PHPSessionHandler: Implement SessionHandlerInterface

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

<?php
/**
 * @defgroup FileBackend File backend
 *
 * File backend is used to interact with file storage systems,
 * such as the local file system, NFS, or cloud storage systems.
 */

/**
 * Base class for all file backends.
 *
 * 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
 */

/**
 * @brief Base class for all file backend classes (including multi-write backends).
 *
 * This class defines the methods as abstract that subclasses must implement.
 * Outside callers can assume that all backends will have these functions.
 *
 * All "storage paths" are of the format "mwstore://<backend>/<container>/<path>".
 * The "backend" portion is unique name for MediaWiki to refer to a backend, while
 * the "container" portion is a top-level directory of the backend. The "path" portion
 * is a relative path that uses UNIX file system (FS) notation, though any particular
 * backend may not actually be using a local filesystem. Therefore, the relative paths
 * are only virtual.
 *
 * Backend contents are stored under wiki-specific container names by default.
 * Global (qualified) backends are achieved by configuring the "wiki ID" to a constant.
 * For legacy reasons, the FSFileBackend class allows manually setting the paths of
 * containers to ones that do not respect the "wiki ID".
 *
 * In key/value (object) stores, containers are the only hierarchy (the rest is emulated).
 * FS-based backends are somewhat more restrictive due to the existence of real
 * directory files; a regular file cannot have the same name as a directory. Other
 * backends with virtual directories may not have this limitation. Callers should
 * store files in such a way that no files and directories are under the same path.
 *
 * In general, this class allows for callers to access storage through the same
 * interface, without regard to the underlying storage system. However, calling code
 * must follow certain patterns and be aware of certain things to ensure compatibility:
 *   - a) Always call prepare() on the parent directory before trying to put a file there;
 *        key/value stores only need the container to exist first, but filesystems need
 *        all the parent directories to exist first (prepare() is aware of all this)
 *   - b) Always call clean() on a directory when it might become empty to avoid empty
 *        directory buildup on filesystems; key/value stores never have empty directories,
 *        so doing this helps preserve consistency in both cases
 *   - c) Likewise, do not rely on the existence of empty directories for anything;
 *        calling directoryExists() on a path that prepare() was previously called on
 *        will return false for key/value stores if there are no files under that path
 *   - d) Never alter the resulting FSFile returned from getLocalReference(), as it could
 *        either be a copy of the source file in /tmp or the original source file itself
 *   - e) Use a file layout that results in never attempting to store files over directories
 *        or directories over files; key/value stores allow this but filesystems do not
 *   - f) Use ASCII file names (e.g. base32, IDs, hashes) to avoid Unicode issues in Windows
 *   - g) Do not assume that move operations are atomic (difficult with key/value stores)
 *   - h) Do not assume that file stat or read operations always have immediate consistency;
 *        various methods have a "latest" flag that should always be used if up-to-date
 *        information is required (this trades performance for correctness as needed)
 *   - i) Do not assume that directory listings have immediate consistency
 *
 * Methods of subclasses should avoid throwing exceptions at all costs.
 * As a corollary, external dependencies should be kept to a minimum.
 *
 * @ingroup FileBackend
 * @since 1.19
 */
abstract class FileBackend {
        /** @var string Unique backend name */
        protected $name;

        /** @var string Unique wiki name */
        protected $wikiId;

        /** @var string Read-only explanation message */
        protected $readOnly;

        /** @var string When to do operations in parallel */
        protected $parallelize;

        /** @var int How many operations can be done in parallel */
        protected $concurrency;

        /** @var LockManager */
        protected $lockManager;

        /** @var FileJournal */
        protected $fileJournal;

        /** Bitfield flags for supported features */
        const ATTR_HEADERS = 1; // files can be tagged with standard HTTP headers
        const ATTR_METADATA = 2; // files can be stored with metadata key/values
        const ATTR_UNICODE_PATHS = 4; // files can have Unicode paths (not just ASCII)

        /**
         * Create a new backend instance from configuration.
         * This should only be called from within FileBackendGroup.
         *
         * @param array $config Parameters include:
         *   - name        : The unique name of this backend.
         *                   This should consist of alphanumberic, '-', and '_' characters.
         *                   This name should not be changed after use (e.g. with journaling).
         *                   Note that the name is *not* used in actual container names.
         *   - wikiId      : Prefix to container names that is unique to this backend.
         *                   It should only consist of alphanumberic, '-', and '_' characters.
         *                   This ID is what avoids collisions if multiple logical backends
         *                   use the same storage system, so this should be set carefully.
         *   - lockManager : LockManager object to use for any file locking.
         *                   If not provided, then no file locking will be enforced.
         *   - fileJournal : FileJournal object to use for logging changes to files.
         *                   If not provided, then change journaling will be disabled.
         *   - readOnly    : Write operations are disallowed if this is a non-empty string.
         *                   It should be an explanation for the backend being read-only.
         *   - parallelize : When to do file operations in parallel (when possible).
         *                   Allowed values are "implicit", "explicit" and "off".
         *   - concurrency : How many file operations can be done in parallel.
         * @throws FileBackendException
         */
        public function __construct( array $config ) {
                $this->name = $config['name'];
                $this->wikiId = $config['wikiId']; // e.g. "my_wiki-en_"
                if ( !preg_match( '!^[a-zA-Z0-9-_]{1,255}$!', $this->name ) ) {
                        throw new FileBackendException( "Backend name '{$this->name}' is invalid." );
                } elseif ( !is_string( $this->wikiId ) ) {
                        throw new FileBackendException( "Backend wiki ID not provided for '{$this->name}'." );
                }
                $this->lockManager = isset( $config['lockManager'] )
                        ? $config['lockManager']
                        : new NullLockManager( array() );
                $this->fileJournal = isset( $config['fileJournal'] )
                        ? $config['fileJournal']
                        : FileJournal::factory( array( 'class' => 'NullFileJournal' ), $this->name );
                $this->readOnly = isset( $config['readOnly'] )
                        ? (string)$config['readOnly']
                        : '';
                $this->parallelize = isset( $config['parallelize'] )
                        ? (string)$config['parallelize']
                        : 'off';
                $this->concurrency = isset( $config['concurrency'] )
                        ? (int)$config['concurrency']
                        : 50;
        }

