After FdGetOsFHandle() failure, lastFileError() is misleading

[nmisch]

The pod documentation mentions lastFileError() reacting to FdGetOsFHandle(),
and File.xs indeed has code to save GetLastError() in that case. I tried
that as a workaround for issue #13. However, _get_osfhandle() does not
document GetLastError() being meaningful. Indeed, my experiments show that a
failing _get_osfhandle() can make GetLastError() return zero, among other
values. Test program:

use strict;
use warnings;
use Test::More;
use Win32API::File();
#use Socket;  # changes the error codes further

my $improbable_error_code = 15614;  # ERROR_PACKAGE_REPOSITORY_CORRUPTED
my($before, $after);

close STDIN;
for my $winsock_active (0, 1) {
  require Socket if $winsock_active;
  # fd 0 is closed but below the highest open fd
  # fd 100000 is closed and above the highest open fd
  # fd 1 is open
  for my $fd (0, 100000, 1) {
    Win32API::File::fileLastError($improbable_error_code);
    $before = 0+Win32API::File::fileLastError();
    Win32API::File::FdGetOsFHandle($fd);
    $after = 0+Win32API::File::fileLastError();
    is($after, $before, "winsock=$winsock_active fd=$fd");
  }
}

Output:

not ok 1 - winsock=0 fd=0
#   Failed test 'winsock=0 fd=0'
#   at //tsclient/remote/demo_closed.pl line 21.
#          got: '6'
#     expected: '15614'
not ok 2 - winsock=0 fd=100000
#   Failed test 'winsock=0 fd=100000'
#   at //tsclient/remote/demo_closed.pl line 21.
#          got: '203'
#     expected: '15614'
ok 3 - winsock=0 fd=1
not ok 4 - winsock=1 fd=0
#   Failed test 'winsock=1 fd=0'
#   at //tsclient/remote/demo_closed.pl line 21.
#          got: '0'
#     expected: '15614'
not ok 5 - winsock=1 fd=100000
#   Failed test 'winsock=1 fd=100000'
#   at //tsclient/remote/demo_closed.pl line 21.
#          got: '203'
#     expected: '15614'
ok 6 - winsock=1 fd=1
# Tests were run but no plan was declared and done_testing() was not seen.

I see the following ways of detecting FdGetOsFHandle() failure today, given
the need to work around issue #13 and $SUBJECT:

1. Test whether Perl is 32-bit. If 32-bit, compare the return value to
   0xffffffff. Otherwise, compare the return value to 0xffffffffffffffff.
   No known disadvantages.

2. Use a sequence like:

   fileLastError($improbable_error_code);
   FdGetOsFHandle(...);
   if ($improbable_error_code == fileLastError()) { # succeeded ...

   In practice, I expect _get_osfhandle() can set just a few of the thousands
   of error codes, making all others viable as $improbable_error_code. Having
   said that, it's infeasible to prove that GetLastError() will never report
   $improbable_error_code after _get_osfhandle().

3. Test $!==9 (EBADF). _get_osfhandle() documents this, and I have observed
   it to work. The risk would be FdGetOsFHandle() somehow making an
   additional library call that clobbers errno. That risk feels significant.

I will likely use (1) for the moment.