caching: fix diskcache implementation

README.md

@@ -40,7 +40,7 @@ Some features of morss:
 - Follow 301/meta redirects
 - Recover xml feeds with corrupt encoding
 - Supports gzip-compressed http content
-- HTTP caching with 3 different backends (in-memory/sqlite/mysql)
+- HTTP caching with different backends (in-memory/sqlite/mysql/redis/diskcache)
 - Works as server/cli tool
 - Deobfuscate various tracking links
 
@@ -60,8 +60,8 @@ Full installation (including optional dependencies)
 pip install git+https://git.pictuga.com/pictuga/morss.git#[full]
 ```
 
-The full install includes mysql and redis (possible cache backends). Otherwise,
-only in-memory and sqlite3 caches are available.
+The full install includes mysql, redis and diskcache (possible cache backends).
+Otherwise, only in-memory and sqlite3 caches are available.
 
 The dependency `lxml` is fairly long to install (especially on Raspberry Pi, as
 C code needs to be compiled). If possible on your distribution, try installing
@@ -390,12 +390,14 @@ will be cleared every time the program is run). Path can be defined with
 environment variables: `MYSQL_USER`, `MYSQL_PWD`, `MYSQL_DB`, `MYSQL_HOST`
 - `CACHE=redis`: Redis cache. Connection can be defined with the following
 environment variables: `REDIS_HOST`, `REDIS_PORT`, `REDIS_DB`, `REDIS_PWD`
+- `CACHE=diskcache`: disk-based cache. Target directory canbe defined with
+`DISKCAHE_DIR`.
 
 To limit the size of the cache:
 
 - `CACHE_SIZE` sets the target number of items in the cache (further items will
 be deleted but the cache might be temporarily bigger than that). Defaults to 1k
-entries.
+entries. NB. When using `diskcache`, this is the cache max size in Bytes.
 - `CACHE_LIFESPAN` (seconds) sets how often the cache must be trimmed (i.e. cut
 down to the number of items set in `CACHE_SIZE`). Defaults to 1min.