Follow up on extended attributes

From: Timothe Litt <litt_at_acm.org>
Date: Sun, 15 Jan 2023 11:16:09 -0500

As I noted in an earlier reply, etags belong with file data and extended
attributes, where available, are a good place to keep them.  Better than
dotfiles, but less portable (which is why I suggested dotfiles.)

Since many people don't know about extended attributes, here is a toy
demo program that creates, deletes, and lists them.

The Perl module supports a number of (but not all the) useful
filesystem, so its source code may be helpful if anyone wants to go that
way with curl.  Windows is a notable exception, but NTFS streams are a
reasonable substitute.  Note that the module has a bug that will keep
this demo from working - my fork (on github
<https://github.com/tlhackque/file-extattr>) has a patch.

e.g.

$ touch myfile
$  ~/ext myfile curl-etag abc-def-hk  # Add an attribute
$ ls -l myfile
-rw-r--r-- 1 litt litt 0 Jan 15 10:44 myfile
$ ~/ext myfile                                       # List all attributes
     curl-etag : "abc-def-hk"

Note that the file is still zero-length; the attribute is stored in the
filesystem metadata, not the file.

FYI, NTFS "streams" are documented here
<https://learn.microsoft.com/en-us/windows/win32/fileio/file-streams>and
here <https://learn.microsoft.com/en-us/windows/win32/fileio/using-streams>.

Enjoy.

|#!/usr/bin/perl||
||
||use warnings;||
||use strict;||
||
||use File::ExtAttr( qw/:all/);||
||
||# Toy demo program for extended file attributes.||
||# The Perl module "File::Extattr" supports several common file systems.||
||
||unless( _at_ARGV ) {||
||    print <<"XXX" ;||
||ext - this help||
||
||ext file - list file attributes of file||
||
||ext file attr - delete attr||
||
||ext file attr val - create or replace attr=val||
||
||ext file attr val create - create attr=value||(can't exist)

||ext file attr val replace - replace attr=value (must exist)||
||||XXX||
||      exit;||
||}||
||
||open( my $fh, '<', $ARGV[0] ) or die( "$ARGV[0]: $!\n");||||
||
||if( _at_ARGV >= 3 ) {||
||    my _at_op = _at_ARGV == 4? $ARGV[3] eq 'replace'? ( replace => 1 ) :
(create => 1) : ();||
||    unless( setfattr( $fh, $ARGV[1], $ARGV[2], { namespace => 'user',
_at_op } ) ) {||
||        print "    set, '$ARGV[1]' => '$ARGV[2]' : $!\n";||
||    }||
||} elsif( _at_ARGV == 2 ) {||
||    unless( delfattr( $fh, $ARGV[1], { namespace => 'user' } ) ) {||
||        print "    del: failed\n";||
||    }||
||} elsif( _at_ARGV == 1 ) {||
||    foreach my $attr ( listfattr( $fh, { namespace => 'user' } ) ) {||
||        my $val = getfattr( $fh, $attr, {namespace => 'user' } );||
||        if( defined $val ) {||
||            print "    $attr : \"$val\"\n";||
||        } else {||
||            print "    $attr : $!\n";||
||        }||
||    }||
||}||
|||

--
Timothe Litt
ACM Distinguished Engineer
--------------------------
This communication may not represent the ACM or my employer's views,
if any, on the matters discussed.

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Received on 2023-01-15