Skip the 004_io_direct.pl test if a pre-flight check fails.

The test previously had a list of OSes that direct I/O was expected to
work on.  That worked well enough for the systems in our build farm, but
didn't survive contact with the Debian build bots running on tmpfs via
overlayfs.  tmpfs does not support O_DIRECT, but we don't want to
exclude Linux generally.

The new approach is to try to create an empty file with O_DIRECT from
Perl first.  If that fails, we'll skip the test and report what the
error was.

Reported-by: Christoph Berg <myon@debian.org>
Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/ZDYd4A78cT2ULxZZ%40msg.df7cb.de

src/test/modules/test_misc/t/004_io_direct.pl

@@ -2,19 +2,38 @@
 
 use strict;
 use warnings;
+use Fcntl;
+use IO::File;
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 use Test::More;
 
-# Systems that we know to have direct I/O support, and whose typical local
-# filesystems support it or at least won't fail with an error.  (illumos should
-# probably be in this list, but perl reports it as solaris.  Solaris should not
-# be in the list because we don't support its way of turning on direct I/O, and
-# even if we did, its version of ZFS rejects it, and OpenBSD just doesn't have
-# it.)
-if (!grep { $^O eq $_ } qw(aix darwin dragonfly freebsd linux MSWin32 netbsd))
+# We know that macOS has F_NOCACHE, and we know that Windows has
+# FILE_FLAG_NO_BUFFERING, and we assume that their typical file systems will
+# accept those flags.  For every other system, we'll probe for O_DIRECT
+# support.
+
+if ($^O ne 'darwin' && $^O ne 'MSWin32')
 {
-	plan skip_all => "no direct I/O support";
+	# Perl's Fcntl module knows if this system has O_DIRECT in <fcntl.h>.
+	if (defined &O_DIRECT)
+	{
+		# Can we open a file in O_DIRECT mode in the file system where
+		# tmp_check lives?
+		my $f = IO::File->new(
+			"${PostgreSQL::Test::Utils::tmp_check}/test_o_direct_file",
+			O_RDWR | O_DIRECT | O_CREAT);
+		if (!$f)
+		{
+			plan skip_all =>
+			  "pre-flight test if we can open a file with O_DIRECT failed: $!";
+		}
+		$f->close;
+	}
+	else
+	{
+		plan skip_all => "no O_DIRECT";
+	}
 }
 
 my $node = PostgreSQL::Test::Cluster->new('main');