[PATCH] multi_git: hoist out common epoch/alternates handling

[Eric Wong <e@80x24.org>]

IMHO, this greatly improves code sharing and organization
between v2, extindex, and lei/store.  Common git-related
logic for these is lightly-refactored and easier to reason
about.

The impetus for this big change was to ensure inboxes
created+managed by public-inbox-{clone,fetch} could have
alternates and configs setup properly without depending on
SQLite (via V2Writable).  This change does that while
making old code shorter and better factored.
---
 MANIFEST                        |   1 +
 lib/PublicInbox/ExtSearchIdx.pm |  85 +++++++-------------
 lib/PublicInbox/Fetch.pm        |  17 ++--
 lib/PublicInbox/LeiMirror.pm    |  15 ++--
 lib/PublicInbox/LeiStore.pm     |  32 ++------
 lib/PublicInbox/MultiGit.pm     | 136 ++++++++++++++++++++++++++++++++
 lib/PublicInbox/V2Writable.pm   |  87 +++-----------------
 script/public-inbox-convert     |   2 +-
 t/lei-mirror.t                  |  15 +++-
 t/v2mirror.t                    |   9 ++-
 t/v2writable.t                  |   2 +-
 11 files changed, 222 insertions(+), 179 deletions(-)
 create mode 100644 lib/PublicInbox/MultiGit.pm

diff --git a/MANIFEST b/MANIFEST
index a1450880..640eabd1 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -270,6 +270,7 @@ lib/PublicInbox/MiscSearch.pm
 lib/PublicInbox/MsgIter.pm
 lib/PublicInbox/MsgTime.pm
 lib/PublicInbox/Msgmap.pm
+lib/PublicInbox/MultiGit.pm
 lib/PublicInbox/NDC_PP.pm
 lib/PublicInbox/NNTP.pm
 lib/PublicInbox/NNTPD.pm
diff --git a/lib/PublicInbox/ExtSearchIdx.pm b/lib/PublicInbox/ExtSearchIdx.pm
index 8cdad23d..e0ba6c32 100644
--- a/lib/PublicInbox/ExtSearchIdx.pm
+++ b/lib/PublicInbox/ExtSearchIdx.pm
@@ -21,6 +21,7 @@ use Carp qw(croak carp);
 use Sys::Hostname qw(hostname);
 use POSIX qw(strftime);
 use File::Glob qw(bsd_glob GLOB_NOSORT);
+use PublicInbox::MultiGit;
 use PublicInbox::Search;
 use PublicInbox::SearchIdx qw(prepare_stack is_ancestor is_bad_blob);
 use PublicInbox::OverIdx;
@@ -1133,88 +1134,60 @@ sub idx_init { # similar to V2Writable
 
 	$self->git->cleanup;
 	my $mode = 0644;
-	my $ALL = $self->git->{git_dir}; # ALL.git
-	my $old = -d $ALL;
+	my $ALL = $self->git->{git_dir}; # topdir/ALL.git
+	my ($has_new, $alt, $seen);
 	if ($opt->{-private}) { # LeiStore
+		my $local = "$self->{topdir}/local"; # lei/store
+		$self->{mg} //= PublicInbox::MultiGit->new($self->{topdir},
+							'ALL.git', 'local');
 		$mode = 0600;
-		if (!$old) {
-			umask 077; # don't bother restoring
+		unless (-d $ALL) {
+			umask 077; # don't bother restoring for lei
 			PublicInbox::Import::init_bare($ALL);
 			$self->git->qx(qw(config core.sharedRepository 0600));
 		}
-	} else {
-		PublicInbox::Import::init_bare($ALL) unless $old;
-	}
-	my $info_dir = "$ALL/objects/info";
-	my $alt = "$info_dir/alternates";
-	my (@old, @new, %seen); # seen: st_dev + st_ino
-	if (-e $alt) {
-		open(my $fh, '<', $alt) or die "open $alt: $!";
-		$mode = (stat($fh))[2] & 07777;
-		while (my $line = <$fh>) {
-			chomp(my $d = $line);
-
-			# expand relative path (/local/ stuff)
-			substr($d, 0, 3) eq '../' and
-				$d = "$ALL/objects/$d";
-			if (my @st = stat($d)) {
-				next if $seen{"$st[0]\0$st[1]"}++;
-			} else {
-				warn "W: stat($d) failed (from $alt): $!\n";
-				next if $opt->{-idx_gc};
-			}
-			push @old, $line;
-		}
+		($alt, $seen) = $self->{mg}->read_alternates(\$mode);
+		$has_new = $self->{mg}->merge_epochs($alt, $seen);
+	} else { # extindex has no epochs
+		$self->{mg} //= PublicInbox::MultiGit->new($self->{topdir},
+							'ALL.git');
+		($alt, $seen) = $self->{mg}->read_alternates(\$mode,
+							$opt->{-idx_gc});
+		PublicInbox::Import::init_bare($ALL);
 	}
 
