Consolidate methods for translating a Perl path to a Windows path.

This fixes some TAP suites when using msys Perl and a builddir located
in an msys mount point other than "/".  For example, builddir=/c/pg
exhibited the problem, since /c/pg falls in mount point "/c".
Back-patch to 9.6, where tests first started to perform such
translations.  In back branches, offer both new and old APIs.

Reviewed by Andrew Dunstan.

Discussion: https://postgr.es/m/20190610045838.GA238501@rfd.leadboat.com

Author: nmisch

src/test/perl/PostgresNode.pm

@@ -103,8 +103,7 @@ our @EXPORT = qw(
 our ($use_tcp, $test_localhost, $test_pghost, $last_host_assigned,
 	$last_port_assigned, @all_nodes);
 
-# Windows path to virtual file system root
-
+# For backward compatibility only.
 our $vfs_path = '';
 if ($Config{osname} eq 'msys')
 {
@@ -854,7 +853,7 @@ standby_mode=on
 sub enable_restoring
 {
 	my ($self, $root_node) = @_;
-	my $path = $vfs_path . $root_node->archive_dir;
+	my $path = TestLib::perl2host($root_node->archive_dir);
 	my $name = $self->name;
 
 	print "### Enabling WAL restore for node \"$name\"\n";
@@ -882,7 +881,7 @@ standby_mode = on
 sub enable_archiving
 {
 	my ($self) = @_;
-	my $path   = $vfs_path . $self->archive_dir;
+	my $path   = TestLib::perl2host($self->archive_dir);
 	my $name   = $self->name;
 
 	print "### Enabling WAL archiving for node \"$name\"\n";

src/test/perl/TestLib.pm

@@ -11,6 +11,7 @@ use strict;
 use warnings;
 
 use Config;
+use Cwd;
 use Exporter 'import';
 use File::Basename;
 use File::Spec;
@@ -156,6 +157,33 @@ sub tempdir_short
 	return File::Temp::tempdir(CLEANUP => 1);
 }
 
+# Translate a Perl file name to a host file name.  Currently, this is a no-op
+# except for the case of Perl=msys and host=mingw32.  The subject need not
+# exist, but its parent directory must exist.
+sub perl2host
+{
+	my ($subject) = @_;
+	return $subject unless $Config{osname} eq 'msys';
+	my $here = cwd;
+	my $leaf;
+	if (chdir $subject)
+	{
+		$leaf = '';
+	}
+	else
+	{
+		$leaf = '/' . basename $subject;
+		my $parent = dirname $subject;
+		chdir $parent or die "could not chdir \"$parent\": $!";
+	}
+
+	# this odd way of calling 'pwd -W' is the only way that seems to work.
+	my $dir = qx{sh -c "pwd -W"};
+	chomp $dir;
+	chdir $here;
+	return $dir . $leaf;
+}
+
 sub system_log
 {
 	print("# Running: " . join(" ", @_) . "\n");