        /**
         * Get the unique backend name.
         * We may have multiple different backends of the same type.
         * For example, we can have two Swift backends using different proxies.
         *
         * @return string
         */
        final public function getName() {
                return $this->name;
        }

        /**
         * Get the wiki identifier used for this backend (possibly empty).
         * Note that this might *not* be in the same format as wfWikiID().
         *
         * @return string
         * @since 1.20
         */
        final public function getWikiId() {
                return $this->wikiId;
        }

        /**
         * Check if this backend is read-only
         *
         * @return bool
         */
        final public function isReadOnly() {
                return ( $this->readOnly != '' );
        }

        /**
         * Get an explanatory message if this backend is read-only
         *
         * @return string|bool Returns false if the backend is not read-only
         */
        final public function getReadOnlyReason() {
                return ( $this->readOnly != '' ) ? $this->readOnly : false;
        }

        /**
         * Get the a bitfield of extra features supported by the backend medium
         *
         * @return int Bitfield of FileBackend::ATTR_* flags
         * @since 1.23
         */
        public function getFeatures() {
                return self::ATTR_UNICODE_PATHS;
        }

        /**
         * Check if the backend medium supports a field of extra features
         *
         * @param int $bitfield Bitfield of FileBackend::ATTR_* flags
         * @return bool
         * @since 1.23
         */
        final public function hasFeatures( $bitfield ) {
                return ( $this->getFeatures() & $bitfield ) === $bitfield;
        }

        /**
         * This is the main entry point into the backend for write operations.
         * Callers supply an ordered list of operations to perform as a transaction.
         * Files will be locked, the stat cache cleared, and then the operations attempted.
         * If any serious errors occur, all attempted operations will be rolled back.
         *
         * $ops is an array of arrays. The outer array holds a list of operations.
         * Each inner array is a set of key value pairs that specify an operation.
         *
         * Supported operations and their parameters. The supported actions are:
         *  - create
         *  - store
         *  - copy
         *  - move
         *  - delete
         *  - describe (since 1.21)
         *  - null
         *
         * a) Create a new file in storage with the contents of a string
         * @code
         *     array(
         *         'op'                  => 'create',
         *         'dst'                 => <storage path>,
         *         'content'             => <string of new file contents>,
         *         'overwrite'           => <boolean>,
         *         'overwriteSame'       => <boolean>,
         *         'headers'             => <HTTP header name/value map> # since 1.21
         *     );
         * @endcode
         *
         * b) Copy a file system file into storage
         * @code
         *     array(
         *         'op'                  => 'store',
         *         'src'                 => <file system path>,
         *         'dst'                 => <storage path>,
         *         'overwrite'           => <boolean>,
         *         'overwriteSame'       => <boolean>,
         *         'headers'             => <HTTP header name/value map> # since 1.21
         *     )
         * @endcode
         *
         * c) Copy a file within storage
         * @code
         *     array(
         *         'op'                  => 'copy',
         *         'src'                 => <storage path>,
         *         'dst'                 => <storage path>,
         *         'overwrite'           => <boolean>,
         *         'overwriteSame'       => <boolean>,
         *         'ignoreMissingSource' => <boolean>, # since 1.21
         *         'headers'             => <HTTP header name/value map> # since 1.21
         *     )
         * @endcode
         *
         * d) Move a file within storage
         * @code
         *     array(
         *         'op'                  => 'move',
         *         'src'                 => <storage path>,
         *         'dst'                 => <storage path>,
         *         'overwrite'           => <boolean>,
         *         'overwriteSame'       => <boolean>,
         *         'ignoreMissingSource' => <boolean>, # since 1.21
         *         'headers'             => <HTTP header name/value map> # since 1.21
         *     )
         * @endcode
         *
         * e) Delete a file within storage
         * @code
         *     array(
         *         'op'                  => 'delete',
         *         'src'                 => <storage path>,
         *         'ignoreMissingSource' => <boolean>
         *     )
         * @endcode
         *
         * f) Update metadata for a file within storage
         * @code
         *     array(
         *         'op'                  => 'describe',
         *         'src'                 => <storage path>,
         *         'headers'             => <HTTP header name/value map>
         *     )
         * @endcode
         *
         * g) Do nothing (no-op)
         * @code
         *     array(
         *         'op'                  => 'null',
         *     )
         * @endcode
         *
         * Boolean flags for operations (operation-specific):
         *   - ignoreMissingSource : The operation will simply succeed and do
         *                           nothing if the source file does not exist.
         *   - overwrite           : Any destination file will be overwritten.
         *   - overwriteSame       : If a file already exists at the destination with the
         *                           same contents, then do nothing to the destination file
         *                           instead of giving an error. This does not compare headers.
         *                           This option is ignored if 'overwrite' is already provided.
         *   - headers             : If supplied, the result of merging these headers with any
         *                           existing source file headers (replacing conflicting ones)
         *                           will be set as the destination file headers. Headers are
         *                           deleted if their value is set to the empty string. When a
         *                           file has headers they are included in responses to GET and
         *                           HEAD requests to the backing store for that file.
         *                           Header values should be no larger than 255 bytes, except for
         *                           Content-Disposition. The system might ignore or truncate any
         *                           headers that are too long to store (exact limits will vary).
         *                           Backends that don't support metadata ignore this. (since 1.21)
         *
         * $opts is an associative of boolean flags, including:
         *   - force               : Operation precondition errors no longer trigger an abort.
         *                           Any remaining operations are still attempted. Unexpected
         *                           failures may still cause remaining operations to be aborted.
         *   - nonLocking          : No locks are acquired for the operations.
         *                           This can increase performance for non-critical writes.
         *                           This has no effect unless the 'force' flag is set.
         *   - nonJournaled        : Don't log this operation batch in the file journal.
         *                           This limits the ability of recovery scripts.
         *   - parallelize         : Try to do operations in parallel when possible.
         *   - bypassReadOnly      : Allow writes in read-only mode. (since 1.20)
         *   - preserveCache       : Don't clear the process cache before checking files.
         *                           This should only be used if all entries in the process
         *                           cache were added after the files were already locked. (since 1.20)
         *
         * @remarks Remarks on locking:
         * File system paths given to operations should refer to files that are
         * already locked or otherwise safe from modification from other processes.
         * Normally these files will be new temp files, which should be adequate.
         *
         * @par Return value:
         *
         * This returns a Status, which contains all warnings and fatals that occurred
         * during the operation. The 'failCount', 'successCount', and 'success' members
         * will reflect each operation attempted.
         *
         * The status will be "OK" unless:
         *   - a) unexpected operation errors occurred (network partitions, disk full...)
         *   - b) significant operation errors occurred and 'force' was not set
         *
         * @param array $ops List of operations to execute in order
         * @param array $opts Batch operation options
         * @return Status
         */
        final public function doOperations( array $ops, array $opts = array() ) {
                if ( empty( $opts['bypassReadOnly'] ) && $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                if ( !count( $ops ) ) {
                        return Status::newGood(); // nothing to do
                }
                if ( empty( $opts['force'] ) ) { // sanity
                        unset( $opts['nonLocking'] );
                }
                /** @noinspection PhpUnusedLocalVariableInspection */
                $scope = $this->getScopedPHPBehaviorForOps(); // try to ignore client aborts
                return $this->doOperationsInternal( $ops, $opts );
        }