-	# for LeiStore, and possibly some mirror-only state
-	if (opendir(my $dh, my $local = "$self->{topdir}/local")) {
-		# highest numbered epoch first
-		for my $n (sort { $b <=> $a } map { substr($_, 0, -4) + 0 }
-				grep(/\A[0-9]+\.git\z/, readdir($dh))) {
-			my $d = "$local/$n.git/objects"; # absolute path
-			if (my @st = stat($d)) {
-				next if $seen{"$st[0]\0$st[1]"}++;
-				# favor relative paths for rename-friendliness
-				push @new, "../../local/$n.git/objects\n";
-			} else {
-				warn "W: stat($d) failed: $!\n";
-			}
-		}
-	}
 	# git-multi-pack-index(1) can speed up "git cat-file" startup slightly
-	my $dh;
 	my $git_midx = 0;
 	my $pd = "$ALL/objects/pack";
-	if (!mkdir($pd) && $!{EEXIST} && opendir($dh, $pd)) {
-		# drop stale symlinks
+	if (opendir(my $dh, $pd)) { # drop stale symlinks
 		while (defined(my $dn = readdir($dh))) {
 			if ($dn =~ /\.(?:idx|pack|promisor|bitmap|rev)\z/) {
 				my $f = "$pd/$dn";
 				unlink($f) if -l $f && !-e $f;
 			}
 		}
-		undef $dh;
+	} elsif ($!{ENOENT}) {
+		mkdir($pd) or die "mkdir($pd): $!";
+	} else {
+		die "opendir($pd): $!";
 	}
+	my $new = '';
 	for my $ibx (@{ibx_sorted($self, 'active')}) {
 		# create symlinks for multi-pack-index
 		$git_midx += symlink_packs($ibx, $pd);
 		# add new lines to our alternates file
-		my $line = $ibx->git->{git_dir} . "/objects\n";
-		chomp(my $d = $line);
+		my $d = $ibx->git->{git_dir} . '/objects';
+		next if exists $alt->{$d};
 		if (my @st = stat($d)) {
-			next if $seen{"$st[0]\0$st[1]"}++;
+			next if $seen->{"$st[0]\0$st[1]"}++;
 		} else {
 			warn "W: stat($d) failed (from $ibx->{inboxdir}): $!\n";
 			next if $opt->{-idx_gc};
 		}
-		push @new, $line;
-	}
-	if (scalar @new) {
-		push @old, @new;
-		my $o = \@old;
-		PublicInbox::V2Writable::write_alternates($info_dir, $mode, $o);
+		$new .= "$d\n";
 	}
+	($has_new || $new ne '') and
+		$self->{mg}->write_alternates($mode, $alt, $new);
 	$git_midx and $self->with_umask(sub {
 		my @cmd = ('multi-pack-index');
 		push @cmd, '--no-progress' if ($opt->{quiet}//0) > 1;
@@ -1226,7 +1199,7 @@ sub idx_init { # similar to V2Writable
 	$self->with_umask(\&_idx_init, $self, $opt);
 	$self->{oidx}->begin_lazy;
 	$self->{oidx}->eidx_prep;
-	$self->{midx}->create_xdb if @new;
+	$self->{midx}->create_xdb if $new ne '';
 }
 
