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From: John Baldwin <jhb@freebsd.org>
To: Gleb Smirnoff <glebius@freebsd.org>
Subject: Re: svn commit: r274494 - head/sys/kern
Date: Fri, 14 Nov 2014 12:31:10 -0500
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On Friday, November 14, 2014 06:02:29 AM Gleb Smirnoff wrote:
> Author: glebius
> Date: Fri Nov 14 06:02:29 2014
> New Revision: 274494
> URL: https://svnweb.freebsd.org/changeset/base/274494
> 
> Log:
>   There should not be zero length mbufs in socket buffers.  The code comes
>   from r1451, and thus can't be explained.  A patch with explicit panic()
>   here survived all tests.

It can be explained. :)

https://svnweb.freebsd.org/csrg?view=revision&revision=17331

It seems even when it was added it wasn't necessary:

Author: 	karels
Date: 	Fri Nov 2 10:39:52 1984 UTC (30 years ago)
Changed paths: 	1
Log Message: 	

drop trailing null mbufs in sbdrop (probably unnecessary now)

-- 
John Baldwin