+}
+
+sub DESTROY {
+	my ($self) = @_;
+	delete $self->{dbh};
+	unlink $self->{tmp_name} if defined $self->{tmp_name};
 }
 
 1;
diff --git a/t/msgmap.t b/t/msgmap.t
index bc22137dd00ca770b31d84c88e6ad0d6aaff0cc2..dce98f46b6f57e48a90653ceba659a3665db247d 100644
--- a/t/msgmap.t
+++ b/t/msgmap.t
@@ -65,4 +65,8 @@ my $orig = $d->mid_insert('spam@1');
 $d->mid_delete('spam@1');
 is($d->mid_insert('spam@2'), 1 + $orig, "last number not recycled");
 
+my $tmp = $d->tmp_clone;
+is_deeply([$d->minmax], [$tmp->minmax], 'Cloned temporary DB matches');
+ok($tmp->mid_delete('spam@2'), 'temporary DB is writable');
+
 done_testing();