Add adjust_conf method to PostgresNode

This method will modify or delete an existing line in the config file
rather than simply appending to the file. This makes adjustment of files
for older versions much simpler and more compact.

src/test/perl/PostgresNode.pm

@@ -19,10 +19,13 @@ PostgresNode - class representing PostgreSQL server instance
   # Start the PostgreSQL server
   $node->start();
 
-  # Change a setting and restart
+  # Add a setting and restart
   $node->append_conf('postgresql.conf', 'hot_standby = on');
   $node->restart();
 
+  # Modify or delete an existing setting
+  $node->adjust_conf('postgresql.conf', 'max_wal_senders', '10');
+
   # run a query with psql, like:
   #   echo 'SELECT 1' | psql -qAXt postgres -v ON_ERROR_STOP=1
   $psql_stdout = $node->safe_psql('postgres', 'SELECT 1');
@@ -544,6 +547,50 @@ sub append_conf
 
 =pod
 
+=item $node->adjust_conf(filename, setting, value, skip_equals)
+
+Modify the named config file setting with the value. If the value is undefined,
+instead delete the setting. If the setting is not present no action is taken.
+
+This will write "$setting = $value\n" in place of the existing line,
+unless skip_equals is true, in which case it will write
+"$setting $value\n". If the value needs to be quoted it is the caller's
+responsibility to do that.
+
+=cut
+
+sub adjust_conf
+{
+	my ($self, $filename, $setting, $value, $skip_equals) = @_;
+
+	my $conffile = $self->data_dir . '/' . $filename;
+
+	my $contents = TestLib::slurp_file($conffile);
+	my @lines    = split(/\n/, $contents);
+	my @result;
+	my $eq = $skip_equals ? '' : '= ';
+	foreach my $line (@lines)
+	{
+		if ($line !~ /^$setting\W/)
+		{
+			push(@result, "$line\n");
+		}
+		elsif (defined $value)
+		{
+			push(@result, "$setting $eq$value\n");
+		}
+	}
+	open my $fh, ">", $conffile
+	  or croak "could not write \"$conffile\": $!";
+	print $fh @result;
+	close $fh;
+
+	chmod($self->group_access() ? 0640 : 0600, $conffile)
+	  or die("unable to set permissions for $conffile");
+}
+
+=pod
+
 =item $node->backup(backup_name)
 
 Create a hot backup with B<pg_basebackup> in subdirectory B<backup_name> of