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/*
* libwebsockets - disk cache helpers
*
* Copyright (C) 2010-2018 Andy Green <andy@warmcat.com>
*
* 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.
*
* 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
*
* included from libwebsockets.h
*/
/*! \defgroup diskcache LWS disk cache
* ## Disk cache API
*
* Lws provides helper apis useful if you need a disk cache containing hashed
* files and need to delete files from it on an LRU basis to keep it below some
* size limit.
*
* The API `lws_diskcache_prepare()` deals with creating the cache dir and
* 256 subdirs, which are used according to the first two chars of the hex
* hash of the cache file.
*
* `lws_diskcache_create()` and `lws_diskcache_destroy()` allocate and free
* an opaque struct that represents the disk cache.
*
* `lws_diskcache_trim()` should be called at eg, 1s intervals to perform the
* cache dir monitoring and LRU autodelete in the background lazily. It can
* be done in its own thread or on a timer... it monitors the directories in a
* stateful way that stats one or more file in the cache per call, and keeps
* a list of the oldest files as it goes. When it completes a scan, if the
* aggregate size is over the limit, it will delete oldest files first to try
* to keep it under the limit.
*
* The cache size monitoring is extremely efficient in time and memory even when
* the cache directory becomes huge.
*
* `lws_diskcache_query()` is used to determine if the file already exists in
* the cache, or if it must be created. If it must be created, then the file
* is opened using a temp name that must be converted to a findable name with
* `lws_diskcache_finalize_name()` when the generation of the file contents are
* complete. Aborted cached files that did not complete generation will be
* flushed by the LRU eventually. If the file already exists, it is 'touched'
* to make it new again and the fd returned.
*
*/
///@{
struct lws_diskcache_scan;
/**
* lws_diskcache_create() - creates an opaque struct representing the disk cache
*
* \param cache_dir_base: The cache dir path, eg `/var/cache/mycache`
* \param cache_size_limit: maximum size on disk the cache is allowed to use
*
* This returns an opaque `struct lws_diskcache_scan *` which represents the
* disk cache, the trim scanning state and so on. You should use
* `lws_diskcache_destroy()` to free it to destroy it.
*/
LWS_VISIBLE LWS_EXTERN struct lws_diskcache_scan *
lws_diskcache_create(const char *cache_dir_base, uint64_t cache_size_limit);
/**
* lws_diskcache_destroy() - destroys the pointer returned by ...create()
*
* \param lds: pointer to the pointer returned by lws_diskcache_create()
*
* Frees *lds and any allocations it did, and then sets *lds to NULL and
* returns.
*/
LWS_VISIBLE LWS_EXTERN void
lws_diskcache_destroy(struct lws_diskcache_scan **lds);
/**
* lws_diskcache_prepare() - ensures the cache dir structure exists on disk
*
* \param cache_base_dir: The cache dir path, eg `/var/cache/mycache`
* \param mode: octal dir mode to enforce, like 0700
* \param uid: uid the cache dir should belong to
*
* This should be called while your app is still privileged. It will create
* the cache directory structure on disk as necessary, enforce the given access
* mode on it and set the given uid as the owner. It won't make any trouble
* if the cache already exists.
*
* Typically the mode is 0700 and the owner is the user that your application
* will transition to use when it drops root privileges.
*/
LWS_VISIBLE LWS_EXTERN int
lws_diskcache_prepare(const char *cache_base_dir, int mode, int uid);
#define LWS_DISKCACHE_QUERY_NO_CACHE 0
#define LWS_DISKCACHE_QUERY_EXISTS 1
#define LWS_DISKCACHE_QUERY_CREATING 2
#define LWS_DISKCACHE_QUERY_ONGOING 3 /* something else is creating it */
/**
* lws_diskcache_query() - ensures the cache dir structure exists on disk
*
* \param lds: The opaque struct representing the disk cache
* \param is_bot: nonzero means the request is from a bot. Don't create new cache contents if so.
* \param hash_hex: hex string representation of the cache object hash
* \param _fd: pointer to the fd to be set
* \param cache: destination string to take the cache filepath
* \param cache_len: length of the buffer at `cache`
* \param extant_cache_len: pointer to a size_t to take any extant cached file size
*
* This function is called when you want to find if the hashed name already
* exists in the cache. The possibilities for the return value are
*
* - LWS_DISKCACHE_QUERY_NO_CACHE: It's not in the cache and you can't create
* it in the cache for whatever reason.
* - LWS_DISKCACHE_QUERY_EXISTS: It exists in the cache. It's open RDONLY and
* *_fd has been set to the file descriptor. *extant_cache_len has been set
* to the size of the cached file in bytes. cache has been set to the
* full filepath of the cached file. Closing _fd is your responsibility.
* - LWS_DISKCACHE_QUERY_CREATING: It didn't exist, but a temp file has been
* created in the cache and *_fd set to a file descriptor opened on it RDWR.
* You should create the contents, and call `lws_diskcache_finalize_name()`
* when it is done. Closing _fd is your responsibility.
* - LWS_DISKCACHE_QUERY_ONGOING: not returned by this api, but you may find it
* desirable to make a wrapper function which can handle another asynchronous
* process that is already creating the cached file. This can be used to
* indicate that situation externally... how to determine the same thing is
* already being generated is out of scope of this api.
*/
LWS_VISIBLE LWS_EXTERN int
lws_diskcache_query(struct lws_diskcache_scan *lds, int is_bot,
const char *hash_hex, int *_fd, char *cache, int cache_len,
size_t *extant_cache_len);
/**
* lws_diskcache_query() - ensures the cache dir structure exists on disk
*
* \param cache: The cache file temp name returned with LWS_DISKCACHE_QUERY_CREATING
*
* This renames the cache file you are creating to its final name. It should
* be called on the temp name returned by `lws_diskcache_query()` if it gave a
* LWS_DISKCACHE_QUERY_CREATING return, after you have filled the cache file and
* closed it.
*/
LWS_VISIBLE LWS_EXTERN int
lws_diskcache_finalize_name(char *cache);
/**
* lws_diskcache_trim() - performs one or more file checks in the cache for size management
*
* \param lds: The opaque object representing the cache
*
* This should be called periodically to statefully walk the cache on disk
* collecting the oldest files. When it has visited every file, if the cache
* is oversize it will delete the oldest files until it's back under size again.
*
* Each time it's called, it will look at one or more dir in the cache. If
* called when the cache is oversize, it increases the amount of work done each
* call until it is reduced again. Typically it will take 256 calls before it
* deletes anything, so if called once per second, it will delete files once
* every 4 minutes. Each call is very inexpensive both in memory and time.
*/
LWS_VISIBLE LWS_EXTERN int
lws_diskcache_trim(struct lws_diskcache_scan *lds);
/**
* lws_diskcache_secs_to_idle() - see how long to idle before calling trim
*
* \param lds: The opaque object representing the cache
*
* If the cache is undersize, there's no need to monitor it immediately. This
* suggests how long to "sleep" before calling `lws_diskcache_trim()` again.
*/
LWS_VISIBLE LWS_EXTERN int
lws_diskcache_secs_to_idle(struct lws_diskcache_scan *lds);
///@}
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