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, 9 insertions, 1 deletions

diff --git a/lib/PublicInbox/Daemon.pm b/lib/PublicInbox/Daemon.pm
index 2b7ac266..2046a7f5 100644
--- a/lib/PublicInbox/Daemon.pm
+++ b/lib/PublicInbox/Daemon.pm
@@ -155,9 +155,9 @@ sub daemon_prepare ($) {
                 my $s = eval { $sock_pkg->new(%o) };
                 warn "error binding $l: $! ($@)\n" unless $s;
                 umask $prev;
-
                 if ($s) {
                         $listener_names{sockname($s)} = $s;
+                        $s->blocking(0);
                         push @listeners, $s;
                 }
         }
@@ -363,6 +363,14 @@ sub inherit () {
         foreach my $fd (3..$end) {
                 my $s = IO::Handle->new_from_fd($fd, 'r');
                 if (my $k = sockname($s)) {
+                        if ($s->blocking) {
+                                $s->blocking(0);
+                                warn <<"";
+Inherited socket (fd=$fd) is blocking, making it non-blocking.
+Set 'NonBlocking = true' in the systemd.service unit to avoid stalled
+processes when multiple service instances start.
+
+                        }
                         $listener_names{$k} = $s;
                         push @rv, $s;
                 } else {