Fixed doc typo

[mediawiki.git] / includes / filebackend / FileOp.php

<?php
/**
 * Helper class for representing operations with transaction support.
 *
 * 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
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup FileBackend
 * @author Aaron Schulz
 */

/**
 * FileBackend helper class for representing operations.
 * Do not use this class from places outside FileBackend.
 *
 * Methods called from FileOpBatch::attempt() should avoid throwing
 * exceptions at all costs. FileOp objects should be lightweight in order
 * to support large arrays in memory and serialization.
 *
 * @ingroup FileBackend
 * @since 1.19
 */
abstract class FileOp {
        /** @var Array */
        protected $params = array();
        /** @var FileBackendStore */
        protected $backend;

        protected $state = self::STATE_NEW; // integer
        protected $failed = false; // boolean
        protected $async = false; // boolean
        protected $batchId; // string

        protected $doOperation = true; // boolean; operation is not a no-op
        protected $sourceSha1; // string
        protected $overwriteSameCase; // boolean
        protected $destExists; // boolean

        /* Object life-cycle */
        const STATE_NEW = 1;
        const STATE_CHECKED = 2;
        const STATE_ATTEMPTED = 3;

        /**
         * Build a new batch file operation transaction
         *
         * @param FileBackendStore $backend
         * @param Array $params
         * @throws MWException
         */
        final public function __construct( FileBackendStore $backend, array $params ) {
                $this->backend = $backend;
                list( $required, $optional ) = $this->allowedParams();
                // @todo normalizeAnyStoragePaths() calls are overzealous, use a parameter list
                foreach ( $required as $name ) {
                        if ( isset( $params[$name] ) ) {
                                // Normalize paths so the paths to the same file have the same string
                                $this->params[$name] = self::normalizeAnyStoragePaths( $params[$name] );
                        } else {
                                throw new MWException( "File operation missing parameter '$name'." );
                        }
                }
                foreach ( $optional as $name ) {
                        if ( isset( $params[$name] ) ) {
                                // Normalize paths so the paths to the same file have the same string
                                $this->params[$name] = self::normalizeAnyStoragePaths( $params[$name] );
                        }
                }
                $this->params = $params;
        }

        /**
         * Normalize $item or anything in $item that is a valid storage path
         *
         * @param string $item|array
         * @return string|Array
         */
        protected function normalizeAnyStoragePaths( $item ) {
                if ( is_array( $item ) ) {
                        $res = array();
                        foreach ( $item as $k => $v ) {
                                $k = self::normalizeIfValidStoragePath( $k );
                                $v = self::normalizeIfValidStoragePath( $v );
                                $res[$k] = $v;
                        }
                        return $res;
                } else {
                        return self::normalizeIfValidStoragePath( $item );
                }
        }

        /**
         * Normalize a string if it is a valid storage path
         *
         * @param string $path
         * @return string
         */
        protected static function normalizeIfValidStoragePath( $path ) {
                if ( FileBackend::isStoragePath( $path ) ) {
                        $res = FileBackend::normalizeStoragePath( $path );
                        return ( $res !== null ) ? $res : $path;
                }
                return $path;
        }

        /**
         * Set the batch UUID this operation belongs to
         *
         * @param string $batchId
         * @return void
         */
        final public function setBatchId( $batchId ) {
                $this->batchId = $batchId;
        }

        /**
         * Get the value of the parameter with the given name
         *
         * @param string $name
         * @return mixed Returns null if the parameter is not set
         */
        final public function getParam( $name ) {
                return isset( $this->params[$name] ) ? $this->params[$name] : null;
        }

        /**
         * Check if this operation failed precheck() or attempt()
         *
         * @return bool
         */
        final public function failed() {
                return $this->failed;
        }

        /**
         * Get a new empty predicates array for precheck()
         *
         * @return Array
         */
        final public static function newPredicates() {
                return array( 'exists' => array(), 'sha1' => array() );
        }

        /**
         * Get a new empty dependency tracking array for paths read/written to
         *
         * @return Array
         */
        final public static function newDependencies() {
                return array( 'read' => array(), 'write' => array() );
        }

        /**
         * Update a dependency tracking array to account for this operation
         *
         * @param array $deps Prior path reads/writes; format of FileOp::newPredicates()
         * @return Array
         */
        final public function applyDependencies( array $deps ) {
                $deps['read'] += array_fill_keys( $this->storagePathsRead(), 1 );
                $deps['write'] += array_fill_keys( $this->storagePathsChanged(), 1 );
                return $deps;
        }

