populateRevisionLength: Read from slaves when possible

The references to revision content stored in 'revision' and 'archive'
tables are immutable (outside of other maintenance scripts), so we can
safely query them from slave databases.

Also:
* Avoid transactions with no operations inside them
* Improve documentation

Bug: T135414
Change-Id: I3e1407e18e09cf01f105c1cae6d6c23bfde747e6

diff --git a/maintenance/populateRevisionLength.php b/maintenance/populateRevisionLength.php
index 10408ec..dcb9933 100644 (file)
--- a/maintenance/populateRevisionLength.php
+++ b/maintenance/populateRevisionLength.php
@@ -1,7 +1,6 @@
 <?php
 /**
- * Populates the rev_len and ar_len fields for old revisions created
- * before MW 1.10.
+ * Populates the rev_len and ar_len fields when they are NULL.
  *
  * 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
@@ -25,8 +24,9 @@
 require_once __DIR__ . '/Maintenance.php';
 
 /**
- * Maintenance script that populates the rev_len and ar_len fields
- * for old revisions created before MW 1.10.
+ * Maintenance script that populates the rev_len and ar_len fields when they are NULL.
+ * This is the case for all revisions created before MW 1.10, as well as those affected
+ * by T18748 (MW 1.10-1.13) and those affected by T135414 (MW 1.21-1.24).
  *
  * @ingroup Maintenance
  */
@@ -42,12 +42,12 @@ class PopulateRevisionLength extends LoggedUpdateMaintenance {
        }
 
        public function doDBUpdates() {
-               $db = $this->getDB( DB_MASTER );
-               if ( !$db->tableExists( 'revision' ) ) {
+               $dbw = $this->getDB( DB_MASTER );
+               if ( !$dbw->tableExists( 'revision' ) ) {
                        $this->error( "revision table does not exist", true );
-               } elseif ( !$db->tableExists( 'archive' ) ) {
+               } elseif ( !$dbw->tableExists( 'archive' ) ) {
                        $this->error( "archive table does not exist", true );
-               } elseif ( !$db->fieldExists( 'revision', 'rev_len', __METHOD__ ) ) {
+               } elseif ( !$dbw->fieldExists( 'revision', 'rev_len', __METHOD__ ) ) {
                        $this->output( "rev_len column does not exist\n\n", true );
 
                        return false;
@@ -73,9 +73,10 @@ class PopulateRevisionLength extends LoggedUpdateMaintenance {
         * @return int
         */
        protected function doLenUpdates( $table, $idCol, $prefix, $fields ) {
-               $db = $this->getDB( DB_MASTER );
-               $start = $db->selectField( $table, "MIN($idCol)", false, __METHOD__ );
-               $end = $db->selectField( $table, "MAX($idCol)", false, __METHOD__ );
+               $dbr = $this->getDB( DB_SLAVE );
+               $dbw = $this->getDB( DB_MASTER );
+               $start = $dbw->selectField( $table, "MIN($idCol)", false, __METHOD__ );
+               $end = $dbw->selectField( $table, "MAX($idCol)", false, __METHOD__ );
                if ( !$start || !$end ) {
                        $this->output( "...$table table seems to be empty.\n" );
 
@@ -89,7 +90,7 @@ class PopulateRevisionLength extends LoggedUpdateMaintenance {
 
                while ( $blockStart <= $end ) {
                        $this->output( "...doing $idCol from $blockStart to $blockEnd\n" );
-                       $res = $db->select(
+                       $res = $dbr->select(
                                $table,
                                $fields,
                                [
@@ -100,14 +101,16 @@ class PopulateRevisionLength extends LoggedUpdateMaintenance {
                                __METHOD__
                        );
 
-                       $this->beginTransaction( $db, __METHOD__ );
-                       # Go through and update rev_len from these rows.
-                       foreach ( $res as $row ) {
-                               if ( $this->upgradeRow( $row, $table, $idCol, $prefix ) ) {
-                                       $count++;
+                       if ( $res->numRows() > 0 ) {
+                               $this->beginTransaction( $dbw, __METHOD__ );
+                               # Go through and update rev_len from these rows.
+                               foreach ( $res as $row ) {
+                                       if ( $this->upgradeRow( $row, $table, $idCol, $prefix ) ) {
+                                               $count++;
+                                       }
                                }
+                               $this->commitTransaction( $dbw, __METHOD__ );
                        }
-                       $this->commitTransaction( $db, __METHOD__ );
 
                        $blockStart += $this->mBatchSize;
                        $blockEnd += $this->mBatchSize;
@@ -125,7 +128,7 @@ class PopulateRevisionLength extends LoggedUpdateMaintenance {
         * @return bool
         */
        protected function upgradeRow( $row, $table, $idCol, $prefix ) {
-               $db = $this->getDB( DB_MASTER );
+               $dbw = $this->getDB( DB_MASTER );
 
                $rev = ( $table === 'archive' )
                        ? Revision::newFromArchiveRow( $row )
@@ -141,7 +144,7 @@ class PopulateRevisionLength extends LoggedUpdateMaintenance {
                }
 
                # Update the row...
-               $db->update( $table,
+               $dbw->update( $table,
                        [ "{$prefix}_len" => $content->getSize() ],
                        [ $idCol => $row->$idCol ],
                        __METHOD__