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bug#10055: [sr #107875] BUG cp -u corrupts 'fs'' information if interupt

From: Paul Eggert
Subject: bug#10055: [sr #107875] BUG cp -u corrupts 'fs'' information if interupted; can't recover on future invoctions
Date: Tue, 15 Nov 2011 12:23:57 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0

Thanks for your thoughtful suggestions.
I like many of the ideas and hope that somebody can find the time
to code them up.  Here are some more-detailed comments.

On 11/15/11 11:07, Linda Walsh wrote:

>   3). use posix_fallocate (if available) to allocate sufficient space for the
> copy

This seems like a good idea, independently of the other points.
That is, if A and B are regular files, "cp A B" could
use A's size to preallocate B's storage, and it could
fail immediately (without trashing B!) if there's not
enough storage.  I like this.

> A) catch INT (& catchable signals), and remove any files that are
> 'incomplete'

That might cause trouble in other cases.  For example, "cp A B" where
B already exists.  In this case it's unwise to remove B if interrupted
-- people won't expect that.  And in general 'cp' has behaved the way
that it does for decades, and we need to be careful about changing its
default behavior in such a fairly-drastic way.

But we could add an option to 'cp' to have this behavior.
Perhaps --remove-destination=signal?  That is --remove-destination
could have an optional list of names of places where the destination
could be removed, where the default is not to remove it, and
plain --remove-destination means --remove-destination=before.

> B) 1). open destination name for write (verifying accesses) w/
>       Exclusive Write;

This could be another new option, though (as you write) it's
orthogonal to the main point.  I would suggest that this option be
called --oflag=excl (by analogy with dd's oflag= option).  We can add
support for the other output flags while we're at it, e.g.,

>   2). open tmp file for actual cp operation.
>   5); rename tmp over original; (closing original before rename on systems
> that don't support separation of names and FD's (Win systems et al).

Yes, that could be another option.  I see (2) and (5) as being the
same feature.  Perhaps --remove-destination=after?

> C) reset DT stamps on newly opened files to '0' (~1969/70?)'

I dunno, this kind of time stamp munging sounds like it'd cause more
trouble than it'd cure.  It's more natural (and easier to debug
failures) if the last-modified time of a file is the time that the
file was last modified.

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