 sub _watch_commit { # PublicInbox::DS::add_timer callback
diff --git a/lib/PublicInbox/Fetch.pm b/lib/PublicInbox/Fetch.pm
index 6a6daee6..9ea55e9d 100644
--- a/lib/PublicInbox/Fetch.pm
+++ b/lib/PublicInbox/Fetch.pm
@@ -6,12 +6,11 @@ use strict;
 use v5.10.1;
 use parent qw(PublicInbox::IPC);
 use URI ();
-use PublicInbox::Spawn qw(popen_rd);
+use PublicInbox::Spawn qw(popen_rd run_die);
 use PublicInbox::Admin;
 use PublicInbox::LEI;
 use PublicInbox::LeiCurl;
 use PublicInbox::LeiMirror;
-use IO::Uncompress::Gunzip qw(gunzip $GunzipError);
 use File::Temp ();
 
 sub new { bless {}, __PACKAGE__ }
@@ -87,15 +86,15 @@ sub do_fetch {
 	my $ibx_ver;
 	$lei->{curl} //= PublicInbox::LeiCurl->new($lei) or return;
 	my $dir = PublicInbox::Admin::resolve_inboxdir($cd, \$ibx_ver);
-	my ($ibx_uri, @git_dir, @epochs);
+	my ($ibx_uri, @git_dir, @epochs, $mg, @new_epoch);
 	if ($ibx_ver == 1) {
 		my $url = remote_url($lei, $dir) //
 			die "E: $dir missing remote.origin.url\n";
 		$ibx_uri = URI->new($url);
 	} else { # v2:
-		opendir my $dh, "$dir/git" or die "opendir $dir/git: $!";
-		@epochs = sort { $b <=> $a } map { substr($_, 0, -4) + 0 }
-					grep(/\A[0-9]+\.git\z/, readdir($dh));
+		require PublicInbox::MultiGit;
+		$mg = PublicInbox::MultiGit->new($dir, 'all.git', 'git');
+		my @epochs = $mg->git_epochs;
 		my ($git_url, $epoch);
 		for my $nr (@epochs) { # try newest epoch, first
 			my $edir = "$dir/git/$nr.git";
@@ -121,9 +120,7 @@ EOM
 	if ($code == 404) {
 		# any pre-manifest.js.gz instances running? Just fetch all
 		# existing ones and unconditionally try cloning the next
-		$v2_epochs = [ map {;
-				"$dir/git/$_.git";
-				} @epochs ];
+		$v2_epochs = [ map { "$dir/git/$_.git" } @epochs ];
 		push @$v2_epochs, "$dir/git/".($epochs[-1] + 1) if @epochs;
 	} else {
 		$code == 200 or die "BUG unexpected code $code\n";
@@ -154,6 +151,7 @@ EOM
 			$cmd = [ @$torsocks,
 				PublicInbox::LeiMirror::clone_cmd($lei, $opt),
 				$$e_uri, $d];
+			push @new_epoch, substr($epath, 5, -4) + 0;
 		}
 		my $cerr = PublicInbox::LeiMirror::run_reap($lei, $cmd, $opt);
 		# do not bail on clone failure if we didn't have a manifest
@@ -162,6 +160,7 @@ EOM
 			return;
 		}
 	}
+	for my $i (@new_epoch) { $mg->epoch_cfg_set($i) }
 	if ($ft) {
 		my $fn = $ft->filename;
 		rename($fn, $mf) or die "E: rename($fn, $mf): $!\n";
diff --git a/lib/PublicInbox/LeiMirror.pm b/lib/PublicInbox/LeiMirror.pm
index bc2e749c..c113c9de 100644
--- a/lib/PublicInbox/LeiMirror.pm
+++ b/lib/PublicInbox/LeiMirror.pm
@@ -1,13 +1,13 @@
 # Copyright (C) 2021 all contributors <meta@public-inbox.org>
 # License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
 
