Make PostgresNode easily subclassable

This module becomes much more useful if we allow it to be used as base
class for external projects.  To achieve this, change the exported
get_new_node function into a class method instead, and use the standard
Perl idiom of accepting the class as first argument.  This method works
as expected for subclasses.  The standalone function is kept for
backwards compatibility, though it could be removed in pg11.

Author: Chap Flackman, based on an earlier patch from Craig Ringer
Discussion: https://postgr.es/m/CAMsr+YF8kO+4+K-_U4PtN==2FndJ+5Bn6A19XHhMiBykEwv0wA@mail.gmail.com

src/test/perl/PostgresNode.pm

@@ -9,7 +9,7 @@ PostgresNode - class representing PostgreSQL server instance
 
   use PostgresNode;
 
-  my $node = get_new_node('mynode');
+  my $node = PostgresNode->get_new_node('mynode');
 
   # Create a data directory with initdb
   $node->init();
@@ -855,20 +855,24 @@ sub _update_pid
 
 =pod
 
-=item get_new_node(node_name)
+=item PostgresNode->get_new_node(node_name)
 
-Build a new PostgresNode object, assigning a free port number. Standalone
-function that's automatically imported.
+Build a new object of class C<PostgresNode> (or of a subclass, if you have
+one), assigning a free port number.  Remembers the node, to prevent its port
+number from being reused for another node, and to ensure that it gets
+shut down when the test script exits.
 
-Remembers the node, to prevent its port number from being reused for another
-node, and to ensure that it gets shut down when the test script exits.
+You should generally use this instead of C<PostgresNode::new(...)>.
 
-You should generally use this instead of PostgresNode::new(...).
+For backwards compatibility, it is also exported as a standalone function,
+which can only create objects of class C<PostgresNode>.
 
 =cut
 
 sub get_new_node
 {
+	my $class = 'PostgresNode';
+	$class = shift if 1 < scalar @_;
 	my $name  = shift;
 	my $found = 0;
 	my $port  = $last_port_assigned;
@@ -913,7 +917,7 @@ sub get_new_node
 	print "# Found free port $port\n";
 
 	# Lock port number found by creating a new node
-	my $node = new PostgresNode($name, $test_pghost, $port);
+	my $node = $class->new($name, $test_pghost, $port);
 
 	# Add node to list of nodes
 	push(@all_nodes, $node);

src/test/perl/README

@@ -48,7 +48,7 @@ Each test script should begin with:
 then it will generally need to set up one or more nodes, run commands
 against them and evaluate the results. For example:
 
-    my $node = get_new_node('master');
+    my $node = PostgresNode->get_new_node('master');
     $node->init;
     $node->start;