From 958a13b0259cada9a78566d0792b40e15075892e Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Tue, 23 Sep 2014 15:38:44 -0700
Subject: [PATCH] Created DatabaseBase::trxTimestamp method and added usage in
 ActiveUsers query

Change-Id: Iba3c83ce8010583908f9dbaa8348f7226f524793
---
 includes/db/Database.php                 | 25 ++++++++++++++++++++++++
 includes/specials/SpecialActiveusers.php | 18 +++++++++--------
 2 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/includes/db/Database.php b/includes/db/Database.php
index 9b783a99d6..29642d0914 100644
--- a/includes/db/Database.php
+++ b/includes/db/Database.php
@@ -272,9 +272,20 @@ abstract class DatabaseBase implements IDatabase, DatabaseType {
 	 * Either a short hexidecimal string if a transaction is active or ""
 	 *
 	 * @var string
+	 * @see DatabaseBase::mTrxLevel
 	 */
 	protected $mTrxShortId = '';
 
+	/**
+	 * The UNIX time that the transaction started. Callers can assume that if
+	 * snapshot isolation is used, then the data is *at least* up to date to that
+	 * point (possibly more up-to-date since the first SELECT defines the snapshot).
+	 *
+	 * @var float|null
+	 * @see DatabaseBase::mTrxLevel
+	 */
+	private $mTrxTimestamp = null;
+
 	/**
 	 * Remembers the function name given for starting the most recent transaction via begin().
 	 * Used to provide additional context for error reporting.
@@ -419,6 +430,19 @@ abstract class DatabaseBase implements IDatabase, DatabaseType {
 		return $this->mTrxLevel;
 	}
 
+	/**
+	 * Get the UNIX timestamp of the time that the transaction was established
+	 *
+	 * This can be used to reason about the staleness of SELECT data
+	 * in REPEATABLE-READ transaction isolation level.
+	 *
+	 * @return float|null Returns null if there is not active transaction
+	 * @since 1.25
+	 */
+	public function trxTimestamp() {
+		return $this->mTrxLevel ? $this->mTrxTimestamp : null;
+	}
+
 	/**
 	 * Get/set the number of errors logged. Only useful when errors are ignored
 	 * @param int $count The count to set, or omitted to leave it unchanged.
@@ -3512,6 +3536,7 @@ abstract class DatabaseBase implements IDatabase, DatabaseType {
 		}
 
 		$this->doBegin( $fname );
+		$this->mTrxTimestamp = microtime( true );
 		$this->mTrxFname = $fname;
 		$this->mTrxDoneWrites = false;
 		$this->mTrxAutomatic = false;
diff --git a/includes/specials/SpecialActiveusers.php b/includes/specials/SpecialActiveusers.php
index 07e1be1933..10ab46a237 100644
--- a/includes/specials/SpecialActiveusers.php
+++ b/includes/specials/SpecialActiveusers.php
@@ -339,21 +339,19 @@ class SpecialActiveUsers extends SpecialPage {
 			return false; // exclusive update (avoids duplicate entries)
 		}
 
-		$now = time();
+		$nowUnix = time();
+		// Get the last-updated timestamp for the cache
 		$cTime = $dbw->selectField( 'querycache_info',
 			'qci_timestamp',
 			array( 'qci_type' => 'activeusers' )
 		);
 		$cTimeUnix = $cTime ? wfTimestamp( TS_UNIX, $cTime ) : 1;
-		// If a transaction was already started, it might have an old
-		// snapshot, so kludge the timestamp range a few seconds back.
-		$cTimeUnix -= 5;
 
 		// Pick the date range to fetch from. This is normally from the last
 		// update to till the present time, but has a limited window for sanity.
 		// If the window is limited, multiple runs are need to fully populate it.
-		$sTimestamp = max( $cTimeUnix, $now - $days * 86400 );
-		$eTimestamp = min( $sTimestamp + $window, $now );
+		$sTimestamp = max( $cTimeUnix, $nowUnix - $days * 86400 );
+		$eTimestamp = min( $sTimestamp + $window, $nowUnix );
 
 		// Get all the users active since the last update
 		$res = $dbw->select(
@@ -381,7 +379,7 @@ class SpecialActiveUsers extends SpecialPage {
 		$dbw->delete( 'querycachetwo',
 			array(
 				'qcc_type' => 'activeusers',
-				'qcc_value < ' . $dbw->addQuotes( $now - $days * 86400 ) // TS_UNIX
+				'qcc_value < ' . $dbw->addQuotes( $nowUnix - $days * 86400 ) // TS_UNIX
 			),
 			__METHOD__
 		);
@@ -425,11 +423,15 @@ class SpecialActiveUsers extends SpecialPage {
 			}
 		}
 
+		// If a transaction was already started, it might have an old
+		// snapshot, so kludge the timestamp range back as needed.
+		$asOfTimestamp = min( $eTimestamp, (int)$dbw->trxTimestamp() );
+
 		// Touch the data freshness timestamp
 		$dbw->replace( 'querycache_info',
 			array( 'qci_type' ),
 			array( 'qci_type' => 'activeusers',
-				'qci_timestamp' => $dbw->timestamp( $eTimestamp ) ), // not always $now
+				'qci_timestamp' => $dbw->timestamp( $asOfTimestamp ) ), // not always $now
 			__METHOD__
 		);
 
-- 
2.20.1