monolog: add MwlogHandler

Introduce a new handler, similar to SyslogHandler, that will be used to
replace udp2log. The main feature of the handler is being able to vary
the 'application name' with each record's channel. This way the channel
can be reconstructed later, e.g. when writing to plaintext files.

See also an overview of the plan here:
https://phabricator.wikimedia.org/T205856#4957430

Bug: T126989
Change-Id: I0c040825b31cd70f197dc8c1d406a103dc0ed1d1

autoload.php

@@ -900,6 +900,7 @@ $wgAutoloadLocalClasses = [
 	'MediaWiki\\Logger\\Monolog\\LegacyHandler' => __DIR__ . '/includes/debug/logger/monolog/LegacyHandler.php',
 	'MediaWiki\\Logger\\Monolog\\LineFormatter' => __DIR__ . '/includes/debug/logger/monolog/LineFormatter.php',
 	'MediaWiki\\Logger\\Monolog\\LogstashFormatter' => __DIR__ . '/includes/debug/logger/monolog/LogstashFormatter.php',
+	'MediaWiki\\Logger\\Monolog\\MwlogHandler' => __DIR__ . '/includes/debug/logger/monolog/MwlogHandler.php',
 	'MediaWiki\\Logger\\Monolog\\SyslogHandler' => __DIR__ . '/includes/debug/logger/monolog/SyslogHandler.php',
 	'MediaWiki\\Logger\\Monolog\\WikiProcessor' => __DIR__ . '/includes/debug/logger/monolog/WikiProcessor.php',
 	'MediaWiki\\Logger\\NullSpi' => __DIR__ . '/includes/debug/logger/NullSpi.php',

includes/debug/logger/monolog/MwlogHandler.php

@@ -0,0 +1,102 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+namespace MediaWiki\Logger\Monolog;
+
+use Monolog\Handler\SyslogUdpHandler;
+use Monolog\Logger;
+
+/**
+ * Log handler that will append the record's channel to a fixed 'application
+ * prefix' given at construction time.
+ *
+ * The use case for this handler is to deprecate udp2log with a localhost
+ * syslog endpoint. The application name is then used to reconstruct the
+ * message's channel.
+ *
+ * @since 1.32
+ * @copyright © 2019 Wikimedia Foundation and contributors
+ */
+class MwlogHandler extends SyslogUdpHandler {
+
+	/**
+	 * @var string $appprefix
+	 */
+	private $appprefix;
+
+	/**
+	 * @var string $hostname
+	 */
+	private $hostname;
+
+	/**
+	 * @param string $appprefix Application prefix to use, channel will be appended.
+	 * @param string $host Syslog host
+	 * @param int $port Syslog port
+	 * @param int $facility Syslog message facility
+	 * @param int $level The minimum logging level at which this handler
+	 *   will be triggered
+	 * @param bool $bubble Whether the messages that are handled can bubble up
+	 *   the stack or not
+	 */
+	public function __construct(
+		$appprefix,
+		$host,
+		$port = 514,
+		$facility = LOG_USER,
+		$level = Logger::DEBUG,
+		$bubble = true
+	) {
+		parent::__construct( $host, $port, $facility, $level, $bubble );
+		$this->appprefix = $appprefix;
+		$this->hostname = php_uname( 'n' );
+	}
+
+	protected function syslogHeader( $severity, $app ) {
+		$pri = $severity + $this->facility;
+
+		// Goofy date format courtesy of RFC 3164 :(
+		// RFC 3164 actually specifies that the day of month should be space
+		// padded rather than unpadded but this seems to work with rsyslog and
+		// Logstash.
+		$timestamp = date( 'M j H:i:s' );
+
+		return "<{$pri}>{$timestamp} {$this->hostname} {$app}: ";
+	}
+
+	private function splitMessageIntoLines( $message ): array {
+		if ( is_array( $message ) ) {
+			$message = implode( "\n", $message );
+		}
+
+		return preg_split( '/$\R?^/m', (string)$message, -1, PREG_SPLIT_NO_EMPTY );
+	}
+
+	protected function write( array $record ) {
+		$lines = $this->splitMessageIntoLines( $record['formatted'] );
+		$header = $this->syslogHeader(
+			$this->logLevels[$record['level']],
+			$this->appprefix . $record['channel'] );
+
+		foreach ( $lines as $line ) {
+			$this->socket->write( $line, $header );
+		}
+	}
+}