-# "lei add-external --mirror" support
+# "lei add-external --mirror" support (also "public-inbox-clone");
 package PublicInbox::LeiMirror;
 use strict;
 use v5.10.1;
 use parent qw(PublicInbox::IPC);
 use IO::Uncompress::Gunzip qw(gunzip $GunzipError);
-use PublicInbox::Spawn qw(popen_rd spawn);
+use PublicInbox::Spawn qw(popen_rd spawn run_die);
 use File::Temp ();
 use Fcntl qw(SEEK_SET);
 
@@ -209,7 +209,6 @@ sub clone_v2 {
 	my $lei = $self->{lei};
 	my $curl = $self->{curl} //= PublicInbox::LeiCurl->new($lei) or return;
 	my $pfx //= $curl->torsocks($lei, $v2_uris->[0]) or return;
-	my @epochs;
 	my $dst = $self->{dst};
 	my @src_edst;
 	for my $uri (@$v2_uris) {
@@ -220,17 +219,21 @@ failed to extract epoch number from $src
 
 		my $nr = $1 + 0;
 		$edst .= "/git/$nr.git";
-		push @src_edst, [ $src, $edst ];
+		push @src_edst, $src, $edst;
 	}
 	my $lk = bless { lock_path => "$dst/inbox.lock" }, 'PublicInbox::Lock';
 	_try_config($self);
 	my $on_destroy = $lk->lock_for_scope($$);
 	my @cmd = clone_cmd($lei, my $opt = {});
-	while (my $pair = shift(@src_edst)) {
-		my $cmd = [ @$pfx, @cmd, @$pair ];
+	while (my ($src, $edst) = splice(@src_edst, 0, 2)) {
+		my $cmd = [ @$pfx, @cmd, $src, $edst ];
 		my $cerr = run_reap($lei, $cmd, $opt);
 		return $lei->child_error($cerr, "@$cmd failed") if $cerr;
 	}
+	require PublicInbox::MultiGit;
+	my $mg = PublicInbox::MultiGit->new($dst, 'all.git', 'git');
+	$mg->fill_alternates;
+	for my $i ($mg->git_epochs) { $mg->epoch_cfg_set($i) }
 	undef $on_destroy; # unlock
 	index_cloned_inbox($self, 2);
 }
diff --git a/lib/PublicInbox/LeiStore.pm b/lib/PublicInbox/LeiStore.pm
index f81a8dae..42f574f2 100644
--- a/lib/PublicInbox/LeiStore.pm
+++ b/lib/PublicInbox/LeiStore.pm
@@ -27,7 +27,6 @@ use PublicInbox::MDA;
 use PublicInbox::Spawn qw(spawn);
 use PublicInbox::MdirReader;
 use PublicInbox::LeiToMail;
-use List::Util qw(max);
 use File::Temp ();
 use POSIX ();
 use IO::Handle (); # ->autoflush
@@ -50,19 +49,6 @@ sub rotate_bytes {
 	$_[0]->{rotate_bytes} // ((1024 * 1024 * 1024) / $_[0]->packing_factor)
 }
 