        /**
         * @see FileBackend::doOperations()
         * @param array $ops
         * @param array $opts
         */
        abstract protected function doOperationsInternal( array $ops, array $opts );

        /**
         * Same as doOperations() except it takes a single operation.
         * If you are doing a batch of operations that should either
         * all succeed or all fail, then use that function instead.
         *
         * @see FileBackend::doOperations()
         *
         * @param array $op Operation
         * @param array $opts Operation options
         * @return Status
         */
        final public function doOperation( array $op, array $opts = array() ) {
                return $this->doOperations( array( $op ), $opts );
        }

        /**
         * Performs a single create operation.
         * This sets $params['op'] to 'create' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param array $params Operation parameters
         * @param array $opts Operation options
         * @return Status
         */
        final public function create( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'create' ) + $params, $opts );
        }

        /**
         * Performs a single store operation.
         * This sets $params['op'] to 'store' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param array $params Operation parameters
         * @param array $opts Operation options
         * @return Status
         */
        final public function store( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'store' ) + $params, $opts );
        }

        /**
         * Performs a single copy operation.
         * This sets $params['op'] to 'copy' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param array $params Operation parameters
         * @param array $opts Operation options
         * @return Status
         */
        final public function copy( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'copy' ) + $params, $opts );
        }

        /**
         * Performs a single move operation.
         * This sets $params['op'] to 'move' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param array $params Operation parameters
         * @param array $opts Operation options
         * @return Status
         */
        final public function move( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'move' ) + $params, $opts );
        }

        /**
         * Performs a single delete operation.
         * This sets $params['op'] to 'delete' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param array $params Operation parameters
         * @param array $opts Operation options
         * @return Status
         */
        final public function delete( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'delete' ) + $params, $opts );
        }

        /**
         * Performs a single describe operation.
         * This sets $params['op'] to 'describe' and passes it to doOperation().
         *
         * @see FileBackend::doOperation()
         *
         * @param array $params Operation parameters
         * @param array $opts Operation options
         * @return Status
         * @since 1.21
         */
        final public function describe( array $params, array $opts = array() ) {
                return $this->doOperation( array( 'op' => 'describe' ) + $params, $opts );
        }

