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, 5 insertions, 2 deletions

diff --git a/t/common.perl b/t/common.perl
index 3f05b68a..91d65c5f 100644
--- a/t/common.perl
+++ b/t/common.perl
@@ -24,15 +24,18 @@ sub tcp_server () {
                 Proto => 'tcp',
                 Type => Socket::SOCK_STREAM(),
                 Listen => 1024,
+                Blocking => 0,
         )
 }
 
 sub unix_server ($) {
-        IO::Socket::UNIX->new(
+        my $s = IO::Socket::UNIX->new(
                 Listen => 1024,
                 Type => Socket::SOCK_STREAM(),
                 Local => $_[0],
-        )
+        );
+        $s->blocking(0);
+        $s;
 }
 
 sub spawn_listener {