-sub git_pfx { "$_[0]->{priv_eidx}->{topdir}/local" };
-
-sub git_epoch_max  {
-	my ($self) = @_;
-	if (opendir(my $dh, $self->git_pfx)) {
-		max(map {
-			substr($_, 0, -4) + 0; # drop ".git" suffix
-		} grep(/\A[0-9]+\.git\z/, readdir($dh))) // 0;
-	} else {
-		$!{ENOENT} ? 0 : die("opendir ${\$self->git_pfx}: $!\n");
-	}
-}
-
 sub git_ident ($) {
 	my ($git) = @_;
 	my $rdr = {};
@@ -91,22 +77,16 @@ sub importer {
 		$im->done;
 		undef $im;
 		$self->checkpoint;
-		$max = $self->git_epoch_max + 1;
+		$max = $self->{priv_eidx}->{mg}->git_epochs + 1;
 	}
 	my (undef, $tl) = eidx_init($self); # acquire lock
-	my $pfx = $self->git_pfx;
-	$max //= $self->git_epoch_max;
+	$max //= $self->{priv_eidx}->{mg}->git_epochs;
 	while (1) {
-		my $latest = "$pfx/$max.git";
-		my $old = -e $latest;
-		PublicInbox::Import::init_bare($latest);
+		my $latest = $self->{priv_eidx}->{mg}->add_epoch($max);
 		my $git = PublicInbox::Git->new($latest);
-		if (!$old) {
-			$git->qx(qw(config core.sharedRepository 0600));
-			$self->done; # unlock
-			# re-acquire lock, update alternates for new epoch
-			(undef, $tl) = eidx_init($self);
-		}
+		$self->done; # unlock
+		# re-acquire lock, update alternates for new epoch
+		(undef, $tl) = eidx_init($self);
 		my $packed_bytes = $git->packed_bytes;
 		my $unpacked_bytes = $packed_bytes / $self->packing_factor;
 		if ($unpacked_bytes >= $self->rotate_bytes) {
diff --git a/lib/PublicInbox/MultiGit.pm b/lib/PublicInbox/MultiGit.pm
new file mode 100644
index 00000000..91d7998a
--- /dev/null
+++ b/lib/PublicInbox/MultiGit.pm
@@ -0,0 +1,136 @@
+# Copyright (C) all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+
+# common git alternates + all.git||ALL.git management code
+package PublicInbox::MultiGit;
+use strict;
+use v5.10.1;
+use PublicInbox::Spawn qw(run_die);
+use PublicInbox::Import;
+use File::Temp 0.19;
+use List::Util qw(max);
+
+sub new {
+	my ($cls, $topdir, $all, $epfx) = @_;
+	bless {
+		topdir => $topdir, # inboxdir || extindex.*.topdir
+		all => $all, # all.git or ALL.git
+		epfx => $epfx, # "git" (inbox) or "local" (lei/store)
+	}, $cls;
+}
+
+sub read_alternates {
+	my ($self, $moderef, $prune) = @_;
+	my $objpfx = "$self->{topdir}/$self->{all}/objects/";
+	my $f = "${objpfx}info/alternates";
+	my %alt; # line => score
+	my %seen; # $st_dev\0$st_ino => count
+	my $other = 0;
+	if (open(my $fh, '<', $f)) {
+		my $is_edir = defined($self->{epfx}) ?
+			qr!\A\Q../../$self->{epfx}\E/([0-9]+)\.git/objects\z! :
+			undef;
+		$$moderef = (stat($fh))[2] & 07777;
+		for my $rel (split(/^/m, do { local $/; <$fh> })) {
+			chomp(my $dir = $rel);
+			my $score;
+			if (defined($is_edir) && $dir =~ $is_edir) {
+				$score = $1 + 0;
+				substr($dir, 0, 0) = $objpfx;
+			} else { # absolute paths, if any (extindex)
+				$score = --$other;
+			}
+			if (my @st = stat($dir)) {
+				next if $seen{"$st[0]\0$st[1]"}++;
+				$alt{$rel} = $score;
+			} else {
+				warn "W: stat($dir) failed: $! ($f)";
+				$alt{$rel} = $score unless $prune;
+			}
+		}
+	} elsif (!$!{ENOENT}) {
+		die "E: open($f): $!";
+	}
+	(\%alt, \%seen);
+}
+
+sub epoch_dir { "$_[0]->{topdir}/$_[0]->{epfx}" }
+
+sub write_alternates {
+	my ($self, $mode, $alt, @new) = @_;
+	my $all_dir = "$self->{topdir}/$self->{all}";
+	PublicInbox::Import::init_bare($all_dir);
+	my $out = join('', sort { $alt->{$b} <=> $alt->{$a} } keys %$alt);
