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Filename : filetest.pm
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package filetest;

our $VERSION = '1.03';

=head1 NAME

filetest - Perl pragma to control the filetest permission operators

=head1 SYNOPSIS

    $can_perhaps_read = -r "file";	# use the mode bits
    {
        use filetest 'access';		# intuit harder
        $can_really_read = -r "file";
    }
    $can_perhaps_read = -r "file";	# use the mode bits again

=head1 DESCRIPTION

This pragma tells the compiler to change the behaviour of the filetest
permission operators, C<-r> C<-w> C<-x> C<-R> C<-W> C<-X>
(see L<perlfunc>).

The default behaviour of file test operators is to use the simple
mode bits as returned by the stat() family of system calls.  However,
many operating systems have additional features to define more complex
access rights, for example ACLs (Access Control Lists).
For such environments, C<use filetest> may help the permission
operators to return results more consistent with other tools.

The C<use filetest> or C<no filetest> statements affect file tests defined in
their block, up to the end of the closest enclosing block (they are lexically
block-scoped).

Currently, only the C<access> sub-pragma is implemented.  It enables (or
disables) the use of access() when available, that is, on most UNIX systems and
other POSIX environments.  See details below.

=head2 Consider this carefully

The stat() mode bits are probably right for most of the files and
directories found on your system, because few people want to use the
additional features offered by access(). But you may encounter surprises
if your program runs on a system that uses ACLs, since the stat()
information won't reflect the actual permissions.

There may be a slight performance decrease in the filetest operations
when the filetest pragma is in effect, because checking bits is very
cheap.

Also, note that using the file tests for security purposes is a lost cause
from the start: there is a window open for race conditions (who is to
say that the permissions will not change between the test and the real
operation?).  Therefore if you are serious about security, just try
the real operation and test for its success - think in terms of atomic
operations.  Filetests are more useful for filesystem administrative
tasks, when you have no need for the content of the elements on disk.

=head2 The "access" sub-pragma

UNIX and POSIX systems provide an abstract access() operating system call,
which should be used to query the read, write, and execute rights. This
function hides various distinct approaches in additional operating system
specific security features, like Access Control Lists (ACLs)

The extended filetest functionality is used by Perl only when the argument
of the operators is a filename, not when it is a filehandle.

=head2 Limitation with regard to C<_>

Because access() does not invoke stat() (at least not in a way visible
to Perl), B<the stat result cache "_" is not set>.  This means that the
outcome of the following two tests is different.  The first has the stat
bits of F</etc/passwd> in C<_>, and in the second case this still
contains the bits of C</etc>.

 { -d '/etc';
   -w '/etc/passwd';
   print -f _ ? 'Yes' : 'No';   # Yes
 }

 { use filetest 'access';
   -d '/etc';
   -w '/etc/passwd';
   print -f _ ? 'Yes' : 'No';   # No
 }

Of course, unless your OS does not implement access(), in which case the
pragma is simply ignored.  Best not to use C<_> at all in a file where
the filetest pragma is active!

As a side effect, as C<_> doesn't work, stacked filetest operators
(C<-f -w $file>) won't work either.

This limitation might be removed in a future version of perl.

=cut

$filetest::hint_bits = 0x00400000; # HINT_FILETEST_ACCESS

sub import {
    if ( $_[1] eq 'access' ) {
	$^H |= $filetest::hint_bits;
    } else {
	die "filetest: the only implemented subpragma is 'access'.\n";
    }
}

sub unimport {
    if ( $_[1] eq 'access' ) {
	$^H &= ~$filetest::hint_bits;
    } else {
	die "filetest: the only implemented subpragma is 'access'.\n";
    }
}

1;