Provide a unique request identifier

When MediaWiki encounters an unhandled exception, the error message it produces
includes a randomly-generated token, which allows the exception details to be
looked up in the error logs. This is useful but narrow: would it not be useful
to have the ability to retrieve all log records associated with a particular
request, rather than just exception details? (Hint: yes.)

So: introduce the notion of a request-global unique ID, retrievable via
WebRequest::getRequestId(). When MediaWiki is behind Apache + mod_unique_id
(which provides the same facility) or some other software which sets a
UNIQUE_ID envvar, the value of that envvar is used as the request ID.
Otherwise, it is a randomly-generated 24-character string.

The request ID supplants exception-specific IDs; MWExceptionHandler::getLogId()
is deprecated, accordingly. The request ID is also added as an annotation to
all Monolog-processed log records, and is exposed client-side as 'wgRequestId'.
This allows developers to associate a page view with log records even when the
page view does not result in an unhandled exception. (For the WMF, I also
intend to add it as an annotation to profiling data).

The request ID is not a tracking token; it does not persist, and it is
associated with a backend request, not with a particular user or a particular
session. Like the data in the NewPP report, the request ID is designed to be
cacheable, so that if, for example, a developer notices something weird in the
HTML, s/he can associate the output with a backend request regardless of
whether the response was served from the cache or directly from the backend.

Some prior art:
* https://httpd.apache.org/docs/2.4/mod/mod_unique_id.html
* http://api.rubyonrails.org/classes/ActionDispatch/RequestId.html
* https://github.com/dabapps/django-log-request-id
* https://packagist.org/packages/php-middleware/request-id
* https://github.com/rhyselsmore/flask-request-id

Change-Id: Iaf90c20c330e0470b9b98627a0228cadefd301d1

RELEASE-NOTES-1.27

@@ -175,6 +175,12 @@ HHVM 3.1.
   be a good idea, but will log out all current sessions.
 * $wgEventRelayerConfig was added, for managing PubSub event relay configuration,
   specifically for reliable CDN url purges.
+* Requests have unique IDs, equal to the UNIQUE_ID environment variable (when
+  MediaWiki is behind Apache+mod_unique_id or something similar) or a randomly-
+  generated 24-character string. This request ID is used to annotate log records
+  and error messages. It is available client-side via mw.config.get( 'wgRequestId' ).
+  The request ID supplants exception IDs. Accordingly, MWExceptionHandler::getLogId()
+  is deprecated.
 
 === External library changes in 1.27 ===
 

includes/OutputPage.php

@@ -3210,6 +3210,7 @@ class OutputPage extends ContextSource {
 			'wgMonthNamesShort' => $lang->getMonthAbbreviationsArray(),
 			'wgRelevantPageName' => $relevantTitle->getPrefixedDBkey(),
 			'wgRelevantArticleId' => $relevantTitle->getArticleID(),
+			'wgRequestId' => WebRequest::getRequestId(),
 		];
 
 		if ( $user->isLoggedIn() ) {

includes/WebRequest.php

@@ -246,6 +246,25 @@ class WebRequest {
 		return microtime( true ) - $this->requestTime;
 	}
 
+	/**
+	 * Get the unique request ID.
+	 * This is either the value of the UNIQUE_ID envvar (if present) or a
+	 * randomly-generated 24-character string.
+	 *
+	 * @return string
+	 * @since 1.27
+	 */
+	public static function getRequestId() {
+		static $reqId;
+
+		if ( !$reqId ) {
+			$reqId = isset( $_SERVER['UNIQUE_ID'] )
+				? $_SERVER['UNIQUE_ID'] : wfRandomString( 24 );
+		}
+
+		return $reqId;
+	}
+
 	/**
 	 * Get the current URL protocol (http or https)
 	 * @return string

includes/api/ApiMain.php

@@ -871,7 +871,7 @@ class ApiMain extends ApiBase {
 
 			$errMessage = [
 				'code' => 'internal_api_error_' . get_class( $e ),
-				'info' => '[' . MWExceptionHandler::getLogId( $e ) . '] ' . $info,
+				'info' => '[' . WebRequest::getRequestId() . '] ' . $info,
 			];
 		}
 		return $errMessage;

includes/debug/logger/monolog/WikiProcessor.php

@@ -21,7 +21,8 @@
 namespace MediaWiki\Logger\Monolog;
 
 /**
- * Injects `wfHostname()`, `wfWikiID()` and `$wgVersion` in all records.
+ * Annotate log records with request-global metadata, such as the hostname,
+ * wiki / request ID, and MediaWiki version.
  *
  * @since 1.25
  * @author Bryan Davis <bd808@wikimedia.org>
@@ -41,6 +42,7 @@ class WikiProcessor {
 				'host' => wfHostname(),
 				'wiki' => wfWikiID(),
 				'mwversion' => $wgVersion,
+				'reqId' => WebRequest::getRequestId(),
 			]
 		);
 		return $record;

includes/exception/MWException.php

@@ -141,7 +141,7 @@ class MWException extends Exception {
 			nl2br( htmlspecialchars( MWExceptionHandler::getRedactedTraceAsString( $this ) ) ) .
 			"</p>\n";
 		} else {
-			$logId = MWExceptionHandler::getLogId( $this );
+			$logId = WebRequest::getRequestId();
 			$type = get_class( $this );
 			return "<div class=\"errorbox\">" .
 			'[' . $logId . '] ' .

includes/exception/MWExceptionHandler.php

@@ -445,14 +445,13 @@ TXT;
 	 * $wgShowExceptionDetails is set to false), to the entry in the debug log.
 	 *
 	 * @since 1.22
+	 * @deprecated since 1.27: Exception IDs are synonymous with request IDs.
 	 * @param Exception|Throwable $e
 	 * @return string
 	 */
 	public static function getLogId( $e ) {
-		if ( !isset( $e->_mwLogId ) ) {
-			$e->_mwLogId = wfRandomString( 8 );
-		}
-		return $e->_mwLogId;
+		wfDeprecated( __METHOD__, '1.27' );
+		return WebRequest::getRequestId();
 	}
 
 	/**
@@ -478,7 +477,7 @@ TXT;
 	 * @return string
 	 */
 	public static function getLogMessage( $e ) {
-		$id = self::getLogId( $e );
+		$id = WebRequest::getRequestId();
 		$type = get_class( $e );
 		$file = $e->getFile();
 		$line = $e->getLine();
@@ -489,9 +488,9 @@ TXT;
 	}
 
 	public static function getPublicLogMessage( Exception $e ) {
-		$logId = self::getLogId( $e );
+		$reqId = WebRequest::getRequestId();
 		$type = get_class( $e );
-		return '[' . $logId . '] '
+		return '[' . $reqId . '] '
 			. gmdate( 'Y-m-d H:i:s' ) . ': '
 			. 'Fatal exception of type ' . $type;
 	}
@@ -509,7 +508,7 @@ TXT;
 	public static function getLogContext( $e ) {
 		return [
 			'exception' => $e,
-			'exception_id' => static::getLogId( $e ),
+			'exception_id' => WebRequest::getRequestId(),
 		];
 	}
 
@@ -527,7 +526,7 @@ TXT;
 	public static function getStructuredExceptionData( $e ) {
 		global $wgLogExceptionBacktrace;
 		$data = [
-			'id' => self::getLogId( $e ),
+			'id' => WebRequest::getRequestId(),
 			'type' => get_class( $e ),
 			'file' => $e->getFile(),
 			'line' => $e->getLine(),