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/**
* @file lldiskcache.h
* @brief The disk cache implementation declarations.
*
* @Description:
* This code implements a disk cache using the following ideas:
* 1/ The metadata for a file can be encapsulated in the filename.
The filenames will be composed of the following fields:
Prefix: Used to identify the file as a part of the cache.
An additional reason for using a prefix is that it
might be possible, either accidentally or maliciously
to end up with the cache dir set to a non-cache
location such as your OS system dir or a work folder.
Purging files from that would obviously be a disaster
so this is an extra step to help avoid that scenario.
ID: Typically the asset ID (UUID) of the asset being
saved but can be anything valid for a filename
Extra Info: A field for use in the future that can be used
to store extra identifiers - e.g. the discard
level of a JPEG2000 file
Asset Type: A text string created from the LLAssetType enum
that identifies the type of asset being stored.
.asset A file extension of .asset is used to help
identify this as a Viewer asset file
* 2/ The time of last access for a file can be updated instantly
* for file reads and automatically as part of the file writes.
* 3/ The purge algorithm collects a list of all files in the
* directory, sorts them by date of last access (write) and then
* deletes any files based on age until the total size of all
* the files is less than the maximum size specified.
* 4/ An LLSingleton idiom is used since there will only ever be
* a single cache and we want to access it from numerous places.
* 5/ Performance on my modest system seems very acceptable. For
* example, in testing, I was able to purge a directory of
* 10,000 files, deleting about half of them in ~ 1700ms. For
* the same sized directory of files, writing the last updated
* time to each took less than 600ms indicating that this
* important part of the mechanism has almost no overhead.
*
* $LicenseInfo:firstyear=2009&license=viewerlgpl$
* Second Life Viewer Source Code
* Copyright (C) 2020, Linden Research, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation;
* version 2.1 of the License only.
*
* This library 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
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*
* Linden Research, Inc., 945 Battery Street, San Francisco, CA 94111 USA
* $/LicenseInfo$
*/
#ifndef _LLDISKCACHE
#define _LLDISKCACHE
#include "llsingleton.h"
class LLDiskCache :
public LLParamSingleton<LLDiskCache>
{
public:
/**
* Since this is using the LLSingleton pattern but we
* want to allow the constructor to be called first
* with various parameters, we also invoke the
* LLParamSingleton idiom and use it to initialize
* the class via a call in LLAppViewer.
*/
LLSINGLETON(LLDiskCache,
/**
* The full name of the cache folder - typically a
* a child of the main Viewer cache directory. Defined
* by the setting at 'DiskCacheDirName'
*/
const std::string cache_dir,
/**
* The maximum size of the cache in bytes - Defined by
* the setting at 'DiskCacheMaxSizeMB' (* 1024 * 1024)
*/
const int max_size_bytes,
/**
* A flag that enables extra cache debugging so that
* if there are bugs, we can ask uses to enable this
* setting and send us their logs
*/
const bool enable_cache_debug_info);
virtual ~LLDiskCache() = default;
public:
/**
* Construct a filename and path to it based on the file meta data
* (id, asset type, additional 'extra' info like discard level perhaps)
* Worth pointing out that this function used to be in LLFileSystem but
* so many things had to be pushed back there to accomodate it, that I
* decided to move it here. Still not sure that's completely right.
*/
const std::string metaDataToFilepath(const std::string id,
LLAssetType::EType at,
const std::string extra_info);
/**
* Update the "last write time" of a file to "now". This must be called whenever a
* file in the cache is read (not written) so that the last time the file was
* accessed is up to date (This is used in the mechanism for purging the cache)
*/
void updateFileAccessTime(const std::string file_path);
/**
* Purge the oldest items in the cache so that the combined size of all files
* is no bigger than mMaxSizeBytes.
*/
void purge();
/**
* Clear the cache by removing all the files in the specified cache
* directory individually. Only the files that contain a prefix defined
* by mCacheFilenamePrefix will be removed.
*/
void clearCache();
/**
* Return some information about the cache for use in About Box etc.
*/
const std::string getCacheInfo();
private:
/**
* Utility function to gather the total size the files in a given
* directory. Primarily used here to determine the directory size
* before and after the cache purge
*/
uintmax_t dirFileSize(const std::string dir);
/**
* Utility function to convert an LLAssetType enum into a
* string that we use as part of the cache file filename
*/
const std::string assetTypeToString(LLAssetType::EType at);
private:
/**
* The maximum size of the cache in bytes. After purge is called, the
* total size of the cache files in the cache directory will be
* less than this value
*/
uintmax_t mMaxSizeBytes;
/**
* The folder that holds the cached files. The consumer of this
* class must avoid letting the user set this location as a malicious
* setting could potentially point it at a non-cache directory (for example,
* the Windows System dir) with disastrous results.
*/
std::string mCacheDir;
/**
* The prefix inserted at the start of a cache file filename to
* help identify it as a cache file. It's probably not required
* (just the presence in the cache folder is enough) but I am
* paranoid about the cache folder being set to something bad
* like the users' OS system dir by mistake or maliciously and
* this will help to offset any damage if that happens.
*/
std::string mCacheFilenamePrefix;
/**
* When enabled, displays additional debugging information in
* various parts of the code
*/
bool mEnableCacheDebugInfo;
};
#endif // _LLDISKCACHE
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