Make $wgShowExceptionDetails=false more feasible for production

* Make the HTML error message prettier, with a nice red box and
  instructions to modify LocalSettings.php hidden in an HTML comment.
* Show the exception class name, since that's pretty safe.
* Show a random "log ID" to the user, and also send it to the exception
  log, to allow easier log correlation.
* Optionally send backtraces to the error log, enabled by default.

Change-Id: Ie92e46032b3d194c4217119567847a38a53be577

RELEASE-NOTES-1.20

@@ -54,6 +54,8 @@ upgrade PHP if you have not done so prior to upgrading MediaWiki.
 * (bug 22887) Add warning and tracking category for preprocessor errors
 * (bug 31704) Allow selection of associated namespace on the watchlist
 * (bug 5445) Now remove autoblocks when a user is unblocked.
+* Added $wgLogExceptionBacktrace, on by default, to allow logging of exception
+  backtraces.
 
 === Bug fixes in 1.20 ===
 * (bug 30245) Use the correct way to construct a log page title.

includes/DefaultSettings.php

@@ -4141,6 +4141,11 @@ $wgShowExceptionDetails = false;
  */
 $wgShowDBErrorBacktrace = false;
 
+/**
+ * If true, send the exception backtrace to the error log
+ */
+$wgLogExceptionBacktrace = true;
+
 /**
  * Expose backend server host names through the API and various HTML comments
  */

includes/Exception.php

@@ -15,6 +15,8 @@
  * @ingroup Exception
  */
 class MWException extends Exception {
+	var $logId;
+
 	/**
 	 * Should the exception use $wgOut to output the error ?
 	 * @return bool
@@ -111,9 +113,14 @@ class MWException extends Exception {
 				'</p><p>Backtrace:</p><p>' . nl2br( htmlspecialchars( $this->getTraceAsString() ) ) .
 				"</p>\n";
 		} else {
-			return "<p>Set <b><tt>\$wgShowExceptionDetails = true;</tt></b> " .
+			return 
+				"<div class=\"errorbox\">" .
+				'[' . $this->getLogId() . '] ' .
+				gmdate( 'Y-m-d H:i:s' ) . 
+				": Fatal exception of type " . get_class( $this ) . "</div>\n" .
+				"<!-- Set \$wgShowExceptionDetails = true; " .
 				"at the bottom of LocalSettings.php to show detailed " .
-				"debugging information.</p>";
+				"debugging information. -->";
 		}
 	}
 
@@ -142,6 +149,13 @@ class MWException extends Exception {
 		return $this->msg( 'internalerror', "Internal error" );
 	}
 
+	function getLogId() {
+		if ( $this->logId === null ) {
+			$this->logId = wfRandomString( 8 );
+		}
+		return $this->logId;
+	}
+
 	/**
 	 * Return the requested URL and point to file and line number from which the
 	 * exception occured
@@ -151,6 +165,7 @@ class MWException extends Exception {
 	function getLogMessage() {
 		global $wgRequest;
 
+		$id = $this->getLogId();
 		$file = $this->getFile();
 		$line = $this->getLine();
 		$message = $this->getMessage();
@@ -164,7 +179,7 @@ class MWException extends Exception {
 			$url = '[no req]';
 		}
 
-		return "$url   Exception from line $line of $file: $message";
+		return "[$id] $url   Exception from line $line of $file: $message";
 	}
 
 	/** Output the exception report using HTML */
@@ -198,11 +213,16 @@ class MWException extends Exception {
 	 * It will be either HTML or plain text based on isCommandLine().
 	 */
 	function report() {
+		global $wgLogExceptionBacktrace;
 		$log = $this->getLogMessage();
 
 		if ( $log ) {
+			if ( $wgLogExceptionBacktrace ) {
+				wfDebugLog( 'exception', $log . "\n" . $this->getTraceAsString() . "\n" );
+			} else {
 				wfDebugLog( 'exception', $log );
 			}
+		}
 
 		if ( defined( 'MW_API' ) ) {
 			// Unhandled API exception, we can't be sure that format printer is alive