+	my $info_dir = "$all_dir/objects/info";
+	my $fh = File::Temp->new(TEMPLATE => 'alt-XXXX', DIR => $info_dir);
+	my $f = $fh->filename;
+	print $fh $out, @new or die "print($f): $!";
+	chmod($mode, $fh) or die "fchmod($f): $!";
+	close $fh or die "close($f): $!";
+	my $fn = "$info_dir/alternates";
+	rename($f, $fn) or die "rename($f, $fn): $!";
+	$fh->unlink_on_destroy(0);
+}
+
+# returns true if new epochs exist
+sub merge_epochs {
+	my ($self, $alt, $seen) = @_;
+	my $epoch_dir = epoch_dir($self);
+	if (opendir my $dh, $epoch_dir) {
+		my $has_new;
+		for my $bn (grep(/\A[0-9]+\.git\z/, readdir($dh))) {
+			my $rel = "../../$self->{epfx}/$bn/objects\n";
+			next if exists($alt->{$rel});
+			if (my @st = stat("$epoch_dir/$bn/objects")) {
+				next if $seen->{"$st[0]\0$st[1]"}++;
+				$alt->{$rel} = substr($bn, 0, -4) + 0;
+				$has_new = 1;
+			} else {
+				warn "E: stat($epoch_dir/$bn/objects): $!";
+			}
+		}
+		$has_new;
+	} else {
+		$!{ENOENT} ? undef : die "opendir($epoch_dir): $!";
+	}
+}
+
+sub fill_alternates {
+	my ($self) = @_;
+	my ($alt, $seen) = read_alternates($self, \(my $mode = 0644));
+	merge_epochs($self, $alt, $seen) and
+		write_alternates($self, $mode, $alt);
+}
+
+sub epoch_cfg_set {
+	my ($self, $epoch_nr) = @_;
+	run_die([qw(git config -f), epoch_dir($self)."/$epoch_nr.git/config",
+		'include.path', "../../$self->{all}/config" ]);
+}
+
+sub add_epoch {
+	my ($self, $epoch_nr) = @_;
+	my $git_dir = epoch_dir($self)."/$epoch_nr.git";
+	my $f = "$git_dir/config";
+	my $existing = -f $f;
+	PublicInbox::Import::init_bare($git_dir);
+	epoch_cfg_set($self, $epoch_nr) unless $existing;
+	fill_alternates($self);
+	$git_dir;
+}
+
+sub git_epochs  {
+	my ($self) = @_;
+	if (opendir(my $dh, epoch_dir($self))) {
+		my @epochs = map {
+			substr($_, 0, -4) + 0; # drop ".git" suffix
+		} grep(/\A[0-9]+\.git\z/, readdir($dh));
+		wantarray ? sort { $b <=> $a } @epochs : (max(@epochs) // 0);
+	} elsif ($!{ENOENT}) {
+		wantarray ? () : 0;
+	} else {
+		die(epoch_dir($self).": $!");
+	}
+}
+
+1;
diff --git a/lib/PublicInbox/V2Writable.pm b/lib/PublicInbox/V2Writable.pm
index 1288f47b..971b007b 100644
--- a/lib/PublicInbox/V2Writable.pm
+++ b/lib/PublicInbox/V2Writable.pm
@@ -12,6 +12,7 @@ use PublicInbox::IPC;
 use PublicInbox::Eml;
 use PublicInbox::Git;
 use PublicInbox::Import;
+use PublicInbox::MultiGit;
 use PublicInbox::MID qw(mids references);
 use PublicInbox::ContentHash qw(content_hash content_digest git_sha);
 use PublicInbox::InboxWritable;
@@ -72,16 +73,14 @@ sub new {
 	$v2ibx = PublicInbox::InboxWritable->new($v2ibx);
 	my $dir = $v2ibx->assert_usable_dir;
 	unless (-d $dir) {
-		if ($creat) {
-			require File::Path;
-			File::Path::mkpath($dir);
-		} else {
-			die "$dir does not exist\n";
-		}
+		die "$dir does not exist\n" if !$creat;
+		require File::Path;
+		File::Path::mkpath($dir);
 	}
 	my $xpfx = "$dir/xap" . PublicInbox::Search::SCHEMA_VERSION;
 	my $self = {
 		ibx => $v2ibx,
+		mg => PublicInbox::MultiGit->new($dir, 'all.git', 'git'),
 		im => undef, #  PublicInbox::Import
 		parallel => 1,
 		transact_bytes => 0,
@@ -110,7 +109,7 @@ sub init_inbox {
 	$self->{mm}->skip_artnum($skip_artnum) if defined $skip_artnum;
 	my $max = $self->{ibx}->max_git_epoch;
 	$max = $skip_epoch if (defined($skip_epoch) && !defined($max));
-	$self->git_init($max // 0);
+	$self->{mg}->add_epoch($max // 0);
 	$self->done;
 }
 
