Implement fileno() support with unique values per open

lib/Test/MockFile.pm

@@ -1422,7 +1422,6 @@ When creating mocked files or directories, we default their stats to:
             'mtime'     => $now,          # stat[9]
             'ctime'     => $now,          # stat[10]
             'blksize'   => 4096,          # stat[11]
-            'fileno'    => undef,         # fileno()
     } );
 
 You'll notice that mode, size, and blocks have been left out of this.
@@ -1464,7 +1463,6 @@ sub _default_mock_attrs {
         'mtime'                  => $now,      # stat[9]
         'ctime'                  => $now,      # stat[10]
         'blksize'                => 4096,      # stat[11]
-        'fileno'                 => undef,     # fileno()
         'tty'                    => 0,
         'readlink'               => '',
         'path'                   => undef,
@@ -1535,8 +1533,6 @@ sub new {
         $self->{'nlink'} = ( $self->{'mode'} & S_IFMT ) == S_IFDIR ? 2 : 1;
     }
 
-    $self->{'fileno'} //= _unused_fileno();
-
     $files_being_mocked{$path} = $self;
     Scalar::Util::weaken( $files_being_mocked{$path} );
 
@@ -2281,10 +2277,6 @@ sub stat {
     );
 }
 
-sub _unused_fileno {
-    return 900;                # TODO
-}
-
 =head2 readlink
 
 Optional Arg: $readlink

lib/Test/MockFile/FileHandle.pm

@@ -20,6 +20,8 @@ my $files_being_mocked;
     $files_being_mocked = \%Test::MockFile::files_being_mocked;
 }
 
+my $_next_fileno = 900;
+
 =head1 NAME
 
 Test::MockFile::FileHandle - Provides a class for L<Test::MockFile> to
@@ -75,6 +77,7 @@ sub TIEHANDLE {
         'read'   => $mode =~ m/r/ ? 1 : 0,
         'write'  => $mode =~ m/w/ ? 1 : 0,
         'append' => $mode =~ m/a/ ? 1 : 0,
+        'fileno' => $_next_fileno++,
     }, $class;
 
     # This ref count can't hold the object from getting released.
@@ -578,19 +581,15 @@ sub OPEN {
 
 =head2 FILENO
 
-B<UNIMPLEMENTED>: Open a ticket in
-L<github|https://github.com/cpanel/Test-MockFile/issues> if you need
-this feature.
-
-No L<perldoc
-documentation|http://perldoc.perl.org/perltie.html#Tying-FileHandles>
-exists on this method.
+Returns the unique file descriptor number assigned to this handle when
+it was opened. Each C<open()> call gets its own fileno, mirroring real
+kernel behavior.
 
 =cut
 
 sub FILENO {
     my ($self) = @_;
-    die 'fileno is purposefully unsupported';
+    return $self->{'fileno'};
 }
 
 =head2 SEEK

t/fileno.t

@@ -14,13 +14,37 @@ my $file = Test::MockFile->file( '/foo', '' );
 my $fh;
 ok( lives( sub { open $fh, '<', '/foo' } ), 'Opened file' );
 
-like(
-    dies( sub { fileno $fh } ),
-    qr/\Qfileno is purposefully unsupported\E/xms,
-    'Refuse to support fileno',
-);
+# fileno should return a value for mocked file handles
+ok( defined fileno($fh), 'fileno returns a defined value for a mocked fh' );
 
-ok( lives( sub { close $fh } ), 'Opened file' );
+ok( lives( sub { close $fh } ), 'Closed file' );
+
+# Each open() should get a unique fileno, even on the same mocked file.
+# This mirrors real behavior where fileno is a property of open(), not the file.
+subtest 'unique fileno per open filehandle' => sub {
+    my $mock_a = Test::MockFile->file( '/unique_a', 'aaa' );
+    my $mock_b = Test::MockFile->file( '/unique_b', 'bbb' );
+
+    open my $fh_a, '<', '/unique_a' or die;
+    open my $fh_b, '<', '/unique_b' or die;
+    open my $fh_a2, '<', '/unique_a' or die;    # second open of same file
+
+    my @filenos = map { fileno($_) } ( $fh_a, $fh_b, $fh_a2 );
+
+    # All three should be defined
+    ok( defined $filenos[0], 'fileno for fh_a is defined' );
+    ok( defined $filenos[1], 'fileno for fh_b is defined' );
+    ok( defined $filenos[2], 'fileno for fh_a2 is defined' );
+
+    # All three should be distinct (even the two opens of /unique_a)
+    isnt( $filenos[0], $filenos[1], 'fh_a and fh_b have different filenos' );
+    isnt( $filenos[0], $filenos[2], 'fh_a and fh_a2 have different filenos' );
+    isnt( $filenos[1], $filenos[2], 'fh_b and fh_a2 have different filenos' );
+
+    close $fh_a;
+    close $fh_b;
+    close $fh_a2;
+};
 
 done_testing();
 exit;