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Subject: Re: svn commit: r349256 - head/libexec/rc/rc.d
To: Conrad Meyer <cem@FreeBSD.org>, src-committers@freebsd.org,
 svn-src-all@freebsd.org, svn-src-head@freebsd.org
References: <201906210237.x5L2bt8I012721@repo.freebsd.org>
From: Eugene Grosbein <eugen@grosbein.net>
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21.06.2019 9:37, Conrad Meyer wrote:

> Author: cem
> Date: Fri Jun 21 02:37:54 2019
> New Revision: 349256
> URL: https://svnweb.freebsd.org/changeset/base/349256
> 
> Log:
>   rc.d/motd: Update motd more robustly
>   
>   Use appropriate fsyncs to persist the rewritten /etc/motd file, when a
>   rewrite is performed.
>   
>   Reported by:	Jonathan Walton <jonathan AT isilon.com>
>   Reviewed by:	allanjude, vangyzen
>   Sponsored by:	Dell EMC Isilon
>   Differential Revision:	https://reviews.freebsd.org/D20701
> 
> Modified:
>   head/libexec/rc/rc.d/motd
> 
> Modified: head/libexec/rc/rc.d/motd
> ==============================================================================
> --- head/libexec/rc/rc.d/motd	Fri Jun 21 00:52:30 2019	(r349255)
> +++ head/libexec/rc/rc.d/motd	Fri Jun 21 02:37:54 2019	(r349256)
> @@ -37,11 +37,15 @@ motd_start()
>  	uname -v | sed -e 's,^\([^#]*\) #\(.* [1-2][0-9][0-9][0-9]\).*/\([^\]*\) $,\1 (\3) #\2,' > ${T}
>  	awk '{if (NR == 1) {if ($1 == "FreeBSD") {next} else {print "\n"$0}} else {print}}' < /etc/motd >> ${T}
>  
> -	cmp -s $T /etc/motd || {
> -		cp $T /etc/motd
> +	if ! cmp -s $T /etc/motd; then
> +		mv -f $T /etc/.motd.tmp
> +		fsync /etc/.motd.tmp
> +		mv -f /etc/.motd.tmp /etc/motd
>  		chmod ${PERMS} /etc/motd
> -	}
> -	rm -f $T
> +		fsync /etc
> +	else
> +		rm -f $T
> +	fi
>  
>  	check_startmsgs && echo '.'
>  }

Why do we need fsync while updating small plain text file?