@@ -641,70 +640,6 @@ sub done {
 	die $err if $err;
 }
 
-sub write_alternates ($$$) {
-	my ($info_dir, $mode, $out) = @_;
-	my $fh = File::Temp->new(TEMPLATE => 'alt-XXXX', DIR => $info_dir);
-	my $tmp = $fh->filename;
-	print $fh @$out or die "print $tmp: $!\n";
-	chmod($mode, $fh) or die "fchmod $tmp: $!\n";
-	close $fh or die "close $tmp $!\n";
-	my $alt = "$info_dir/alternates";
-	rename($tmp, $alt) or die "rename $tmp => $alt: $!\n";
-	$fh->unlink_on_destroy(0);
-}
-
-sub fill_alternates ($$) {
-	my ($self, $epoch) = @_;
-
-	my $pfx = "$self->{ibx}->{inboxdir}/git";
-	my $all = "$self->{ibx}->{inboxdir}/all.git";
-	PublicInbox::Import::init_bare($all) unless -d $all;
-	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";
-		$mode = (stat($fh))[2] & 07777;
-
-		# we assign a sort score to every alternate and favor
-		# the newest (highest numbered) one because loose objects
-		# require scanning epochs and only the latest epoch is
-		# expected to see loose objects
-		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";
-		if (!exists($alt{$dir}) && -d "$pfx/$i.git") {
-			$alt{$dir} = $i;
-			$new = 1;
-		}
-	}
-	return unless $new;
-	write_alternates($info_dir, $mode,
-		[join("\n", sort { $alt{$b} <=> $alt{$a} } keys %alt), "\n"]);
-}
-
-sub git_init {
-	my ($self, $epoch) = @_;
-	my $git_dir = "$self->{ibx}->{inboxdir}/git/$epoch.git";
-	PublicInbox::Import::init_bare($git_dir);
-	run_die([qw(git config), "--file=$git_dir/config",
-		qw(include.path ../../all.git/config)]);
-	fill_alternates($self, $epoch);
-	$git_dir
-}
-
 sub importer {
 	my ($self) = @_;
 	my $im = $self->{im};
@@ -716,8 +651,8 @@ sub importer {
 			$im->done;
 			$im = undef;
 			$self->checkpoint;
-			my $git_dir = $self->git_init(++$self->{epoch_max});
-			my $git = PublicInbox::Git->new($git_dir);
+			my $dir = $self->{mg}->add_epoch(++$self->{epoch_max});
+			my $git = PublicInbox::Git->new($dir);
 			return $self->import_init($git, 0);
 		}
 	}
@@ -737,8 +672,8 @@ sub importer {
 		}
 	}
 	$self->{epoch_max} = $epoch;
-	$latest = $self->git_init($epoch);
-	$self->import_init(PublicInbox::Git->new($latest), 0);
+	my $dir = $self->{mg}->add_epoch($epoch);
+	$self->import_init(PublicInbox::Git->new($dir), 0);
 }
 
 sub import_init {
@@ -1335,7 +1270,7 @@ sub index_sync {
 	local $self->{ibx}->{indexlevel} = 'basic' if $seq;
 
