[PATCH 1/2] lei: avoid deadlock on inotify/EVFILT_VNODE wakeups

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

Enqueuing "note-event" requests from the DS event loop must
not wait on workers being able to drain the queue quickly
enough.  Thus we make the SOCK_SEQPACKET writes nonblocking
and rely on the lei-daemon event loop to enqueue writes.

This is a unique problem for "note-event" since it reuses
workers in between commands, while most lei commands currently
fork off new workers.
---
 MANIFEST                        |  1 +
 lib/PublicInbox/IPC.pm          | 26 +++++++++++++++--
 lib/PublicInbox/LeiNoteEvent.pm |  5 ++--
 lib/PublicInbox/WQBlocked.pm    | 49 +++++++++++++++++++++++++++++++++
 4 files changed, 77 insertions(+), 4 deletions(-)
 create mode 100644 lib/PublicInbox/WQBlocked.pm

diff --git a/MANIFEST b/MANIFEST
index 2cbe66b7..923f5147 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -337,6 +337,7 @@ lib/PublicInbox/V2Writable.pm
 lib/PublicInbox/View.pm
 lib/PublicInbox/ViewDiff.pm
 lib/PublicInbox/ViewVCS.pm
+lib/PublicInbox/WQBlocked.pm
 lib/PublicInbox/WQWorker.pm
 lib/PublicInbox/WWW.pm
 lib/PublicInbox/WWW.pod
diff --git a/lib/PublicInbox/IPC.pm b/lib/PublicInbox/IPC.pm
index 67e86a43..74862673 100644
--- a/lib/PublicInbox/IPC.pm
+++ b/lib/PublicInbox/IPC.pm
@@ -1,4 +1,4 @@
-# Copyright (C) 2020-2021 all contributors <meta@public-inbox.org>
+# Copyright (C) all contributors <meta@public-inbox.org>
 # License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
 
 # base class for remote IPC calls and workqueues, requires Storable or Sereal
@@ -43,7 +43,7 @@ if ($enc && $dec) { # should be custom ops
 }
 
 my $recv_cmd = PublicInbox::Spawn->can('recv_cmd4');
-my $send_cmd = PublicInbox::Spawn->can('send_cmd4') // do {
+our $send_cmd = PublicInbox::Spawn->can('send_cmd4') // do {
 	require PublicInbox::CmdIPC4;
 	$recv_cmd //= PublicInbox::CmdIPC4->can('recv_cmd4');
 	PublicInbox::CmdIPC4->can('send_cmd4');
@@ -348,6 +348,24 @@ sub wq_do {
 	}
 }
 
+sub prepare_nonblock {
+	($_[0]->{-wq_s1} // die 'BUG: no {-wq_s1}')->blocking(0);
+	$_[0]->{-reap_async} or die 'BUG: {-reap_async} needed for nonblock';
+	require PublicInbox::WQBlocked;
+}
+
+sub wq_nonblock_do { # always async
+	my ($self, $sub, @args) = @_;
+	my $buf = ipc_freeze([$sub, @args]);
+	if ($self->{wqb}) { # saturated once, assume saturated forever
+		$self->{wqb}->flush_send($buf);
+	} else {
+		$send_cmd->($self->{-wq_s1}, [], $buf, MSG_EOR) //
+			($!{EAGAIN} ? PublicInbox::WQBlocked->new($self, $buf)
+					: croak("sendmsg: $!"));
+	}
+}
+
 sub _wq_worker_start ($$$$) {
 	my ($self, $oldset, $fields, $one) = @_;
 	my ($bcast1, $bcast2);
@@ -405,6 +423,10 @@ sub wq_workers_start {
 
 sub wq_close {
 	my ($self) = @_;
+	if (my $wqb = delete $self->{wqb}) {
+		$self->{-reap_async} or die 'BUG: {-reap_async} unset';
+		$wqb->enq_close;
+	}
 	delete @$self{qw(-wq_s1 -wq_s2)} or return;
 	return if $self->{-reap_async};
 	my @pids = keys %{$self->{-wq_workers}};
diff --git a/lib/PublicInbox/LeiNoteEvent.pm b/lib/PublicInbox/LeiNoteEvent.pm
index db387633..93f80116 100644
--- a/lib/PublicInbox/LeiNoteEvent.pm
+++ b/lib/PublicInbox/LeiNoteEvent.pm
@@ -58,7 +58,7 @@ sub eml_event ($$$$) {
 	}
 }
 