        /**
         * Perform a set of independent file operations on some files.
         *
         * This does no locking, nor journaling, and possibly no stat calls.
         * Any destination files that already exist will be overwritten.
         * This should *only* be used on non-original files, like cache files.
         *
         * Supported operations and their parameters:
         *  - create
         *  - store
         *  - copy
         *  - move
         *  - delete
         *  - describe (since 1.21)
         *  - null
         *
         * a) Create a new file in storage with the contents of a string
         * @code
         *     array(
         *         'op'                  => 'create',
         *         'dst'                 => <storage path>,
         *         'content'             => <string of new file contents>,
         *         'headers'             => <HTTP header name/value map> # since 1.21
         *     )
         * @endcode
         *
         * b) Copy a file system file into storage
         * @code
         *     array(
         *         'op'                  => 'store',
         *         'src'                 => <file system path>,
         *         'dst'                 => <storage path>,
         *         'headers'             => <HTTP header name/value map> # since 1.21
         *     )
         * @endcode
         *
         * c) Copy a file within storage
         * @code
         *     array(
         *         'op'                  => 'copy',
         *         'src'                 => <storage path>,
         *         'dst'                 => <storage path>,
         *         'ignoreMissingSource' => <boolean>, # since 1.21
         *         'headers'             => <HTTP header name/value map> # since 1.21
         *     )
         * @endcode
         *
         * d) Move a file within storage
         * @code
         *     array(
         *         'op'                  => 'move',
         *         'src'                 => <storage path>,
         *         'dst'                 => <storage path>,
         *         'ignoreMissingSource' => <boolean>, # since 1.21
         *         'headers'             => <HTTP header name/value map> # since 1.21
         *     )
         * @endcode
         *
         * e) Delete a file within storage
         * @code
         *     array(
         *         'op'                  => 'delete',
         *         'src'                 => <storage path>,
         *         'ignoreMissingSource' => <boolean>
         *     )
         * @endcode
         *
         * f) Update metadata for a file within storage
         * @code
         *     array(
         *         'op'                  => 'describe',
         *         'src'                 => <storage path>,
         *         'headers'             => <HTTP header name/value map>
         *     )
         * @endcode
         *
         * g) Do nothing (no-op)
         * @code
         *     array(
         *         'op'                  => 'null',
         *     )
         * @endcode
         *
         * @par Boolean flags for operations (operation-specific):
         *   - ignoreMissingSource : The operation will simply succeed and do
         *                           nothing if the source file does not exist.
         *   - headers             : If supplied with a header name/value map, the backend will
         *                           reply with these headers when GETs/HEADs of the destination
         *                           file are made. Header values should be smaller than 256 bytes.
         *                           Content-Disposition headers can be longer, though the system
         *                           might ignore or truncate ones that are too long to store.
         *                           Existing headers will remain, but these will replace any
         *                           conflicting previous headers, and headers will be removed
         *                           if they are set to an empty string.
         *                           Backends that don't support metadata ignore this. (since 1.21)
         *
         * $opts is an associative of boolean flags, including:
         *   - bypassReadOnly      : Allow writes in read-only mode (since 1.20)
         *
         * @par Return value:
         * This returns a Status, which contains all warnings and fatals that occurred
         * during the operation. The 'failCount', 'successCount', and 'success' members
         * will reflect each operation attempted for the given files. The status will be
         * considered "OK" as long as no fatal errors occurred.
         *
         * @param array $ops Set of operations to execute
         * @param array $opts Batch operation options
         * @return Status
         * @since 1.20
         */
        final public function doQuickOperations( array $ops, array $opts = array() ) {
                if ( empty( $opts['bypassReadOnly'] ) && $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                if ( !count( $ops ) ) {
                        return Status::newGood(); // nothing to do
                }
                foreach ( $ops as &$op ) {
                        $op['overwrite'] = true; // avoids RTTs in key/value stores
                }
                /** @noinspection PhpUnusedLocalVariableInspection */
                $scope = $this->getScopedPHPBehaviorForOps(); // try to ignore client aborts
                return $this->doQuickOperationsInternal( $ops );
        }

        /**
         * @see FileBackend::doQuickOperations()
         * @param array $ops
         * @since 1.20
         */
        abstract protected function doQuickOperationsInternal( array $ops );

        /**
         * Same as doQuickOperations() except it takes a single operation.
         * If you are doing a batch of operations, then use that function instead.
         *
         * @see FileBackend::doQuickOperations()
         *
         * @param array $op Operation
         * @return Status
         * @since 1.20
         */
        final public function doQuickOperation( array $op ) {
                return $this->doQuickOperations( array( $op ) );
        }

        /**
         * Performs a single quick create operation.
         * This sets $params['op'] to 'create' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param array $params Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickCreate( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'create' ) + $params );
        }

        /**
         * Performs a single quick store operation.
         * This sets $params['op'] to 'store' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param array $params Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickStore( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'store' ) + $params );
        }

        /**
         * Performs a single quick copy operation.
         * This sets $params['op'] to 'copy' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param array $params Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickCopy( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'copy' ) + $params );
        }

        /**
         * Performs a single quick move operation.
         * This sets $params['op'] to 'move' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param array $params Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickMove( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'move' ) + $params );
        }

        /**
         * Performs a single quick delete operation.
         * This sets $params['op'] to 'delete' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param array $params Operation parameters
         * @return Status
         * @since 1.20
         */
        final public function quickDelete( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'delete' ) + $params );
        }

        /**
         * Performs a single quick describe operation.
         * This sets $params['op'] to 'describe' and passes it to doQuickOperation().
         *
         * @see FileBackend::doQuickOperation()
         *
         * @param array $params Operation parameters
         * @return Status
         * @since 1.21
         */
        final public function quickDescribe( array $params ) {
                return $this->doQuickOperation( array( 'op' => 'describe' ) + $params );
        }

        /**
         * Concatenate a list of storage files into a single file system file.
         * The target path should refer to a file that is already locked or
         * otherwise safe from modification from other processes. Normally,
         * the file will be a new temp file, which should be adequate.
         *
         * @param array $params Operation parameters, include:
         *   - srcs        : ordered source storage paths (e.g. chunk1, chunk2, ...)
         *   - dst         : file system path to 0-byte temp file
         *   - parallelize : try to do operations in parallel when possible
         * @return Status
         */
        abstract public function concatenate( array $params );

        /**
         * Prepare a storage directory for usage.
         * This will create any required containers and parent directories.
         * Backends using key/value stores only need to create the container.
         *
         * The 'noAccess' and 'noListing' parameters works the same as in secure(),
         * except they are only applied *if* the directory/container had to be created.
         * These flags should always be set for directories that have private files.
         * However, setting them is not guaranteed to actually do anything.
         * Additional server configuration may be needed to achieve the desired effect.
         *
         * @param array $params Parameters include:
         *   - dir            : storage directory
         *   - noAccess       : try to deny file access (since 1.20)
         *   - noListing      : try to deny file listing (since 1.20)
         *   - bypassReadOnly : allow writes in read-only mode (since 1.20)
         * @return Status
         */
        final public function prepare( array $params ) {
                if ( empty( $params['bypassReadOnly'] ) && $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                /** @noinspection PhpUnusedLocalVariableInspection */
                $scope = $this->getScopedPHPBehaviorForOps(); // try to ignore client aborts
                return $this->doPrepare( $params );
        }

