imap: break giant inboxes into sub-inboxes of 50K messages

This limit on mailbox size should keep users of tools like
mbsync (isync) and offlineimap happy, since typical filesystems
struggle with giant Maildirs.

I chose 50K since it's a bit more than what LKML typically sees
in a month and still manages to give acceptable performance on
my ancient Centrino laptop.

There were also no responses to my original proposal at:
<https://public-inbox.org/meta/20200519090000.GA24273@dcvr/>
so no objections, either :>

1 files changed, 100 insertions, 34 deletions

diff --git a/lib/PublicInbox/IMAP.pm b/lib/PublicInbox/IMAP.pm
index ffa76bb0..959dcfbe 100644
--- a/lib/PublicInbox/IMAP.pm
+++ b/lib/PublicInbox/IMAP.pm
@@ -16,7 +16,7 @@ package PublicInbox::IMAP;
 use strict;
 use base qw(PublicInbox::DS);
 use fields qw(imapd logged_in ibx long_cb -login_tag
-        -idle_tag -idle_max);
+        uid_min -idle_tag -idle_max);
 use PublicInbox::Eml;
 use PublicInbox::EmlContentFoo qw(parse_content_disposition);
 use PublicInbox::DS qw(now);
@@ -34,6 +34,9 @@ die "neither Email::Address::XS nor Mail::Address loaded: $@" if !$Address;
 
 sub LINE_MAX () { 512 } # does RFC 3501 have a limit like RFC 977?
 
+# changing this will cause grief for clients which cache
+sub UID_BLOCK () { 50_000 }
+
 my %FETCH_NEED_BLOB = ( # for future optimization
         'BODY[HEADER]' => 1,
         'BODY[TEXT]' => 1,
@@ -128,6 +131,7 @@ sub cmd_login ($$$$) {
 
 sub cmd_close ($$) {
         my ($self, $tag) = @_;
+        delete $self->{uid_min};
         delete $self->{ibx} ? "$tag OK Close done\r\n"
                                 : "$tag BAD No mailbox\r\n";
 }
@@ -188,12 +192,67 @@ sub cmd_done ($$) {
         "$idle_tag OK Idle done\r\n";
 }
 
