From f40f4695c0cb576a4e00819da45c1bea8f548aec Mon Sep 17 00:00:00 2001
From: Eric Wong <e@80x24.org>
Date: Wed, 20 Jul 2022 09:24:13 +0000
Subject: [PATCH] pop3: advertise STLS in CAPA if appropriate

This is documented in RFC 2595, and POP3 clients may rely on
seeing "STLS" in CAPA output to initiate TLS negotiation.
---
 Documentation/standards.perl | 1 +
 lib/PublicInbox/POP3.pm      | 6 ++++--
 t/pop3d.t                    | 7 +++++++
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/Documentation/standards.perl b/Documentation/standards.perl
index 835de3a2..c36afb5d 100755
--- a/Documentation/standards.perl
+++ b/Documentation/standards.perl
@@ -69,6 +69,7 @@ my $rfcs = [
 	1081 => 'Post Office Protocol – Version 3',
 	1939 => 'Post Office Protocol – Version 3 (STD 53)',
 	2449 => 'POP3 extension mechanism',
+	2595 => 'STARTTLS for IMAP and POP3',
 	2384 => 'POP URL Scheme',
 
 	# TODO: flesh this out
diff --git a/lib/PublicInbox/POP3.pm b/lib/PublicInbox/POP3.pm
index 2c20c84b..ec73893c 100644
--- a/lib/PublicInbox/POP3.pm
+++ b/lib/PublicInbox/POP3.pm
@@ -343,15 +343,17 @@ sub cmd_dele {
 # RFC 2449
 sub cmd_capa {
 	my ($self) = @_;
+	my $STLS = !$self->{ibx} && !$self->{sock}->can('stop_SSL') &&
+			$self->{pop3d}->{accept_tls} ? "\nSTLS\r" : '';
 	$self->{expire} = ''; # "EXPIRE 0" allows clients to avoid DELE commands
-	\<<EOM;
+	<<EOM;
 +OK Capability list follows\r
 TOP\r
 USER\r
 PIPELINING\r
 UIDL\r
 EXPIRE 0\r
-RESP-CODES\r
+RESP-CODES\r$STLS
 .\r
 EOM
 }
diff --git a/t/pop3d.t b/t/pop3d.t
index 3d70935f..9eb110d6 100644
--- a/t/pop3d.t
+++ b/t/pop3d.t
@@ -106,6 +106,8 @@ for my $args (
 	my @p3s_args = ($pop3s->sockhost,
 			Port => $pop3s->sockport, SSL => 1, %o);
 	my $p3s = Net::POP3->new(@p3s_args);
+	my $capa = $p3s->capa;
+	ok(!exists $capa->{STLS}, 'no STLS CAPA for POP3S');
 	ok($p3s->quit, 'QUIT works w/POP3S');
 	{
 		$p3s = Net::POP3->new(@p3s_args);
@@ -127,7 +129,11 @@ for my $args (
 	my $np3 = Net::POP3->new(@np3_args);
 	ok($np3->quit, 'plain QUIT works');
 	$np3 = Net::POP3->new(@np3_args, %o);
+	$capa = $np3->capa;
+	ok(exists $capa->{STLS}, 'STLS CAPA advertised before STLS');
 	ok($np3->starttls, 'STLS works');
+	$capa = $np3->capa;
+	ok(!exists $capa->{STLS}, 'STLS CAPA not advertised after STLS');
 	ok($np3->quit, 'QUIT works after STLS');
 
 	for my $mailbox (('x'x32)."\@$group", $group, ('a'x32)."\@z.$group") {
@@ -239,6 +245,7 @@ EOF
 	my $capa = $oldc->capa;
 	ok(defined($capa->{PIPELINING}), 'pipelining supported by CAPA');
 	is($capa->{EXPIRE}, 0, 'EXPIRE 0 set');
+	ok(!exists $capa->{STLS}, 'STLS unset w/o daemon certs');
 
 	# ensure TOP doesn't trigger "EXPIRE 0" like RETR does (cf. RFC2449)
 	my $list = $oldc->list;
-- 
2.44.0