        /**
         * @see FileBackend::prepare()
         * @param array $params
         */
        abstract protected function doPrepare( array $params );

        /**
         * Take measures to block web access to a storage directory and
         * the container it belongs to. FS backends might add .htaccess
         * files whereas key/value store backends might revoke container
         * access to the storage user representing end-users in web requests.
         *
         * This is not guaranteed to actually make files or listings publically hidden.
         * Additional server configuration may be needed to achieve the desired effect.
         *
         * @param array $params Parameters include:
         *   - dir            : storage directory
         *   - noAccess       : try to deny file access
         *   - noListing      : try to deny file listing
         *   - bypassReadOnly : allow writes in read-only mode (since 1.20)
         * @return Status
         */
        final public function secure( array $params ) {
                if ( empty( $params['bypassReadOnly'] ) && $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                /** @noinspection PhpUnusedLocalVariableInspection */
                $scope = $this->getScopedPHPBehaviorForOps(); // try to ignore client aborts
                return $this->doSecure( $params );
        }

        /**
         * @see FileBackend::secure()
         * @param array $params
         */
        abstract protected function doSecure( array $params );

        /**
         * Remove measures to block web access to a storage directory and
         * the container it belongs to. FS backends might remove .htaccess
         * files whereas key/value store backends might grant container
         * access to the storage user representing end-users in web requests.
         * This essentially can undo the result of secure() calls.
         *
         * This is not guaranteed to actually make files or listings publically viewable.
         * Additional server configuration may be needed to achieve the desired effect.
         *
         * @param array $params Parameters include:
         *   - dir            : storage directory
         *   - access         : try to allow file access
         *   - listing        : try to allow file listing
         *   - bypassReadOnly : allow writes in read-only mode (since 1.20)
         * @return Status
         * @since 1.20
         */
        final public function publish( array $params ) {
                if ( empty( $params['bypassReadOnly'] ) && $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                /** @noinspection PhpUnusedLocalVariableInspection */
                $scope = $this->getScopedPHPBehaviorForOps(); // try to ignore client aborts
                return $this->doPublish( $params );
        }

        /**
         * @see FileBackend::publish()
         * @param array $params
         */
        abstract protected function doPublish( array $params );

        /**
         * Delete a storage directory if it is empty.
         * Backends using key/value stores may do nothing unless the directory
         * is that of an empty container, in which case it will be deleted.
         *
         * @param array $params Parameters include:
         *   - dir            : storage directory
         *   - recursive      : recursively delete empty subdirectories first (since 1.20)
         *   - bypassReadOnly : allow writes in read-only mode (since 1.20)
         * @return Status
         */
        final public function clean( array $params ) {
                if ( empty( $params['bypassReadOnly'] ) && $this->isReadOnly() ) {
                        return Status::newFatal( 'backend-fail-readonly', $this->name, $this->readOnly );
                }
                /** @noinspection PhpUnusedLocalVariableInspection */
                $scope = $this->getScopedPHPBehaviorForOps(); // try to ignore client aborts
                return $this->doClean( $params );
        }

        /**
         * @see FileBackend::clean()
         * @param array $params
         */
        abstract protected function doClean( array $params );

        /**
         * Enter file operation scope.
         * This just makes PHP ignore user aborts/disconnects until the return
         * value leaves scope. This returns null and does nothing in CLI mode.
         *
         * @return ScopedCallback|null
         */
        final protected function getScopedPHPBehaviorForOps() {
                if ( PHP_SAPI != 'cli' ) { // http://bugs.php.net/bug.php?id=47540
                        $old = ignore_user_abort( true ); // avoid half-finished operations
                        return new ScopedCallback( function () use ( $old ) {
                                ignore_user_abort( $old );
                        } );
                }

                return null;
        }

        /**
         * Check if a file exists at a storage path in the backend.
         * This returns false if only a directory exists at the path.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return bool|null Returns null on failure
         */
        abstract public function fileExists( array $params );

        /**
         * Get the last-modified timestamp of the file at a storage path.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return string|bool TS_MW timestamp or false on failure
         */
        abstract public function getFileTimestamp( array $params );

        /**
         * Get the contents of a file at a storage path in the backend.
         * This should be avoided for potentially large files.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return string|bool Returns false on failure
         */
        final public function getFileContents( array $params ) {
                $contents = $this->getFileContentsMulti(
                        array( 'srcs' => array( $params['src'] ) ) + $params );

                return $contents[$params['src']];
        }

        /**
         * Like getFileContents() except it takes an array of storage paths
         * and returns a map of storage paths to strings (or null on failure).
         * The map keys (paths) are in the same order as the provided list of paths.
         *
         * @see FileBackend::getFileContents()
         *
         * @param array $params Parameters include:
         *   - srcs        : list of source storage paths
         *   - latest      : use the latest available data
         *   - parallelize : try to do operations in parallel when possible
         * @return array Map of (path name => string or false on failure)
         * @since 1.20
         */
        abstract public function getFileContentsMulti( array $params );

        /**
         * Get metadata about a file at a storage path in the backend.
         * If the file does not exist, then this returns false.
         * Otherwise, the result is an associative array that includes:
         *   - headers  : map of HTTP headers used for GET/HEAD requests (name => value)
         *   - metadata : map of file metadata (name => value)
         * Metadata keys and headers names will be returned in all lower-case.
         * Additional values may be included for internal use only.
         *
         * Use FileBackend::hasFeatures() to check how well this is supported.
         *
         * @param array $params
         * $params include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return array|bool Returns false on failure
         * @since 1.23
         */
        abstract public function getFileXAttributes( array $params );