-sub maildir_event { # via wq_io_do
+sub maildir_event { # via wq_nonblock_do
 	my ($self, $fn, $vmd, $state) = @_;
 	if (my $eml = PublicInbox::InboxWritable::eml_from_path($fn)) {
 		eml_event($self, $eml, $vmd, $state);
@@ -93,6 +93,7 @@ sub lei_note_event {
 		my ($op_c, $ops) = $lei->workers_start($wq, $jobs);
 		$lei->wait_wq_events($op_c, $ops);
 		note_event_arm_done($lei);
+		$wq->prepare_nonblock;
 		$lei->{lne} = $wq;
 	};
 	if ($folder =~ /\Amaildir:/i) {
@@ -101,7 +102,7 @@ sub lei_note_event {
 		return if index($fl, 'T') >= 0;
 		my $kw = PublicInbox::MdirReader::flags2kw($fl);
 		my $vmd = { kw => $kw, sync_info => [ $folder, \$bn ] };
-		$self->wq_do('maildir_event', $fn, $vmd, $state);
+		$self->wq_nonblock_do('maildir_event', $fn, $vmd, $state);
 	} # else: TODO: imap
 }
 
diff --git a/lib/PublicInbox/WQBlocked.pm b/lib/PublicInbox/WQBlocked.pm
new file mode 100644
index 00000000..fbb43600
--- /dev/null
+++ b/lib/PublicInbox/WQBlocked.pm
@@ -0,0 +1,49 @@
+# Copyright (C) all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+
+# non-blocking workqueues, currently used by LeiNoteEvent to track renames
+package PublicInbox::WQBlocked;
+use v5.12;
+use parent qw(PublicInbox::DS);
+use PublicInbox::Syscall qw(EPOLLOUT EPOLLONESHOT);
+use PublicInbox::IPC;
+use Carp ();
+use Socket qw(MSG_EOR);
+
+sub new {
+	my ($cls, $wq, $buf) = @_;
+	my $self = bless { msgq => [$buf], }, $cls;
+	$wq->{wqb} = $self->SUPER::new($wq->{-wq_s1}, EPOLLOUT|EPOLLONESHOT);
+}
+
+sub flush_send {
+	my ($self) = @_;
+	push(@{$self->{msgq}}, $_[1]) if defined($_[1]);
+	while (defined(my $buf = shift @{$self->{msgq}})) {
+		if (ref($buf) eq 'CODE') {
+			$buf->($self); # could be \&PublicInbox::DS::close
+		} else {
+			my $wq_s1 = $self->{sock};
+			my $n = $PublicInbox::IPC::send_cmd->($wq_s1, [], $buf,
+								MSG_EOR);
+			next if defined($n);
+			Carp::croak("sendmsg: $!") unless $!{EAGAIN};
+			PublicInbox::DS::epwait($wq_s1, EPOLLOUT|EPOLLONESHOT);
+			unshift @{$self->{msgq}}, $buf;
+			last; # wait for ->event_step
+		}
+	}
+}
+
+sub enq_close { flush_send($_[0], $_[0]->can('close')) }
+
+sub event_step { # called on EPOLLOUT wakeup
+	my ($self) = @_;
+	eval { flush_send($self) } if $self->{sock};
+	if ($@) {
+		warn $@;
+		$self->close;
+	}
+}
+
+1;