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

[Artem Chuprina <ran@lasgalen.net>]

Glenn Morris -> Artem Chuprina  @ Fri, 13 Dec 2013 17:51:34 -0500:

 >> Having org on ext4 filesystem with 0664 permission on org/work.org, and
 >> c1 on FAT filesystem with file permissions 0075 (in fact, SD card on
 >> Android system, so such strange permissions) I do
 >>
 >> M-: copy-file("org/work.org" "c1/work.org" t)
 >>
 >> and get an error
 >>
 >> Debugger entered--Lisp error: (file-error "Doing chmod" "operation not
 >> permitted" $

 GM> I'm not sure this is a bug.
 GM> The file is copied, but there is an error trying to preserve permissions.
 GM> Because eval-expression-debug-on-error is non-nil by default, the
 GM> debugger pops up.

 GM> Emacs's behaviour seems the same as using

 GM> cp -a org/work.org c1/work.org

 GM> on the command-line. "-a" is appropriate because C-h f copy-file says:

 GM>   This function always sets the file modes of the output file to match
 GM>   the input file.

The problem is that now it is impossible to use other functions that use
copy-file.  I've encountered this bug trying to use org-mobile-push
that, among other, copies several files to its target directory (which I
want to be on FAT partition for Android's MobileOrg).  On the first such
file, copy-file throws an error, and whole operation stops there and so
fails.  org-mobile-push catches error and don't enter a debugger, but it
don't work either, thinking that it failed to copy file.  Yet file is
indeed copied, just (totally unneeded in this case) chmod failed.

I think that it is a good idea to _attempt to_ copy permissions.  But it
is a bad idea to throw an error if file is copied well but we cannot set
permissions.  These days it is very common to copy files between
incompatible file systems.  Most of us use FAT, many use CIFS, there are
also NFS, davfs, sshfs etc.  So I think that a reasonable behavior would
be to try to set permissions, but don't fail on chmod error.  At least,
on copy-file level (if chmod function itself throws an error, this is
ok, but copy-file should not).