        /**
         * Get the size (bytes) of a file at a storage path in the backend.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return int|bool Returns false on failure
         */
        abstract public function getFileSize( array $params );

        /**
         * Get quick information about a file at a storage path in the backend.
         * If the file does not exist, then this returns false.
         * Otherwise, the result is an associative array that includes:
         *   - mtime  : the last-modified timestamp (TS_MW)
         *   - size   : the file size (bytes)
         * Additional values may be included for internal use only.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return array|bool|null Returns null on failure
         */
        abstract public function getFileStat( array $params );

        /**
         * Get a SHA-1 hash of the file at a storage path in the backend.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return string|bool Hash string or false on failure
         */
        abstract public function getFileSha1Base36( array $params );

        /**
         * Get the properties of the file at a storage path in the backend.
         * This gives the result of FSFile::getProps() on a local copy of the file.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return array Returns FSFile::placeholderProps() on failure
         */
        abstract public function getFileProps( array $params );

        /**
         * Stream the file at a storage path in the backend.
         * If the file does not exists, an HTTP 404 error will be given.
         * Appropriate HTTP headers (Status, Content-Type, Content-Length)
         * will be sent if streaming began, while none will be sent otherwise.
         * Implementations should flush the output buffer before sending data.
         *
         * @param array $params Parameters include:
         *   - src     : source storage path
         *   - headers : list of additional HTTP headers to send on success
         *   - latest  : use the latest available data
         * @return Status
         */
        abstract public function streamFile( array $params );

        /**
         * Returns a file system file, identical to the file at a storage path.
         * The file returned is either:
         *   - a) A local copy of the file at a storage path in the backend.
         *        The temporary copy will have the same extension as the source.
         *   - b) An original of the file at a storage path in the backend.
         * Temporary files may be purged when the file object falls out of scope.
         *
         * Write operations should *never* be done on this file as some backends
         * may do internal tracking or may be instances of FileBackendMultiWrite.
         * In that later case, there are copies of the file that must stay in sync.
         * Additionally, further calls to this function may return the same file.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return FSFile|null Returns null on failure
         */
        final public function getLocalReference( array $params ) {
                $fsFiles = $this->getLocalReferenceMulti(
                        array( 'srcs' => array( $params['src'] ) ) + $params );

                return $fsFiles[$params['src']];
        }

        /**
         * Like getLocalReference() except it takes an array of storage paths
         * and returns a map of storage paths to FSFile objects (or null on failure).
         * The map keys (paths) are in the same order as the provided list of paths.
         *
         * @see FileBackend::getLocalReference()
         *
         * @param array $params Parameters include:
         *   - srcs        : list of source storage paths
         *   - latest      : use the latest available data
         *   - parallelize : try to do operations in parallel when possible
         * @return array Map of (path name => FSFile or null on failure)
         * @since 1.20
         */
        abstract public function getLocalReferenceMulti( array $params );

        /**
         * Get a local copy on disk of the file at a storage path in the backend.
         * The temporary copy will have the same file extension as the source.
         * Temporary files may be purged when the file object falls out of scope.
         *
         * @param array $params Parameters include:
         *   - src    : source storage path
         *   - latest : use the latest available data
         * @return TempFSFile|null Returns null on failure
         */
        final public function getLocalCopy( array $params ) {
                $tmpFiles = $this->getLocalCopyMulti(
                        array( 'srcs' => array( $params['src'] ) ) + $params );

                return $tmpFiles[$params['src']];
        }

        /**
         * Like getLocalCopy() except it takes an array of storage paths and
         * returns a map of storage paths to TempFSFile objects (or null on failure).
         * The map keys (paths) are in the same order as the provided list of paths.
         *
         * @see FileBackend::getLocalCopy()
         *
         * @param array $params Parameters include:
         *   - srcs        : list of source storage paths
         *   - latest      : use the latest available data
         *   - parallelize : try to do operations in parallel when possible
         * @return array Map of (path name => TempFSFile or null on failure)
         * @since 1.20
         */
        abstract public function getLocalCopyMulti( array $params );

        /**
         * Return an HTTP URL to a given file that requires no authentication to use.
         * The URL may be pre-authenticated (via some token in the URL) and temporary.
         * This will return null if the backend cannot make an HTTP URL for the file.
         *
         * This is useful for key/value stores when using scripts that seek around
         * large files and those scripts (and the backend) support HTTP Range headers.
         * Otherwise, one would need to use getLocalReference(), which involves loading
         * the entire file on to local disk.
         *
         * @param array $params Parameters include:
         *   - src : source storage path
         *   - ttl : lifetime (seconds) if pre-authenticated; default is 1 day
         * @return string|null
         * @since 1.21
         */
        abstract public function getFileHttpUrl( array $params );

        /**
         * Check if a directory exists at a given storage path.
         * Backends using key/value stores will check if the path is a
         * virtual directory, meaning there are files under the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * @param array $params Parameters include:
         *   - dir : storage directory
         * @return bool|null Returns null on failure
         * @since 1.20
         */
        abstract public function directoryExists( array $params );

        /**
         * Get an iterator to list *all* directories under a storage directory.
         * If the directory is of the form "mwstore://backend/container",
         * then all directories in the container will be listed.
         * If the directory is of form "mwstore://backend/container/dir",
         * then all directories directly under that directory will be listed.
         * Results will be storage directories relative to the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * Failures during iteration can result in FileBackendError exceptions (since 1.22).
         *
         * @param array $params Parameters include:
         *   - dir     : storage directory
         *   - topOnly : only return direct child dirs of the directory
         * @return Traversable|array|null Returns null on failure
         * @since 1.20
         */
        abstract public function getDirectoryList( array $params );