+sub ensure_old_ranges_exist ($$$) {
+        my ($self, $ibx, $uid_min) = @_;
+        my $groups = $self->{imapd}->{groups};
+        my $mailbox = $ibx->{newsgroup};
+        my @created;
+        $uid_min -= UID_BLOCK;
+        my $uid_end = $uid_min + UID_BLOCK - 1;
+        while ($uid_min > 0) {
+                my $sub_mailbox = "$mailbox.$uid_min-$uid_end";
+                last if exists $groups->{$sub_mailbox};
+                $groups->{$sub_mailbox} = $ibx;
+                $uid_end -= UID_BLOCK;
+                $uid_min -= UID_BLOCK;
+                push @created, $sub_mailbox;
+        }
+        return unless @created;
+        my $l = $self->{imapd}->{inboxlist};
+        grep {
+                / \Q$mailbox\E\r\n\z/ and s/\(\\HasNoChildren/\(\\HasChildren/;
+        } @$l;
+        push @$l, map { qq[* LIST (\\HasNoChildren) "." $_\r\n] } @created;
+}
+
 sub cmd_examine ($$$) {
         my ($self, $tag, $mailbox) = @_;
-        my $ibx = $self->{imapd}->{groups}->{$mailbox} or
-                return "$tag NO Mailbox doesn't exist: $mailbox\r\n";
-        my $mm = $ibx->mm;
-        my $max = $mm->max // 0;
+        my ($ibx, $mm, $max);
+
+        if ($mailbox =~ /\A(.+)\.([0-9]+)-([0-9]+)\z/) {
+                # old mail: inbox.comp.foo.$uid_min-$uid_end
+                my ($mb_top, $uid_min, $uid_end) = ($1, $2 + 0, $3 + 0);
+                $ibx = $self->{imapd}->{groups}->{lc $mb_top};
+                if (!$ibx || ($uid_end % UID_BLOCK) != 0 ||
+                                ($uid_min + UID_BLOCK - 1) != $uid_end) {
+                        return "$tag NO Mailbox doesn't exist: $mailbox\r\n";
+                }
+                $mm = $ibx->mm;
+                $max = $mm->max // 0;
+
+                # don't let users create inboxes w/ not-yet-possible range:
+                $uid_min > $max and
+                        return "$tag NO Mailbox doesn't exist: $mailbox\r\n";
+
+                $max = $uid_min + UID_BLOCK + 1;
+                $self->{uid_min} = $uid_min;
+                ensure_old_ranges_exist($self, $ibx, $uid_min);
+        } else { # current mailbox (most recent UID_BLOCK messages)
+                $ibx = $self->{imapd}->{groups}->{lc $mailbox} or
+                        return "$tag NO Mailbox doesn't exist: $mailbox\r\n";
+
+                $mm = $ibx->mm;
+                $max = $mm->max // 0;
+
+                my $uid_min = UID_BLOCK * int($max/UID_BLOCK) + 1;
+                if ($uid_min == 1) { # normal inbox with <UID_BLOCK messages
+                        delete $self->{uid_min}; # implicit cmd_close
+                } else { # we have a giant inbox:
+                        $self->{uid_min} = $uid_min;
+                        ensure_old_ranges_exist($self, $ibx, $uid_min);
+                }
+        }
+
         # RFC 3501 2.3.1.1 -  "A good UIDVALIDITY value to use in
         # this case is a 32-bit representation of the creation
         # date/time of the mailbox"
@@ -208,11 +267,11 @@ sub cmd_examine ($$$) {
 * $max RECENT\r
 * FLAGS (\\Seen)\r
 * OK [PERMANENTFLAGS ()] Read-only mailbox\r
+* OK [UNSEEN $max]\r
+* OK [UIDNEXT $uidnext]\r
+* OK [UIDVALIDITY $uidvalidity]\r
+$tag OK [READ-ONLY] EXAMINE/SELECT done\r
 EOF
-        $ret .= "* OK [UNSEEN $max]\r\n" if $max;
-        $ret .= "* OK [UIDNEXT $uidnext]\r\n" if defined $uidnext;
-        $ret .= "* OK [UIDVALIDITY $uidvalidity]\r\n" if defined $uidvalidity;
-        $ret .= "$tag OK [READ-ONLY] EXAMINE/SELECT done\r\n";
 }
 
 sub _esc ($) {
@@ -357,12 +416,12 @@ sub fetch_body ($;$) {
 }
 
 sub dummy_message ($$) {
-        my ($seqno, $ibx) = @_;
+        my ($self, $seqno) = @_;
         my $ret = <<EOF;
 From: nobody\@localhost\r
 To: nobody\@localhost\r
 Date: Thu, 01 Jan 1970 00:00:00 +0000\r
-Message-ID: <dummy-$seqno\@$ibx->{newsgroup}>\r
+Message-ID: <dummy-$seqno\@$self->{ibx}->{newsgroup}>\r
 Subject: dummy message #$seqno\r
 \r
 You're seeing this message because your IMAP client didn't use UIDs.\r
@@ -390,13 +449,13 @@ sub requeue_once ($) {
 