 	$self->idx_init($opt); # acquire lock
-	fill_alternates($self, $epoch_max);
+	$self->{mg}->fill_alternates;
 	$self->{oidx}->rethread_prepare($opt);
 	my $sync = {
 		need_checkpoint => \(my $bool = 0),
diff --git a/script/public-inbox-convert b/script/public-inbox-convert
index fec6b624..01af846a 100755
--- a/script/public-inbox-convert
+++ b/script/public-inbox-convert
@@ -179,7 +179,7 @@ if (my $old_mm = $old->mm) {
 	$v2w->idx_init($opt);
 	$v2w->{mm}->{dbh}->sqlite_backup_from_file($old_mm);
 
-	my $epoch0 = PublicInbox::Git->new($v2w->git_init(0));
+	my $epoch0 = PublicInbox::Git->new($v2w->{mg}->add_epoch(0));
 	chop(my $cmt = $epoch0->qx(qw(rev-parse --verify), $head));
 	$v2w->last_epoch_commit(0, $cmt);
 }
diff --git a/t/lei-mirror.t b/t/lei-mirror.t
index 44acbe95..5238b67c 100644
--- a/t/lei-mirror.t
+++ b/t/lei-mirror.t
@@ -95,7 +95,20 @@ SKIP: {
 
 	ok(run_script([qw(-clone -q -C), $d, "$http/t2"], undef, $opt),
 		'-clone succeeds on v2');
-	ok(-d "$d/t2/git/0.git", 'epoch cloned');
+	ok(-f "$d/t2/git/0.git/config", 'epoch cloned');
+
+	# writeBitmaps is the default for bare repos in git 2.22+,
+	# so we may stop setting it ourselves.
+	0 and is(xqx(['git', "--git-dir=$d/t2/git/0.git", 'config',
+		qw(--bool repack.writeBitmaps)]), "true\n",
+		'write bitmaps set (via include.path=all.git/config');
+
+	is(xqx(['git', "--git-dir=$d/t2/git/0.git", 'config',
+		qw(include.path)]), "../../all.git/config\n",
+		'include.path set');
+
+	ok(-s "$d/t2/all.git/objects/info/alternates",
+		'all.git alternates created');
 	ok(-f "$d/t2/manifest.js.gz", 'manifest saved');
 	ok(!-e "$d/t2/mirror.done", 'no leftover mirror.done');
 	ok(run_script([qw(-fetch -C), "$d/t2"], undef, $opt),
diff --git a/t/v2mirror.t b/t/v2mirror.t
index 8bcffc29..54ad6945 100644
--- a/t/v2mirror.t
+++ b/t/v2mirror.t
@@ -228,10 +228,13 @@ EOF
 	is(scalar($mset->items), 0, 'large message not re-indexed');
 }
 ok(scalar(@new_epochs), 'new epochs were created and fetched');
+for my $d (@new_epochs) {
+	is(xqx(['git', "--git-dir=$d", 'config', qw(include.path)]),
+		"../../all.git/config\n",
+		'include.path set');
+}
 
 ok($td->kill, 'killed httpd');
 $td->join;
 
-done_testing();
-
-1;
+done_testing;
diff --git a/t/v2writable.t b/t/v2writable.t
index d9e7b980..477621e2 100644
--- a/t/v2writable.t
+++ b/t/v2writable.t
@@ -308,7 +308,7 @@ ok($@, 'V2Writable fails on non-existent dir');
 	open $fh, '<', $alt or die $!;
 	my $before = do { local $/; <$fh> };
 
-	ok($v2w->git_init(3), 'init a new epoch');
+	ok($v2w->{mg}->add_epoch(3), 'init a new epoch');
 	open $fh, '<', $alt or die $!;
 	my $after = do { local $/; <$fh> };
 	ok(index($after, $before) > 0,