ipc: fix IO::FDPass use with a worker limit of 1

IO::FDPass is our last choice for implementing the workqueue
because its lack of atomicity makes it impossible to guarantee
all requests of a single group hit a single worker out of many.

So the only way to use IO::FDPass for workqueues it to only have
a single worker.  A single worker still buys us a small amount
of parallelism because of the parent process.

2 files changed, 42 insertions, 10 deletions

diff --git a/lib/PublicInbox/IPC.pm b/lib/PublicInbox/IPC.pm
index b0a0bfb5..e6a1082c 100644
--- a/lib/PublicInbox/IPC.pm
+++ b/lib/PublicInbox/IPC.pm
@@ -12,6 +12,7 @@ use POSIX qw(WNOHANG);
 use Socket qw(AF_UNIX MSG_EOR);
 my $SEQPACKET = eval { Socket::SOCK_SEQPACKET() }; # portable enough?
 use constant PIPE_BUF => $^O eq 'linux' ? 4096 : POSIX::_POSIX_PIPE_BUF();
+my $WQ_MAX_WORKERS = 4096;
 my ($enc, $dec);
 # ->imports at BEGIN turns sereal_*_with_object into custom ops on 5.14+
 # and eliminate method call overhead
@@ -36,17 +37,39 @@ if ($enc && $dec) { # should be custom ops
         } // warn("Storable (part of Perl) missing: $@\n");
 }
 
+my $recv_cmd1; # PublicInbox::CmdIPC1::recv_cmd1;
 my $recv_cmd = PublicInbox::Spawn->can('recv_cmd4');
 my $send_cmd = PublicInbox::Spawn->can('send_cmd4') // do {
         require PublicInbox::CmdIPC4;
         $recv_cmd //= PublicInbox::CmdIPC4->can('recv_cmd4');
         PublicInbox::CmdIPC4->can('send_cmd4');
 } // do {
+        # IO::FDPass only allows sending a single FD at-a-time, which
+        # means we can't guarantee all packets end up on the same worker,
+        # so we cap WQ_MAX_WORKERS
         require PublicInbox::CmdIPC1;
-        $recv_cmd = PublicInbox::CmdIPC1->can('recv_cmd1');
+        $recv_cmd1 = PublicInbox::CmdIPC1->can('recv_cmd1');
+        $WQ_MAX_WORKERS = 1 if $recv_cmd1;
+        wq_set_recv_fds(3);
         PublicInbox::CmdIPC1->can('send_cmd1');
 };
 
+# needed to tell recv_cmd1 how many times to loop IO::FDPass::recv
+sub wq_set_recv_fds {
+        return unless $recv_cmd1;
+        my $nfds = pop;
+        my $sub = sub {
+                my ($sock, $fds, undef, $flags) = @_;
+                $recv_cmd1->($sock, $fds, $_[2], $flags, $nfds);
+        };
+        my $self = pop;
+        if (ref $self) {
+                $self->{-wq_recv_cmd} = $sub;
+        } else {
+                $recv_cmd = $sub;
+        }
+}
+
 sub _get_rec ($) {
         my ($r) = @_;
         defined(my $len = <$r>) or return;
@@ -237,8 +260,9 @@ sub wq_worker_loop ($) {
         local $SIG{PIPE} = sub {
                 die(bless(\"$_[0]", __PACKAGE__.'::PIPE')) if $sub;
         };
+        my $rcv = $self->{-wq_recv_cmd} // $recv_cmd;
         until ($self->{-wq_quit}) {
-                my (@fds) = $recv_cmd->($s2, $buf, $len) or return; # EOF
+                my (@fds) = $rcv->($s2, $buf, $len) or return; # EOF
                 my $i = 0;
                 my @m = @{$self->{wq_open_modes} // [qw( +<&= >&= >&= )]};
                 for my $fd (@fds) {
@@ -305,6 +329,7 @@ sub wq_workers_start {
         socketpair($s1, $s2, AF_UNIX, $SEQPACKET, 0) or die "socketpair: $!";
         $self->ipc_atfork_parent;
         $nr_workers //= 4;
+        $nr_workers = $WQ_MAX_WORKERS if $nr_workers > $WQ_MAX_WORKERS;
         my $sigset = $oldset // PublicInbox::DS::block_signals();
         $self->{-wq_workers} = {};
         $self->{-wq_ident} = $ident;
@@ -318,6 +343,7 @@ sub wq_workers_start {
 sub wq_worker_incr { # SIGTTIN handler
         my ($self, $oldset) = @_;
         $self->{-wq_s2} or return;
+        return if wq_workers($self) >= $WQ_MAX_WORKERS;
         $self->ipc_atfork_parent;
         my $sigset = $oldset // PublicInbox::DS::block_signals();
         _wq_worker_start($self, $sigset);
@@ -331,7 +357,7 @@ sub wq_exit { # wakes up wq_worker_decr_wait
 
 sub wq_worker_decr { # SIGTTOU handler, kills first idle worker
         my ($self) = @_;
-        my $workers = $self->{-wq_workers} or return;
+        return unless wq_workers($self);
         my $s2 = $self->{-wq_s2} // die 'BUG: no wq_s2';
         $self->wq_do('wq_exit', [ $s2, $s2, $s2 ]);
         $self->{-wq_exit_pending}++;
@@ -377,6 +403,8 @@ sub wq_close {
         }
 }
 
+sub WQ_MAX_WORKERS { $WQ_MAX_WORKERS }
+
 sub DESTROY {
         wq_close($_[0]);
         ipc_worker_stop($_[0]);
diff --git a/t/ipc.t b/t/ipc.t
index d2b6ad4f..fd290809 100644
--- a/t/ipc.t
+++ b/t/ipc.t
@@ -171,13 +171,17 @@ is($warn[2], $warn[1], 'worker did not die');
 $SIG{__WARN__} = 'DEFAULT';
 is($ipc->wq_workers_start('wq', 1), $$, 'workers started again');
 is($ipc->wq_workers, 1, '1 worker started');
-$ipc->wq_worker_incr;
-is($ipc->wq_workers, 2, 'worker count bumped');
-$ipc->wq_worker_decr;
-$ipc->wq_worker_decr_wait(10);
-is($ipc->wq_workers, 1, 'worker count lowered');
-is($ipc->wq_workers(2), 2, 'worker count set');
-is($ipc->wq_workers, 2, 'worker count stayed set');
+SKIP: {
+        $ipc->WQ_MAX_WORKERS > 1 or
+                skip 'Inline::C or Socket::MsgHdr not available', 4;
+        $ipc->wq_worker_incr;
+        is($ipc->wq_workers, 2, 'worker count bumped');
+        $ipc->wq_worker_decr;
+        $ipc->wq_worker_decr_wait(10);
+        is($ipc->wq_workers, 1, 'worker count lowered');
+        is($ipc->wq_workers(2), 2, 'worker count set');
+        is($ipc->wq_workers, 2, 'worker count stayed set');
+}
 $ipc->wq_close;
 is($ipc->wq_workers, undef, 'workers undef after close');