bug#16133: 24.3; copy-file fails on chmod when copying to FAT filesystem

[Artem Chuprina]

Paul Eggert -> Artem Chuprina  @ Mon, 23 Dec 2013 15:58:29 -0800:

 >> As you appeal to GNU cp, see
 >> its default behavior: BY DEFAULT it TRIES to save permissions and
 >> owner/group

 PE> No, by default GNU cp does not try to copy either owner/group
 PE> or permissions to an existing destination.  It does not invoke
 PE> chmod or chown unless you use something like 'cp -p'.

You are wrong.

zsh% umask 
002
zsh% touch testfile
zsh% ls -l testfile
-rw-rw-r-- 1 ran ran 0 Дек 24 10:37 testfile
zsh% chmod 600 testfile
zsh% cp testfile testfile.copy
zsh% ls -l testfile.copy 
-rw------- 1 ran ran 0 Дек 24 10:38 testfile.copy

Debian GNU/Linux 7.3 (wheezy)

 >> I can create files there, but I cannot change
 >> their metainfo, because they are not mine.

 PE> This is an unusual setup, at least for me.
 PE> If I create a file, I should be able to change its metainformation.

Key phrase: "at least for me".  This is usual setup with FAT filesystems
on every multiuser system.  Because FAT cannot keep owner and there are
multiple users, it is mounted so that all the files belong to root or
another system user, not to some real user.

 PE> I expect this setup will cause problems with other applications,
 PE> not just Emacs.  GNU tar would be one example.

You are wrong again.

zsh% tar tvf ~/testfiles.tar
-rw------- ran/ran           0 2013-12-24 10:37 testfile
-rw------- ran/ran           0 2013-12-24 10:38 testfile.copy
zsh% cd /fat 
zsh% tar xvf ~/testfiles.tar 
testfile
testfile.copy
zsh% echo $?
0
zsh% ls -l testfile*
-rw-rw-rw- 1 root root 0 Дек 24 10:37 testfile
-rw-rw-rw- 1 root root 0 Дек 24 10:38 testfile.copy

 PE> That being said, we should be able to work around the problem
 PE> by having copy-file behave more like 'cp'.  That is, copy-file
 PE> should not invoke chmod by default; it should invoke chmod only
 PE> if it's told to preserve permissions (or preserve ownership,
 PE> since that often involves temporarily revoking permissions for
 PE> security reasons).  That way, plain copy-file should work with
 PE> your setup, although you'll still have trouble with
 PE> copy-file with the last arg t (which asks to copy permissions).

This is also reasonable behavior, but not the best one, and it is
inconsistent with such of GNU cp and GNU tar, as proved above.

 PE> Attached is a proposed patch to do that, against trunk bzr 115721.
 PE> Does it solve your problem?

I'll try to check it today.