        /**
         * Same as FileBackend::getDirectoryList() except only lists
         * directories that are immediately under the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * Failures during iteration can result in FileBackendError exceptions (since 1.22).
         *
         * @param array $params Parameters include:
         *   - dir : storage directory
         * @return Traversable|array|null Returns null on failure
         * @since 1.20
         */
        final public function getTopDirectoryList( array $params ) {
                return $this->getDirectoryList( array( 'topOnly' => true ) + $params );
        }

        /**
         * Get an iterator to list *all* stored files under a storage directory.
         * If the directory is of the form "mwstore://backend/container",
         * then all files in the container will be listed.
         * If the directory is of form "mwstore://backend/container/dir",
         * then all files under that directory will be listed.
         * Results will be storage paths relative to the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * Failures during iteration can result in FileBackendError exceptions (since 1.22).
         *
         * @param array $params Parameters include:
         *   - dir        : storage directory
         *   - topOnly    : only return direct child files of the directory (since 1.20)
         *   - adviseStat : set to true if stat requests will be made on the files (since 1.22)
         * @return Traversable|array|null Returns null on failure
         */
        abstract public function getFileList( array $params );

        /**
         * Same as FileBackend::getFileList() except only lists
         * files that are immediately under the given directory.
         *
         * Storage backends with eventual consistency might return stale data.
         *
         * Failures during iteration can result in FileBackendError exceptions (since 1.22).
         *
         * @param array $params Parameters include:
         *   - dir        : storage directory
         *   - adviseStat : set to true if stat requests will be made on the files (since 1.22)
         * @return Traversable|array|null Returns null on failure
         * @since 1.20
         */
        final public function getTopFileList( array $params ) {
                return $this->getFileList( array( 'topOnly' => true ) + $params );
        }

        /**
         * Preload persistent file stat cache and property cache into in-process cache.
         * This should be used when stat calls will be made on a known list of a many files.
         *
         * @see FileBackend::getFileStat()
         *
         * @param array $paths Storage paths
         */
        abstract public function preloadCache( array $paths );

        /**
         * Invalidate any in-process file stat and property cache.
         * If $paths is given, then only the cache for those files will be cleared.
         *
         * @see FileBackend::getFileStat()
         *
         * @param array $paths Storage paths (optional)
         */
        abstract public function clearCache( array $paths = null );

        /**
         * Preload file stat information (concurrently if possible) into in-process cache.
         *
         * This should be used when stat calls will be made on a known list of a many files.
         * This does not make use of the persistent file stat cache.
         *
         * @see FileBackend::getFileStat()
         *
         * @param array $params Parameters include:
         *   - srcs        : list of source storage paths
         *   - latest      : use the latest available data
         * @return bool All requests proceeded without I/O errors (since 1.24)
         * @since 1.23
         */
        abstract public function preloadFileStat( array $params );

        /**
         * Lock the files at the given storage paths in the backend.
         * This will either lock all the files or none (on failure).
         *
         * Callers should consider using getScopedFileLocks() instead.
         *
         * @param array $paths Storage paths
         * @param int $type LockManager::LOCK_* constant
         * @param int $timeout Timeout in seconds (0 means non-blocking) (since 1.24)
         * @return Status
         */
        final public function lockFiles( array $paths, $type, $timeout = 0 ) {
                $paths = array_map( 'FileBackend::normalizeStoragePath', $paths );

                return $this->lockManager->lock( $paths, $type, $timeout );
        }

        /**
         * Unlock the files at the given storage paths in the backend.
         *
         * @param array $paths Storage paths
         * @param int $type LockManager::LOCK_* constant
         * @return Status
         */
        final public function unlockFiles( array $paths, $type ) {
                $paths = array_map( 'FileBackend::normalizeStoragePath', $paths );

                return $this->lockManager->unlock( $paths, $type );
        }

        /**
         * Lock the files at the given storage paths in the backend.
         * This will either lock all the files or none (on failure).
         * On failure, the status object will be updated with errors.
         *
         * Once the return value goes out scope, the locks will be released and
         * the status updated. Unlock fatals will not change the status "OK" value.
         *
         * @see ScopedLock::factory()
         *
         * @param array $paths List of storage paths or map of lock types to path lists
         * @param int|string $type LockManager::LOCK_* constant or "mixed"
         * @param Status $status Status to update on lock/unlock
         * @param int $timeout Timeout in seconds (0 means non-blocking) (since 1.24)
         * @return ScopedLock|null Returns null on failure
         */
        final public function getScopedFileLocks( array $paths, $type, Status $status, $timeout = 0 ) {
                if ( $type === 'mixed' ) {
                        foreach ( $paths as &$typePaths ) {
                                $typePaths = array_map( 'FileBackend::normalizeStoragePath', $typePaths );
                        }
                } else {
                        $paths = array_map( 'FileBackend::normalizeStoragePath', $paths );
                }

                return ScopedLock::factory( $this->lockManager, $paths, $type, $status, $timeout );
        }

        /**
         * Get an array of scoped locks needed for a batch of file operations.
         *
         * Normally, FileBackend::doOperations() handles locking, unless
         * the 'nonLocking' param is passed in. This function is useful if you
         * want the files to be locked for a broader scope than just when the
         * files are changing. For example, if you need to update DB metadata,
         * you may want to keep the files locked until finished.
         *
         * @see FileBackend::doOperations()
         *
         * @param array $ops List of file operations to FileBackend::doOperations()
         * @param Status $status Status to update on lock/unlock
         * @return ScopedLock|null
         * @since 1.20
         */
        abstract public function getScopedLocksForOps( array $ops, Status $status );

        /**
         * Get the root storage path of this backend.
         * All container paths are "subdirectories" of this path.
         *
         * @return string Storage path
         * @since 1.20
         */
        final public function getRootStoragePath() {
                return "mwstore://{$this->name}";
        }