        /**
         * Check if this operation changes files listed in $paths
         *
         * @param array $paths Prior path reads/writes; format of FileOp::newPredicates()
         * @return boolean
         */
        final public function dependsOn( array $deps ) {
                foreach ( $this->storagePathsChanged() as $path ) {
                        if ( isset( $deps['read'][$path] ) || isset( $deps['write'][$path] ) ) {
                                return true; // "output" or "anti" dependency
                        }
                }
                foreach ( $this->storagePathsRead() as $path ) {
                        if ( isset( $deps['write'][$path] ) ) {
                                return true; // "flow" dependency
                        }
                }
                return false;
        }

        /**
         * Get the file journal entries for this file operation
         *
         * @param array $oPredicates Pre-op info about files (format of FileOp::newPredicates)
         * @param array $nPredicates Post-op info about files (format of FileOp::newPredicates)
         * @return Array
         */
        final public function getJournalEntries( array $oPredicates, array $nPredicates ) {
                if ( !$this->doOperation ) {
                        return array(); // this is a no-op
                }
                $nullEntries = array();
                $updateEntries = array();
                $deleteEntries = array();
                $pathsUsed = array_merge( $this->storagePathsRead(), $this->storagePathsChanged() );
                foreach ( array_unique( $pathsUsed ) as $path ) {
                        $nullEntries[] = array( // assertion for recovery
                                'op' => 'null',
                                'path' => $path,
                                'newSha1' => $this->fileSha1( $path, $oPredicates )
                        );
                }
                foreach ( $this->storagePathsChanged() as $path ) {
                        if ( $nPredicates['sha1'][$path] === false ) { // deleted
                                $deleteEntries[] = array(
                                        'op' => 'delete',
                                        'path' => $path,
                                        'newSha1' => ''
                                );
                        } else { // created/updated
                                $updateEntries[] = array(
                                        'op' => $this->fileExists( $path, $oPredicates ) ? 'update' : 'create',
                                        'path' => $path,
                                        'newSha1' => $nPredicates['sha1'][$path]
                                );
                        }
                }
                return array_merge( $nullEntries, $updateEntries, $deleteEntries );
        }

        /**
         * Check preconditions of the operation without writing anything.
         * This must update $predicates for each path that the op can change
         * except when a failing status object is returned.
         *
         * @param Array $predicates
         * @return Status
         */
        final public function precheck( array &$predicates ) {
                if ( $this->state !== self::STATE_NEW ) {
                        return Status::newFatal( 'fileop-fail-state', self::STATE_NEW, $this->state );
                }
                $this->state = self::STATE_CHECKED;
                $status = $this->doPrecheck( $predicates );
                if ( !$status->isOK() ) {
                        $this->failed = true;
                }
                return $status;
        }

        /**
         * @return Status
         */
        protected function doPrecheck( array &$predicates ) {
                return Status::newGood();
        }

        /**
         * Attempt the operation
         *
         * @return Status
         */
        final public function attempt() {
                if ( $this->state !== self::STATE_CHECKED ) {
                        return Status::newFatal( 'fileop-fail-state', self::STATE_CHECKED, $this->state );
                } elseif ( $this->failed ) { // failed precheck
                        return Status::newFatal( 'fileop-fail-attempt-precheck' );
                }
                $this->state = self::STATE_ATTEMPTED;
                if ( $this->doOperation ) {
                        $status = $this->doAttempt();
                        if ( !$status->isOK() ) {
                                $this->failed = true;
                                $this->logFailure( 'attempt' );
                        }
                } else { // no-op
                        $status = Status::newGood();
                }
                return $status;
        }

        /**
         * @return Status
         */
        protected function doAttempt() {
                return Status::newGood();
        }

        /**
         * Attempt the operation in the background
         *
         * @return Status
         */
        final public function attemptAsync() {
                $this->async = true;
                $result = $this->attempt();
                $this->async = false;
                return $result;
        }

        /**
         * Get the file operation parameters
         *
         * @return Array (required params list, optional params list)
         */
        protected function allowedParams() {
                return array( array(), array() );
        }

