daemon: warn on inheriting blocking listeners

For users relying on socket activation via service manager (e.g.
systemd) and running multiple service instances (@1, @2),
we need to ensure configuration of the socket is NonBlocking.
Otherwise, service managers such as systemd may clear the
O_NONBLOCK flag for a small window where accept/accept4
blocks:

public-inbox-nntpd@1      |systemd         |public-inbox-nntpd@2
--------------------------+----------------+--------------------
F_SETFL,O_NONBLOCK|O_RDWR |                | (not running, yet)
                          |F_SETFL, O_RDWR |
                          |fork+exec @2... |
accept(...) # blocks!     |                |(started by systemd)
                          |                |F_SETFL,O_NONBLOCK|O_RDWR
                          |                |accept(...) non-blocking

It's a very small window where O_NONBLOCK can be cleared,
but it exists, and I finally hit it after many years.

1 files changed, 0 insertions, 1 deletions

diff --git a/lib/PublicInbox/Listener.pm b/lib/PublicInbox/Listener.pm
index 594dabb8..60428934 100644
--- a/lib/PublicInbox/Listener.pm
+++ b/lib/PublicInbox/Listener.pm
@@ -16,7 +16,6 @@ sub new ($$$) {
         setsockopt($s, SOL_SOCKET, SO_KEEPALIVE, 1);
         setsockopt($s, IPPROTO_TCP, TCP_NODELAY, 1); # ignore errors on non-TCP
         listen($s, 1024);
-        IO::Handle::blocking($s, 0);
         my $self = fields::new($class);
         $self->SUPER::new($s, EPOLLIN|EPOLLET|EPOLLEXCLUSIVE);
         $self->{post_accept} = $cb;