 sub uid_fetch_cb { # called by git->cat_async via git_async_cat
         my ($bref, $oid, $type, $size, $fetch_m_arg) = @_;
-        my ($self, undef, $ibx, $msgs, undef, $want) = @$fetch_m_arg;
+        my ($self, undef, $msgs, undef, $want) = @$fetch_m_arg;
         my $smsg = shift @$msgs or die 'BUG: no smsg';
         if (!defined($oid)) {
                 # it's possible to have TOCTOU if an admin runs
                 # public-inbox-(edit|purge), just move onto the next message
                 return requeue_once($self) unless defined $want->{-seqno};
-                $bref = dummy_message($smsg->{num}, $ibx);
+                $bref = dummy_message($self, $smsg->{num});
         } else {
                 $smsg->{blob} eq $oid or die "BUG: $smsg->{blob} != $oid";
         }
@@ -446,7 +505,7 @@ sub uid_fetch_cb { # called by git->cat_async via git_async_cat
 }
 
 sub range_step ($$) {
-        my ($ibx, $range_csv) = @_;
+        my ($self, $range_csv) = @_;
         my ($beg, $end, $range);
         if ($$range_csv =~ s/\A([^,]+),//) {
                 $range = $1;
@@ -458,39 +517,46 @@ sub range_step ($$) {
                 ($beg, $end) = ($1 + 0, $2 + 0);
         } elsif ($range =~ /\A([0-9]+):\*\z/) {
                 $beg = $1 + 0;
-                $end = $ibx->mm->max // 0;
+                $end = $self->{ibx}->mm->max // 0;
                 $beg = $end if $beg > $end;
         } elsif ($range =~ /\A[0-9]+\z/) {
                 $beg = $end = $range + 0;
+                undef $range;
         } else {
                 return 'BAD fetch range';
         }
+        if (defined($range) && (my $uid_min = $self->{uid_min})) {
+                my $uid_end = $uid_min + UID_BLOCK - 1;
+                $beg = $uid_min if $beg < $uid_min;
+                $end = $uid_end if $end > $uid_end;
+        }
         [ $beg, $end, $$range_csv ];
 }
 
 sub refill_range ($$$) {
-        my ($ibx, $msgs, $range_info) = @_;
+        my ($self, $msgs, $range_info) = @_;
         my ($beg, $end, $range_csv) = @$range_info;
-        if (scalar(@$msgs = @{$ibx->over->query_xover($beg, $end)})) {
+        if (scalar(@$msgs = @{$self->{ibx}->over->query_xover($beg, $end)})) {
                 $range_info->[0] = $msgs->[-1]->{num} + 1;
                 return;
         }
         return 'OK Fetch done' if !$range_csv;
-        my $next_range = range_step($ibx, \$range_csv);
+        my $next_range = range_step($self, \$range_csv);
         return $next_range if !ref($next_range); # error
         @$range_info = @$next_range;
         undef; # keep looping
 }
 
 sub uid_fetch_m { # long_response
-        my ($self, $tag, $ibx, $msgs, $range_info, $want) = @_;
+        my ($self, $tag, $msgs, $range_info, $want) = @_;
         while (!@$msgs) { # rare
-                if (my $end = refill_range($ibx, $msgs, $range_info)) {
+                if (my $end = refill_range($self, $msgs, $range_info)) {
                         $self->write(\"$tag $end\r\n");
                         return;
                 }
         }
-        git_async_cat($ibx->git, $msgs->[0]->{blob}, \&uid_fetch_cb, \@_);
+        git_async_cat($self->{ibx}->git, $msgs->[0]->{blob},
+                        \&uid_fetch_cb, \@_);
 }
 
 sub cmd_status ($$$;@) {
@@ -698,9 +764,9 @@ sub fetch_common ($$$$) {
                 } sort keys %partial ];
         }
         $range_csv = 'bad' if $range_csv !~ $valid_range;
-        my $range_info = range_step($ibx, \$range_csv);
+        my $range_info = range_step($self, \$range_csv);
         return "$tag $range_info\r\n" if !ref($range_info);
-        [ $tag, $ibx, [], $range_info, \%want ];
+        [ $tag, [], $range_info, \%want ];
 }
 