        /**
         * Adjust params to FileBackendStore internal file calls
         *
         * @param Array $params
         * @return Array (required params list, optional params list)
         */
        protected function setFlags( array $params ) {
                return array( 'async' => $this->async ) + $params;
        }

        /**
         * Get a list of storage paths read from for this operation
         *
         * @return Array
         */
        public function storagePathsRead() {
                return array();
        }

        /**
         * Get a list of storage paths written to for this operation
         *
         * @return Array
         */
        public function storagePathsChanged() {
                return array();
        }

        /**
         * Check for errors with regards to the destination file already existing.
         * Also set the destExists, overwriteSameCase and sourceSha1 member variables.
         * A bad status will be returned if there is no chance it can be overwritten.
         *
         * @param Array $predicates
         * @return Status
         */
        protected function precheckDestExistence( array $predicates ) {
                $status = Status::newGood();
                // Get hash of source file/string and the destination file
                $this->sourceSha1 = $this->getSourceSha1Base36(); // FS file or data string
                if ( $this->sourceSha1 === null ) { // file in storage?
                        $this->sourceSha1 = $this->fileSha1( $this->params['src'], $predicates );
                }
                $this->overwriteSameCase = false;
                $this->destExists = $this->fileExists( $this->params['dst'], $predicates );
                if ( $this->destExists ) {
                        if ( $this->getParam( 'overwrite' ) ) {
                                return $status; // OK
                        } elseif ( $this->getParam( 'overwriteSame' ) ) {
                                $dhash = $this->fileSha1( $this->params['dst'], $predicates );
                                // Check if hashes are valid and match each other...
                                if ( !strlen( $this->sourceSha1 ) || !strlen( $dhash ) ) {
                                        $status->fatal( 'backend-fail-hashes' );
                                } elseif ( $this->sourceSha1 !== $dhash ) {
                                        // Give an error if the files are not identical
                                        $status->fatal( 'backend-fail-notsame', $this->params['dst'] );
                                } else {
                                        $this->overwriteSameCase = true; // OK
                                }
                                return $status; // do nothing; either OK or bad status
                        } else {
                                $status->fatal( 'backend-fail-alreadyexists', $this->params['dst'] );
                                return $status;
                        }
                }
                return $status;
        }

        /**
         * precheckDestExistence() helper function to get the source file SHA-1.
         * Subclasses should overwride this iff the source is not in storage.
         *
         * @return string|bool Returns false on failure
         */
        protected function getSourceSha1Base36() {
                return null; // N/A
        }

        /**
         * Check if a file will exist in storage when this operation is attempted
         *
         * @param string $source Storage path
         * @param Array $predicates
         * @return bool
         */
        final protected function fileExists( $source, array $predicates ) {
                if ( isset( $predicates['exists'][$source] ) ) {
                        return $predicates['exists'][$source]; // previous op assures this
                } else {
                        $params = array( 'src' => $source, 'latest' => true );
                        return $this->backend->fileExists( $params );
                }
        }

        /**
         * Get the SHA-1 of a file in storage when this operation is attempted
         *
         * @param string $source Storage path
         * @param Array $predicates
         * @return string|bool False on failure
         */
        final protected function fileSha1( $source, array $predicates ) {
                if ( isset( $predicates['sha1'][$source] ) ) {
                        return $predicates['sha1'][$source]; // previous op assures this
                } elseif ( isset( $predicates['exists'][$source] ) && !$predicates['exists'][$source] ) {
                        return false; // previous op assures this
                } else {
                        $params = array( 'src' => $source, 'latest' => true );
                        return $this->backend->getFileSha1Base36( $params );
                }
        }

        /**
         * Get the backend this operation is for
         *
         * @return FileBackendStore
         */
        public function getBackend() {
                return $this->backend;
        }

        /**
         * Log a file operation failure and preserve any temp files
         *
         * @param string $action
         * @return void
         */
        final public function logFailure( $action ) {
                $params = $this->params;
                $params['failedAction'] = $action;
                try {
                        wfDebugLog( 'FileOperation', get_class( $this ) .
                                " failed (batch #{$this->batchId}): " . FormatJson::encode( $params ) );
                } catch ( Exception $e ) {
                        // bad config? debug log error?
                }
        }
}

