From owner-freebsd-current@FreeBSD.ORG  Sat Aug  2 09:40:04 2014
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: freebsd-current@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTPS id C5D9C5DB;
 Sat,  2 Aug 2014 09:40:04 +0000 (UTC)
Received: from mail-qg0-x234.google.com (mail-qg0-x234.google.com
 [IPv6:2607:f8b0:400d:c04::234])
 (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits))
 (Client CN "smtp.gmail.com",
 Issuer "Google Internet Authority G2" (verified OK))
 by mx1.freebsd.org (Postfix) with ESMTPS id 78BC92839;
 Sat,  2 Aug 2014 09:40:04 +0000 (UTC)
Received: by mail-qg0-f52.google.com with SMTP id f51so6730197qge.39
 for <multiple recipients>; Sat, 02 Aug 2014 02:40:03 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113;
 h=mime-version:sender:from:date:message-id:subject:to:cc:content-type;
 bh=8IAdaKslnIZn10Sm9Ta60kc8Pk9V/U4k1e16mrD0R2o=;
 b=YQssxVmrSypbMDOS7TzHIT5W2uzngo/RCMfJ1dvkT/OYLQyQXt4+k+bHpZ/k8tec6T
 eRQOr+SoiJwUrSVVRP9Lgg35iev7CST2re6svk8CaS3Y20doLkTEvenaYDcNSlEgt7kG
 DOfjm8CpwGgcXqiLoxjhQuUtx69+R71y3Dq1SkeovyP6DK66BKIrim/jB9z+GXl/O+NS
 HLl2289xRyd720lDUUzyrhps1a5hOtk7o+TUK+dnsmCt7bCElyqgOvxIu0VjHuCOrMKf
 aHzqizo3q5Dd0gT58GHIyWHlQ7LmHTZp5bAE8HhLGwuRD2CLXeQSy+Xpmk/D6qkjOLQ2
 z9Og==
X-Received: by 10.229.211.132 with SMTP id go4mr17470792qcb.0.1406972403527;
 Sat, 02 Aug 2014 02:40:03 -0700 (PDT)
MIME-Version: 1.0
Sender: fedor.indutny@gmail.com
Received: by 10.96.198.227 with HTTP; Sat, 2 Aug 2014 02:39:43 -0700 (PDT)
From: Fedor Indutny <fedor@indutny.com>
Date: Sat, 2 Aug 2014 13:39:43 +0400
X-Google-Sender-Auth: 9llRlbfxgOIvFJt-6R3cRu5BLQE
Message-ID: <CAEv2VfLoPiDtEmwJyWMGJuTOaPUTUJoD3B6VQDBE=4A6U4o4-A@mail.gmail.com>
Subject: KQueue 0-length UDP packet
To: freebsd-current@freebsd.org
Content-Type: text/plain; charset=UTF-8
X-Content-Filtered-By: Mailman/MimeDel 2.1.18
Cc: Mark Johnston <markj@freebsd.org>
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.18
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
 <freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-current>, 
 <mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current/>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
 <mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Sat, 02 Aug 2014 09:40:05 -0000

Hello!

I'm trying to figure out, why this code:

https://github.com/indutny/0-udp

Which basically sends a 0-length UDP packet to a server and polls
kqueue events on the server fd.

Return 1 kevent on FreeBSD, and blocks indefinitely without
returning any events on OSX.

So far I could see that FreeBSD and OSX are treating NOTE_LOWAT
differently:

*
https://github.com/opensource-apple/xnu/blob/2fa84067f6cdeb23267f877ca4fd26201316da1b/bsd/kern/uipc_socket.c#L4461
*
https://github.com/freebsd/freebsd/blob/6901832d8588537c81afbdb91d1a22deb5582c47/sys/kern/uipc_socket.c#L3163-L3164

FreeBSD's NOTE_LOWAT is overriding SO_RCVLOWAT, and OSX is using
SO_RVCLOWAT as a minimum value. But, since NOTE_LOWAT is not
involved here by default, I'm failing to see where exactly this
event could pass through kqueue filter.

Could anyone with UDP and/or KQueue implementation knowledge
share some insights with me?

Thank you very much!
Fedor.

(NOTE: Duplicate, first email wasn't posted, because I wasn't subscribed to
the ML)