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Only apply DB_MASTER fallback in Revision::fetchText() if READ_LATEST

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Add support to DBAccessObjectUtils for fallback logic to make
this simple for other callers too.

Change-Id: I58ab7bd7d31a481f9dc9a773392ea90feb1ebeac
This commit is contained in:
Aaron Schulz 2016-09-07 12:42:03 -07:00
parent 5f42db0a6d
commit 7cc14e02d6
2 changed files with 63 additions and 28 deletions
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41
includes/Revision.php
View file

@ -1608,25 +1608,38 @@ class Revision implements IDBAccessObject {
$row = null;
}
// Callers doing updates will pass in READ_LATEST as usual. Since the text/blob tables
// do not normally get rows changed around, set READ_LATEST_IMMUTABLE in those cases.
$flags = $this->mQueryFlags;
$flags |= DBAccessObjectUtils::hasFlags( $flags, self::READ_LATEST )
? self::READ_LATEST_IMMUTABLE
: 0;
list( $index, $options, $fallbackIndex, $fallbackOptions ) =
DBAccessObjectUtils::getDBOptions( $flags );
if ( !$row ) {
// Text data is immutable; check replica DBs first.
$dbr = wfGetDB( DB_REPLICA );
$row = $dbr->selectRow( 'text',
[ 'old_text', 'old_flags' ],
[ 'old_id' => $textId ],
__METHOD__ );
}
// Fallback to the master in case of replica DB lag. Also use FOR UPDATE if it was
// used to fetch this revision to avoid missing the row due to REPEATABLE-READ.
$forUpdate = ( $this->mQueryFlags & self::READ_LOCKING == self::READ_LOCKING );
if ( !$row && ( $forUpdate || wfGetLB()->getServerCount() > 1 ) ) {
$dbw = wfGetDB( DB_MASTER );
$row = $dbw->selectRow( 'text',
$row = wfGetDB( $index )->selectRow(
'text',
[ 'old_text', 'old_flags' ],
[ 'old_id' => $textId ],
__METHOD__,
$forUpdate ? [ 'FOR UPDATE' ] : [] );
$options
);
}
// Fallback to DB_MASTER in some cases if the row was not found
if ( !$row && $fallbackIndex !== null ) {
// Use FOR UPDATE if it was used to fetch this revision. This avoids missing the row
// due to REPEATABLE-READ. Also fallback to the master if READ_LATEST is provided.
$row = wfGetDB( $fallbackIndex )->selectRow(
'text',
[ 'old_text', 'old_flags' ],
[ 'old_id' => $textId ],
__METHOD__,
$fallbackOptions
);
}
if ( !$row ) {

50
includes/dao/DBAccessObjectUtils.php
View file

@ -26,7 +26,7 @@
*
* @since 1.26
*/
class DBAccessObjectUtils {
class DBAccessObjectUtils implements IDBAccessObject {
/**
* @param integer $bitfield
* @param integer $flags IDBAccessObject::READ_* constant
@ -37,23 +37,45 @@ class DBAccessObjectUtils {
}
/**
* Get an appropriate DB index and options for a query
* Get an appropriate DB index, options, and fallback DB index for a query
*
* @param integer $bitfield
* @return array (DB_MASTER/DB_REPLICA, SELECT options array)
* The fallback DB index and options are to be used if the entity is not found
* with the initial DB index, typically querying the master DB to avoid lag
*
* @param integer $bitfield Bitfield of IDBAccessObject::READ_* constants
* @return array List of DB indexes and options in this order:
* - DB_MASTER or DB_REPLICA constant for the initial query
* - SELECT options array for the initial query
* - DB_MASTER constant for the fallback query; null if no fallback should happen
* - SELECT options array for the fallback query; empty if no fallback should happen
*/
public static function getDBOptions( $bitfield ) {
$index = self::hasFlags( $bitfield, IDBAccessObject::READ_LATEST )
? DB_MASTER
: DB_REPLICA;
$options = [];
if ( self::hasFlags( $bitfield, IDBAccessObject::READ_EXCLUSIVE ) ) {
$options[] = 'FOR UPDATE';
} elseif ( self::hasFlags( $bitfield, IDBAccessObject::READ_LOCKING ) ) {
$options[] = 'LOCK IN SHARE MODE';
if ( self::hasFlags( $bitfield, self::READ_LATEST_IMMUTABLE ) ) {
$index = DB_REPLICA; // override READ_LATEST if set
$fallbackIndex = DB_MASTER;
} elseif ( self::hasFlags( $bitfield, self::READ_LATEST ) ) {
$index = DB_MASTER;
$fallbackIndex = null;
} else {
$index = DB_REPLICA;
$fallbackIndex = null;
}
return [ $index, $options ];
$lockingOptions = [];
if ( self::hasFlags( $bitfield, self::READ_EXCLUSIVE ) ) {
$lockingOptions[] = 'FOR UPDATE';
} elseif ( self::hasFlags( $bitfield, self::READ_LOCKING ) ) {
$lockingOptions[] = 'LOCK IN SHARE MODE';
}
if ( $fallbackIndex !== null ) {
$options = []; // locks on DB_REPLICA make no sense
$fallbackOptions = $lockingOptions;
} else {
$options = $lockingOptions;
$fallbackOptions = []; // no fallback
}
return [ $index, $options, $fallbackIndex, $fallbackOptions ];
}
}