/**
 * Create a file in the backend with the given content.
 * Parameters for this operation are outlined in FileBackend::doOperations().
 */
class CreateFileOp extends FileOp {
        protected function allowedParams() {
                return array( array( 'content', 'dst' ),
                        array( 'overwrite', 'overwriteSame', 'headers' ) );
        }

        protected function doPrecheck( array &$predicates ) {
                $status = Status::newGood();
                // Check if the source data is too big
                if ( strlen( $this->getParam( 'content' ) ) > $this->backend->maxFileSizeInternal() ) {
                        $status->fatal( 'backend-fail-maxsize',
                                $this->params['dst'], $this->backend->maxFileSizeInternal() );
                        $status->fatal( 'backend-fail-create', $this->params['dst'] );
                        return $status;
                // Check if a file can be placed/changed at the destination
                } elseif ( !$this->backend->isPathUsableInternal( $this->params['dst'] ) ) {
                        $status->fatal( 'backend-fail-usable', $this->params['dst'] );
                        $status->fatal( 'backend-fail-create', $this->params['dst'] );
                        return $status;
                }
                // Check if destination file exists
                $status->merge( $this->precheckDestExistence( $predicates ) );
                $this->params['dstExists'] = $this->destExists; // see FileBackendStore::setFileCache()
                if ( $status->isOK() ) {
                        // Update file existence predicates
                        $predicates['exists'][$this->params['dst']] = true;
                        $predicates['sha1'][$this->params['dst']] = $this->sourceSha1;
                }
                return $status; // safe to call attempt()
        }

        protected function doAttempt() {
                if ( !$this->overwriteSameCase ) {
                        // Create the file at the destination
                        return $this->backend->createInternal( $this->setFlags( $this->params ) );
                }
                return Status::newGood();
        }

        protected function getSourceSha1Base36() {
                return wfBaseConvert( sha1( $this->params['content'] ), 16, 36, 31 );
        }

        public function storagePathsChanged() {
                return array( $this->params['dst'] );
        }
}

/**
 * Store a file into the backend from a file on the file system.
 * Parameters for this operation are outlined in FileBackend::doOperations().
 */
class StoreFileOp extends FileOp {
        protected function allowedParams() {
                return array( array( 'src', 'dst' ),
                        array( 'overwrite', 'overwriteSame', 'headers' ) );
        }

        protected function doPrecheck( array &$predicates ) {
                $status = Status::newGood();
                // Check if the source file exists on the file system
                if ( !is_file( $this->params['src'] ) ) {
                        $status->fatal( 'backend-fail-notexists', $this->params['src'] );
                        return $status;
                // Check if the source file is too big
                } elseif ( filesize( $this->params['src'] ) > $this->backend->maxFileSizeInternal() ) {
                        $status->fatal( 'backend-fail-maxsize',
                                $this->params['dst'], $this->backend->maxFileSizeInternal() );
                        $status->fatal( 'backend-fail-store', $this->params['src'], $this->params['dst'] );
                        return $status;
                // Check if a file can be placed/changed at the destination
                } elseif ( !$this->backend->isPathUsableInternal( $this->params['dst'] ) ) {
                        $status->fatal( 'backend-fail-usable', $this->params['dst'] );
                        $status->fatal( 'backend-fail-store', $this->params['src'], $this->params['dst'] );
                        return $status;
                }
                // Check if destination file exists
                $status->merge( $this->precheckDestExistence( $predicates ) );
                $this->params['dstExists'] = $this->destExists; // see FileBackendStore::setFileCache()
                if ( $status->isOK() ) {
                        // Update file existence predicates
                        $predicates['exists'][$this->params['dst']] = true;
                        $predicates['sha1'][$this->params['dst']] = $this->sourceSha1;
                }
                return $status; // safe to call attempt()
        }

        protected function doAttempt() {
                if ( !$this->overwriteSameCase ) {
                        // Store the file at the destination
                        return $this->backend->storeInternal( $this->setFlags( $this->params ) );
                }
                return Status::newGood();
        }

        protected function getSourceSha1Base36() {
                wfSuppressWarnings();
                $hash = sha1_file( $this->params['src'] );
                wfRestoreWarnings();
                if ( $hash !== false ) {
                        $hash = wfBaseConvert( $hash, 16, 36, 31 );
                }
                return $hash;
        }

        public function storagePathsChanged() {
                return array( $this->params['dst'] );
        }
}

