Date: Mon, 19 Nov 2012 18:13:31 +0000 From: Chris Rees <utisoft@gmail.com> To: Bruce Evans <brde@optusnet.com.au> Cc: svn-src-head@freebsd.org, Ed Schouten <ed@80386.nl>, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r243228 - head/etc Message-ID: <CADLo83_SmUpbU0sQnSAr33DBQ7TURtQ%2BngxoOV3t29%2BSbbBBJw@mail.gmail.com> In-Reply-To: <20121119175936.J1085@besplex.bde.org> References: <201211181421.qAIEL5KT042019@svn.freebsd.org> <CAJOYFBBWmm_eoS9qOYMxigtS%2BSjafXXVeT_BmgdWFgEF69j%2BNw@mail.gmail.com> <CADLo83-RM8dQTQZ7HdQPHvyZ1aHQpqFfGOp6x51-gTtqGL84=g@mail.gmail.com> <20121119175936.J1085@besplex.bde.org>
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On 19 November 2012 08:03, Bruce Evans <brde@optusnet.com.au> wrote:
> On Sun, 18 Nov 2012, Chris Rees wrote:
>
>> On 18 Nov 2012 20:39, "Ed Schouten" <ed@80386.nl> wrote:
>>>
>>>
>>> Hi Chris,
>>>
>>> 2012/11/18 Chris Rees <crees@freebsd.org>:
>>>>
>>>> Modified: head/etc/rc.initdiskless
>>>>
>>
>> ==============================================================================
>>>>
>>>> --- head/etc/rc.initdiskless Sun Nov 18 14:05:28 2012
>>
>> (r243227)
>>>>
>>>> +++ head/etc/rc.initdiskless Sun Nov 18 14:21:05 2012
>>
>> (r243228)
>>>>
>>>> @@ -354,7 +354,7 @@ for i in ${templates} ; do
>>>> subdir=${j##*/}
>>>> if [ -d $j -a ! -f $j.cpio.gz ]; then
>>>> create_md $subdir
>>>> - cp -Rp $j/ /$subdir
>>>> + (cd $j && pax -rw . /$subdir)
>>>> fi
>>>> done
>>>> for j in /conf/$i/*.cpio.gz ; do
>>>
>>>
>>> Are you sure that this bug wasn't already fixed? The original version
>>> of the code in the bug report used the following line:
>>>
>>> - cp -Rp $j/* /$subdir
>>>
>>> The old version of the code you changed didn't have this asterisk,
>>> meaning dotfiles would already be copied. Still, you could argue that
>>> your version is nicer, as our behaviour of cp with the trailing slash
>>> contradicts POSIX.
>
>
> I don't think POSIX is that broken.
>
>
>> You are correct, and the second to point it out :)
>>
>> As you say however, pax is technically how it should be done anyway, and
>> has the nice effect of also preserving hard links. If no-one objects I
>> think it should stay in.
>
>
> Not perserving hard links is a bug in cp -R.
pax/tar/cpio have always been recommended over cp -R for this very
reason-- I would imagine that the fix is non-trivial if this is a bug
at all (which I don't think it is).
> Another bug in cp -Rp is that
> it doesn't preserve mtimes for directories. But a non-broken cp -Rp would
> be nicer than non-broken use of pax, and even a broken cp -Rp is better than
> a broken use of pax. The above use of pax is semantically very different
> from cp -Rp, and introduces the following bugs:
>
> - no error checking for cd. We have just checked that $j is a directory.
> If it should somehow go away, then the errors from cp -Rp of it are more
> fail-safe than the errors from not checking for cd failure.
$ cp -Rp spam /eggs
cp: spam: No such file or directory
$ cd spam && pax -rw . /eggs
cd: spam: No such file or directory
$
I'm not seeing a huge difference. How is cp more fail safe?
> - cp -R creates the target directory /$subdir if it doesn't already exist
> (and its path prefix does exist and is a directory or a symlink to a
> directory), but pax doesn't If /$subdir does already exist, then there
> are races similar to the ones for the source directory if the target
> directory goes away, and pax handles them differently.
In that case the cp -R has the wrong behaviour-- /$subdir must exist,
or it should fail with an error.
> - -p was used in 'cp -Rp' to preserve all attributes. The corresponding
> flags are not used in 'pax -rw'. They are '-p e'. By default,
> pax preserves file times (so '-p am' is part of the default). pax's
> man page seems to say that the file mode is not preserved by default
> and that '-p p' (or '-p e') must be used to preserve it, but in my
> tests under FreeBSD-~5.2 it was preserved. The uid and gid can only
> be preserved by root, and it is the default to not preserve them even
> for root. Root should use '-p e' or '-p o' to preseve them, just like
> -p was used with cp -Rp.
This I'll concede.
> pax is little used and poorly maintained. It has no support for acls,
> while cp has some. Pax hasn't caught up with the creation of utimes(2)
> in 4.2BSD, so it still clobbers the tv_nsec part of file times when it
> "preserves" them (though it uses lutimes(2)), while cp only clobbers
> the last 3 decimal digits in the tv_nsec part of file times when it
> "preserves" them. So even with '-p e', pax often clobbers file times
> and never preserves acls even. Maybe this doesn't matter here. But
> pax is unusable in general. I normally use cp -pR when I don't care
> about links or file times (which is rarely), else gnu tar if I care
> about links but not file times, else bsd tar occasionally for its
> better handling of file times (tar format just can't handle all the
> times well, and bsd tar handles them slightly less badly), else
> gnu tar following by a fixup program to duplicate all the times.
> gnu cp -a should work best, but I haven't used it lately.
How are any of these alternatives good in an rc script? Using tar
requires two threads (tar c | tar x), which is why I replaced it with
pax in the first place.
Diskless init means that we use RAM as disks-- for your own
entertainment, I suggest making a copy of /rescue with pax, and then
with cp -R. Which would you prefer in your ramdisk? For those
concerned about size, this also applies to you-- 96k of pax vs any
hardlink duplication.
Additional flags to pax are in the patch at [1]. With approval from
someone, I'll commit it.
Chris
[1] http://www.bayofrum.net/~crees/patches/rc-initdiskless-paxflags.diff
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