[PATCH 2/2] imap: EXAMINE: avoid potential race conditions

[Eric Wong <e@yhbt.net>]

We need to rely on num_highwater for UIDNEXT since the
highest `num' stored in over.sqlite3 may be rolled back
if the most recent messages were spam.

We also need to load the uo2m immediately on EXAMINE to ensure
EXISTS responses are always consistent with regard to future
updates.
---
 lib/PublicInbox/IMAP.pm | 61 ++++++++++++++++++++++-------------------
 lib/PublicInbox/Over.pm | 18 ++----------
 2 files changed, 36 insertions(+), 43 deletions(-)

diff --git a/lib/PublicInbox/IMAP.pm b/lib/PublicInbox/IMAP.pm
index 888c9becfe0..9ad74de837f 100644
--- a/lib/PublicInbox/IMAP.pm
+++ b/lib/PublicInbox/IMAP.pm
@@ -192,8 +192,8 @@ sub cmd_capability ($$) {
 
 # uo2m: UID Offset to MSN, this is an arrayref by default,
 # but uo2m_hibernate can compact and deduplicate it
-sub uo2m_ary_new ($) {
-	my ($self) = @_;
+sub uo2m_ary_new ($;$) {
+	my ($self, $exists) = @_;
 	my $base = $self->{uid_base};
 	my $uids = $self->{ibx}->over->uid_range($base + 1, $base + UID_SLICE);
 
@@ -202,6 +202,7 @@ sub uo2m_ary_new ($) {
 	my $msn = 0;
 	++$base;
 	$tmp[$_ - $base] = ++$msn for @$uids;
+	$$exists = $msn if $exists;
 	\@tmp;
 }
 
@@ -385,44 +386,48 @@ sub ensure_slices_exist ($$$) {
 	push @$l, map { qq[* LIST (\\HasNoChildren) "." $_\r\n] } @created;
 }
 
-sub inbox_lookup ($$) {
-	my ($self, $mailbox) = @_;
-	my ($ibx, $exists, $uidnext, $uid_base);
-	if ($mailbox =~ /\A(.+)\.([0-9]+)\z/) {
-		# old mail: inbox.comp.foo.$SLICE_IDX
-		my $mb_top = $1;
-		$uid_base = $2 * UID_SLICE;
-		$ibx = $self->{imapd}->{mailboxes}->{lc $mailbox} or return;
-		my $max;
-		($exists, $uidnext, $max) = $ibx->over->imap_status($uid_base,
-							$uid_base + UID_SLICE);
-		ensure_slices_exist($self->{imapd}, $ibx, $max);
-	} else { # check for dummy inboxes
-		$mailbox = lc $mailbox;
-		$ibx = $self->{imapd}->{mailboxes}->{$mailbox} or return;
-
+sub inbox_lookup ($$;$) {
+	my ($self, $mailbox, $examine) = @_;
+	my ($ibx, $exists, $uidmax, $uid_base) = (undef, 0, 0, 0);
+	$mailbox = lc $mailbox;
+	$ibx = $self->{imapd}->{mailboxes}->{$mailbox} or return;
+	my $over = $ibx->over;
+	if ($over != $ibx) { # not a dummy
+		$mailbox =~ /\.([0-9]+)\z/ or
+				die "BUG: unexpected dummy mailbox: $mailbox\n";
+		$uid_base = $1 * UID_SLICE;
+
+		# ->num_highwater caches for writers, so use ->meta_accessor
+		$uidmax = $ibx->mm->meta_accessor('num_highwater') // 0;
+		if ($examine) {
+			$self->{uid_base} = $uid_base;
+			$self->{ibx} = $ibx;
+			$self->{uo2m} = uo2m_ary_new($self, \$exists);
+		} else {
+			$exists = $over->imap_exists;
+		}
+		ensure_slices_exist($self->{imapd}, $ibx, $over->max);
+	} else {
+		if ($examine) {
+			$self->{uid_base} = $uid_base;
+			$self->{ibx} = $ibx;
+			delete $self->{uo2m};
+		}
 		# if "INBOX.foo.bar" is selected and "INBOX.foo.bar.0",
 		# check for new UID ranges (e.g. "INBOX.foo.bar.1")
 		if (my $z = $self->{imapd}->{mailboxes}->{"$mailbox.0"}) {
 			ensure_slices_exist($self->{imapd}, $z, $z->over->max);
 		}
-
-		$uid_base = $exists = 0;
-		$uidnext = 1;
 	}
-	($ibx, $exists, $uidnext, $uid_base);
+	($ibx, $exists, $uidmax + 1, $uid_base);
 }
 
 sub cmd_examine ($$$) {
 	my ($self, $tag, $mailbox) = @_;
-	my ($ibx, $exists, $uidnext, $base) = inbox_lookup($self, $mailbox);
-	return "$tag NO Mailbox doesn't exist: $mailbox\r\n" if !$ibx;
-	$self->{uid_base} = $base;
-	delete $self->{uo2m};
-
 	# XXX: do we need this? RFC 5162/7162
 	my $ret = $self->{ibx} ? "* OK [CLOSED] previous closed\r\n" : '';
-	$self->{ibx} = $ibx;
+	my ($ibx, $exists, $uidnext, $base) = inbox_lookup($self, $mailbox, 1);
+	return "$tag NO Mailbox doesn't exist: $mailbox\r\n" if !$ibx;
 	$ret .= <<EOF;
 * $exists EXISTS\r
 * $exists RECENT\r
diff --git a/lib/PublicInbox/Over.pm b/lib/PublicInbox/Over.pm
index 74c8fb86270..e5a980d59ea 100644
--- a/lib/PublicInbox/Over.pm
+++ b/lib/PublicInbox/Over.pm
@@ -248,25 +248,13 @@ SELECT MAX(num) FROM over WHERE num > 0
 	$sth->fetchrow_array // 0;
 }
 
-sub imap_status {
+sub imap_exists {
 	my ($self, $uid_base, $uid_end) = @_;
-	my $dbh = $self->connect;
-	my $sth = $dbh->prepare_cached(<<'', undef, 1);
+	my $sth = $self->connect->prepare_cached(<<'', undef, 1);
 SELECT COUNT(num) FROM over WHERE num > ? AND num <= ?
 
 	$sth->execute($uid_base, $uid_end);
-	my $exists = $sth->fetchrow_array;
-
-	$sth = $dbh->prepare_cached(<<'', undef, 1);
-SELECT MAX(num) + 1 FROM over WHERE num <= ?
-
-	$sth->execute($uid_end);
-	my $uidnext = $sth->fetchrow_array;
-
-	$sth = $dbh->prepare_cached(<<'', undef, 1);
-SELECT MAX(num) FROM over WHERE num > 0
-
-	($exists, $uidnext, $sth->fetchrow_array // 0);
+	$sth->fetchrow_array;
 }
 
 sub check_inodes {