/**
 * Copy a file from one storage path to another in the backend.
 * Parameters for this operation are outlined in FileBackend::doOperations().
 */
class CopyFileOp extends FileOp {
        protected function allowedParams() {
                return array( array( 'src', 'dst' ),
                        array( 'overwrite', 'overwriteSame', 'ignoreMissingSource', 'headers' ) );
        }

        protected function doPrecheck( array &$predicates ) {
                $status = Status::newGood();
                // Check if the source file exists
                if ( !$this->fileExists( $this->params['src'], $predicates ) ) {
                        if ( $this->getParam( 'ignoreMissingSource' ) ) {
                                $this->doOperation = false; // no-op
                                // Update file existence predicates (cache 404s)
                                $predicates['exists'][$this->params['src']] = false;
                                $predicates['sha1'][$this->params['src']] = false;
                                return $status; // nothing to do
                        } else {
                                $status->fatal( 'backend-fail-notexists', $this->params['src'] );
                                return $status;
                        }
                // Check if a file can be placed/changed at the destination
                } elseif ( !$this->backend->isPathUsableInternal( $this->params['dst'] ) ) {
                        $status->fatal( 'backend-fail-usable', $this->params['dst'] );
                        $status->fatal( 'backend-fail-copy', $this->params['src'], $this->params['dst'] );
                        return $status;
                }
                // Check if destination file exists
                $status->merge( $this->precheckDestExistence( $predicates ) );
                $this->params['dstExists'] = $this->destExists; // see FileBackendStore::setFileCache()
                if ( $status->isOK() ) {
                        // Update file existence predicates
                        $predicates['exists'][$this->params['dst']] = true;
                        $predicates['sha1'][$this->params['dst']] = $this->sourceSha1;
                }
                return $status; // safe to call attempt()
        }

        protected function doAttempt() {
                if ( $this->overwriteSameCase ) {
                        $status = Status::newGood(); // nothing to do
                } elseif ( $this->params['src'] === $this->params['dst'] ) {
                        // Just update the destination file headers
                        $headers = $this->getParam( 'headers' ) ?: array();
                        $status = $this->backend->describeInternal( $this->setFlags( array(
                                'src' => $this->params['dst'], 'headers' => $headers
                        ) ) );
                } else {
                        // Copy the file to the destination
                        $status = $this->backend->copyInternal( $this->setFlags( $this->params ) );
                }
                return $status;
        }

        public function storagePathsRead() {
                return array( $this->params['src'] );
        }

        public function storagePathsChanged() {
                return array( $this->params['dst'] );
        }
}

/**
 * Move a file from one storage path to another in the backend.
 * Parameters for this operation are outlined in FileBackend::doOperations().
 */
class MoveFileOp extends FileOp {
        protected function allowedParams() {
                return array( array( 'src', 'dst' ),
                        array( 'overwrite', 'overwriteSame', 'ignoreMissingSource', 'headers' ) );
        }

        protected function doPrecheck( array &$predicates ) {
                $status = Status::newGood();
                // Check if the source file exists
                if ( !$this->fileExists( $this->params['src'], $predicates ) ) {
                        if ( $this->getParam( 'ignoreMissingSource' ) ) {
                                $this->doOperation = false; // no-op
                                // Update file existence predicates (cache 404s)
                                $predicates['exists'][$this->params['src']] = false;
                                $predicates['sha1'][$this->params['src']] = false;
                                return $status; // nothing to do
                        } else {
                                $status->fatal( 'backend-fail-notexists', $this->params['src'] );
                                return $status;
                        }
                // Check if a file can be placed/changed at the destination
                } elseif ( !$this->backend->isPathUsableInternal( $this->params['dst'] ) ) {
                        $status->fatal( 'backend-fail-usable', $this->params['dst'] );
                        $status->fatal( 'backend-fail-move', $this->params['src'], $this->params['dst'] );
                        return $status;
                }
                // Check if destination file exists
                $status->merge( $this->precheckDestExistence( $predicates ) );
                $this->params['dstExists'] = $this->destExists; // see FileBackendStore::setFileCache()
                if ( $status->isOK() ) {
                        // Update file existence predicates
                        $predicates['exists'][$this->params['src']] = false;
                        $predicates['sha1'][$this->params['src']] = false;
                        $predicates['exists'][$this->params['dst']] = true;
                        $predicates['sha1'][$this->params['dst']] = $this->sourceSha1;
                }
                return $status; // safe to call attempt()
        }