        /**
         * Get the storage path for the given container for this backend
         *
         * @param string $container Container name
         * @return string Storage path
         * @since 1.21
         */
        final public function getContainerStoragePath( $container ) {
                return $this->getRootStoragePath() . "/{$container}";
        }

        /**
         * Get the file journal object for this backend
         *
         * @return FileJournal
         */
        final public function getJournal() {
                return $this->fileJournal;
        }

        /**
         * Check if a given path is a "mwstore://" path.
         * This does not do any further validation or any existence checks.
         *
         * @param string $path
         * @return bool
         */
        final public static function isStoragePath( $path ) {
                return ( strpos( $path, 'mwstore://' ) === 0 );
        }

        /**
         * Split a storage path into a backend name, a container name,
         * and a relative file path. The relative path may be the empty string.
         * This does not do any path normalization or traversal checks.
         *
         * @param string $storagePath
         * @return array (backend, container, rel object) or (null, null, null)
         */
        final public static function splitStoragePath( $storagePath ) {
                if ( self::isStoragePath( $storagePath ) ) {
                        // Remove the "mwstore://" prefix and split the path
                        $parts = explode( '/', substr( $storagePath, 10 ), 3 );
                        if ( count( $parts ) >= 2 && $parts[0] != '' && $parts[1] != '' ) {
                                if ( count( $parts ) == 3 ) {
                                        return $parts; // e.g. "backend/container/path"
                                } else {
                                        return array( $parts[0], $parts[1], '' ); // e.g. "backend/container"
                                }
                        }
                }

                return array( null, null, null );
        }

        /**
         * Normalize a storage path by cleaning up directory separators.
         * Returns null if the path is not of the format of a valid storage path.
         *
         * @param string $storagePath
         * @return string|null
         */
        final public static function normalizeStoragePath( $storagePath ) {
                list( $backend, $container, $relPath ) = self::splitStoragePath( $storagePath );
                if ( $relPath !== null ) { // must be for this backend
                        $relPath = self::normalizeContainerPath( $relPath );
                        if ( $relPath !== null ) {
                                return ( $relPath != '' )
                                        ? "mwstore://{$backend}/{$container}/{$relPath}"
                                        : "mwstore://{$backend}/{$container}";
                        }
                }

                return null;
        }

        /**
         * Get the parent storage directory of a storage path.
         * This returns a path like "mwstore://backend/container",
         * "mwstore://backend/container/...", or null if there is no parent.
         *
         * @param string $storagePath
         * @return string|null
         */
        final public static function parentStoragePath( $storagePath ) {
                $storagePath = dirname( $storagePath );
                list( , , $rel ) = self::splitStoragePath( $storagePath );

                return ( $rel === null ) ? null : $storagePath;
        }

        /**
         * Get the final extension from a storage or FS path
         *
         * @param string $path
         * @param string $case One of (rawcase, uppercase, lowercase) (since 1.24)
         * @return string
         */
        final public static function extensionFromPath( $path, $case = 'lowercase' ) {
                $i = strrpos( $path, '.' );
                $ext = $i ? substr( $path, $i + 1 ) : '';

                if ( $case === 'lowercase' ) {
                        $ext = strtolower( $ext );
                } elseif ( $case === 'uppercase' ) {
                        $ext = strtoupper( $ext );
                }

                return $ext;
        }

        /**
         * Check if a relative path has no directory traversals
         *
         * @param string $path
         * @return bool
         * @since 1.20
         */
        final public static function isPathTraversalFree( $path ) {
                return ( self::normalizeContainerPath( $path ) !== null );
        }

        /**
         * Build a Content-Disposition header value per RFC 6266.
         *
         * @param string $type One of (attachment, inline)
         * @param string $filename Suggested file name (should not contain slashes)
         * @throws FileBackendError
         * @return string
         * @since 1.20
         */
        final public static function makeContentDisposition( $type, $filename = '' ) {
                $parts = array();

                $type = strtolower( $type );
                if ( !in_array( $type, array( 'inline', 'attachment' ) ) ) {
                        throw new FileBackendError( "Invalid Content-Disposition type '$type'." );
                }
                $parts[] = $type;

                if ( strlen( $filename ) ) {
                        $parts[] = "filename*=UTF-8''" . rawurlencode( basename( $filename ) );
                }

                return implode( ';', $parts );
        }

        /**
         * Validate and normalize a relative storage path.
         * Null is returned if the path involves directory traversal.
         * Traversal is insecure for FS backends and broken for others.
         *
         * This uses the same traversal protection as Title::secureAndSplit().
         *
         * @param string $path Storage path relative to a container
         * @return string|null
         */
        final protected static function normalizeContainerPath( $path ) {
                // Normalize directory separators
                $path = strtr( $path, '\\', '/' );
                // Collapse any consecutive directory separators
                $path = preg_replace( '![/]{2,}!', '/', $path );
                // Remove any leading directory separator
                $path = ltrim( $path, '/' );
                // Use the same traversal protection as Title::secureAndSplit()
                if ( strpos( $path, '.' ) !== false ) {
                        if (
                                $path === '.' ||
                                $path === '..' ||
                                strpos( $path, './' ) === 0 ||
                                strpos( $path, '../' ) === 0 ||
                                strpos( $path, '/./' ) !== false ||
                                strpos( $path, '/../' ) !== false
                        ) {
                                return null;
                        }
                }

                return $path;
        }
}

/**
 * Generic file backend exception for checked and unexpected (e.g. config) exceptions
 *
 * @ingroup FileBackend
 * @since 1.23
 */
class FileBackendException extends Exception {
}

/**
 * File backend exception for checked exceptions (e.g. I/O errors)
 *
 * @ingroup FileBackend
 * @since 1.22
 */
class FileBackendError extends FileBackendException {
}