Warn when a log context accidentally overrides a log processor

Bug: T145133
Change-Id: I6677dbf608aac1d7fd6036f5e51977d56ce2fc57

includes/debug/logger/monolog/WikiProcessor.php

@@ -29,6 +29,17 @@ namespace MediaWiki\Logger\Monolog;
  * @copyright © 2013 Bryan Davis and Wikimedia Foundation.
  */
 class WikiProcessor {
+	/** @var array Keys which should not be used in log context */
+	protected $reservedKeys = [
+		// from monolog:src/Monolog/Formatter/LogstashFormatter.php#L71-L88
+		'message', 'channel', 'level', 'type',
+		// from WebProcessor
+		'url', 'ip', 'http_method', 'server', 'referrer',
+		// from WikiProcessor
+		'host', 'wiki', 'reqId', 'mwversion',
+		// from config magic
+		'normalized_message',
+	];
 
 	/**
 	 * @param array $record
@@ -36,6 +47,15 @@ class WikiProcessor {
 	 */
 	public function __invoke( array $record ) {
 		global $wgVersion;
+
+		// some log aggregators such as Logstash will merge the log context into the main
+		// metadata and end up overwriting the data coming from processors
+		foreach ( $this->reservedKeys as $key ) {
+			if ( isset( $record['context'][$key] ) ) {
+				wfLogWarning( __METHOD__ . ": '$key' key overwritten in log context." );
+			}
+		}
+
 		$record['extra'] = array_merge(
 			$record['extra'],
 			[
@@ -47,5 +67,4 @@ class WikiProcessor {
 		);
 		return $record;
 	}
-
 }