        protected function doAttempt() {
                if ( $this->overwriteSameCase ) {
                        if ( $this->params['src'] === $this->params['dst'] ) {
                                // Do nothing to the destination (which is also the source)
                                $status = Status::newGood();
                        } else {
                                // Just delete the source as the destination file needs no changes
                                $status = $this->backend->deleteInternal( $this->setFlags(
                                        array( 'src' => $this->params['src'] )
                                ) );
                        }
                } elseif ( $this->params['src'] === $this->params['dst'] ) {
                        // Just update the destination file headers
                        $headers = $this->getParam( 'headers' ) ?: array();
                        $status = $this->backend->describeInternal( $this->setFlags(
                                array( 'src' => $this->params['dst'], 'headers' => $headers )
                        ) );
                } else {
                        // Move the file to the destination
                        $status = $this->backend->moveInternal( $this->setFlags( $this->params ) );
                }
                return $status;
        }

        public function storagePathsRead() {
                return array( $this->params['src'] );
        }

        public function storagePathsChanged() {
                return array( $this->params['src'], $this->params['dst'] );
        }
}

/**
 * Delete a file at the given storage path from the backend.
 * Parameters for this operation are outlined in FileBackend::doOperations().
 */
class DeleteFileOp extends FileOp {
        protected function allowedParams() {
                return array( array( 'src' ), array( 'ignoreMissingSource' ) );
        }

        protected function doPrecheck( array &$predicates ) {
                $status = Status::newGood();
                // Check if the source file exists
                if ( !$this->fileExists( $this->params['src'], $predicates ) ) {
                        if ( $this->getParam( 'ignoreMissingSource' ) ) {
                                $this->doOperation = false; // no-op
                                // Update file existence predicates (cache 404s)
                                $predicates['exists'][$this->params['src']] = false;
                                $predicates['sha1'][$this->params['src']] = false;
                                return $status; // nothing to do
                        } else {
                                $status->fatal( 'backend-fail-notexists', $this->params['src'] );
                                return $status;
                        }
                // Check if a file can be placed/changed at the source
                } elseif ( !$this->backend->isPathUsableInternal( $this->params['src'] ) ) {
                        $status->fatal( 'backend-fail-usable', $this->params['src'] );
                        $status->fatal( 'backend-fail-delete', $this->params['src'] );
                        return $status;
                }
                // Update file existence predicates
                $predicates['exists'][$this->params['src']] = false;
                $predicates['sha1'][$this->params['src']] = false;
                return $status; // safe to call attempt()
        }

        protected function doAttempt() {
                // Delete the source file
                return $this->backend->deleteInternal( $this->setFlags( $this->params ) );
        }

        public function storagePathsChanged() {
                return array( $this->params['src'] );
        }
}

/**
 * Change metadata for a file at the given storage path in the backend.
 * Parameters for this operation are outlined in FileBackend::doOperations().
 */
class DescribeFileOp extends FileOp {
        protected function allowedParams() {
                return array( array( 'src' ), array( 'headers' ) );
        }

        protected function doPrecheck( array &$predicates ) {
                $status = Status::newGood();
                // Check if the source file exists
                if ( !$this->fileExists( $this->params['src'], $predicates ) ) {
                        $status->fatal( 'backend-fail-notexists', $this->params['src'] );
                        return $status;
                // Check if a file can be placed/changed at the source
                } elseif ( !$this->backend->isPathUsableInternal( $this->params['src'] ) ) {
                        $status->fatal( 'backend-fail-usable', $this->params['src'] );
                        $status->fatal( 'backend-fail-describe', $this->params['src'] );
                        return $status;
                }
                // Update file existence predicates
                $predicates['exists'][$this->params['src']] =
                        $this->fileExists( $this->params['src'], $predicates );
                $predicates['sha1'][$this->params['src']] =
                        $this->fileSha1( $this->params['src'], $predicates );
                return $status; // safe to call attempt()
        }

        protected function doAttempt() {
                // Update the source file's metadata
                return $this->backend->describeInternal( $this->setFlags( $this->params ) );
        }

        public function storagePathsChanged() {
                return array( $this->params['src'] );
        }
}

/**
 * Placeholder operation that has no params and does nothing
 */
class NullFileOp extends FileOp {}