 sub cmd_uid_fetch ($$$;@) {
@@ -712,9 +778,9 @@ sub cmd_uid_fetch ($$$;@) {
 }
 
 sub seq_fetch_m { # long_response
-        my ($self, $tag, $ibx, $msgs, $range_info, $want) = @_;
+        my ($self, $tag, $msgs, $range_info, $want) = @_;
         while (!@$msgs) { # rare
-                if (my $end = refill_range($ibx, $msgs, $range_info)) {
+                if (my $end = refill_range($self, $msgs, $range_info)) {
                         $self->write(\"$tag $end\r\n");
                         return;
                 }
@@ -722,13 +788,13 @@ sub seq_fetch_m { # long_response
         my $seq = $want->{-seqno}++;
         my $cur_num = $msgs->[0]->{num};
         if ($cur_num == $seq) { # as expected
-                git_async_cat($ibx->git, $msgs->[0]->{blob},
+                git_async_cat($self->{ibx}->git, $msgs->[0]->{blob},
                                 \&uid_fetch_cb, \@_);
         } elsif ($cur_num > $seq) {
                 # send dummy messages until $seq catches up to $cur_num
                 my $smsg = bless { num => $seq, ts => 0 }, 'PublicInbox::Smsg';
                 unshift @$msgs, $smsg;
-                my $bref = dummy_message($seq, $ibx);
+                my $bref = dummy_message($self, $seq);
                 uid_fetch_cb($bref, undef, undef, undef, \@_);
                 $smsg; # blessed response since uid_fetch_cb requeues
         } else { # should not happen
@@ -741,14 +807,14 @@ sub cmd_fetch ($$$;@) {
         my $args = fetch_common($self, $tag, $range_csv, \@want);
         ref($args) eq 'ARRAY' ? do {
                 my $want = $args->[-1];
-                $want->{-seqno} = $args->[3]->[0]; # $beg == $range_info->[0];
+                $want->{-seqno} = $args->[2]->[0]; # $beg == $range_info->[0];
                 long_response($self, \&seq_fetch_m, @$args)
         } : $args; # error
 }
 
 sub uid_search_all { # long_response
-        my ($self, $tag, $ibx, $num) = @_;
-        my $uids = $ibx->mm->ids_after($num);
+        my ($self, $tag, $num) = @_;
+        my $uids = $self->{ibx}->mm->ids_after($num);
         if (scalar(@$uids)) {
                 $self->msg_more(join(' ', '', @$uids));
         } else {
@@ -758,8 +824,8 @@ sub uid_search_all { # long_response
 }
 
 sub uid_search_uid_range { # long_response
-        my ($self, $tag, $ibx, $beg, $end) = @_;
-        my $uids = $ibx->mm->msg_range($beg, $end, 'num');
+        my ($self, $tag, $beg, $end) = @_;
+        my $uids = $self->{ibx}->mm->msg_range($beg, $end, 'num');
         if (@$uids) {
                 $self->msg_more(join('', map { " $_->[0]" } @$uids));
         } else {
@@ -775,14 +841,14 @@ sub cmd_uid_search ($$$;) {
         if ($arg eq 'ALL' && !@rest) {
                 $self->msg_more('* SEARCH');
                 my $num = 0;
-                long_response($self, \&uid_search_all, $tag, $ibx, \$num);
+                long_response($self, \&uid_search_all, $tag, \$num);
         } elsif ($arg eq 'UID' && scalar(@rest) == 1) {
                 if ($rest[0] =~ /\A([0-9]+):([0-9]+|\*)\z/s) {
                         my ($beg, $end) = ($1, $2);
                         $end = $ibx->mm->max if $end eq '*';
                         $self->msg_more('* SEARCH');
                         long_response($self, \&uid_search_uid_range,
-                                        $tag, $ibx, \$beg, $end);
+                                        $tag, \$beg, $end);
                 } elsif ($rest[0] =~ /\A[0-9]+\z/s) {
                         my $uid = $rest[0];
                         $uid = $ibx->over->get_art($uid) ? " $uid" : '';