This commit introduces a new utility module called `fscache`. It
implements a cache module that stores data on the file system. It
supports parallel access and protects data with file-system locks. It
provides three basic functions:
FsCache.load("<name>"):
Loads the cache entry with the specified name, acquires a
read-lock and yields control to the caller to use the entry.
Once control returns, the entry is unlocked again.
If the entry cannot be found, a cache miss is signalled via
FsCache.MissError.
FsCache.store("<name>"):
Creates a new anonymous cache entry and yields control to the
caller to fill in. Once control returns, the entry is renamed
to the specified name, thus committing it to the object store.
FsCache.stage():
Create a new anonymous staging entry and yield control to the
caller. Once control returns, the entry is completely
discarded.
This is primarily used to create a working directory for osbuild
pipeline operations. The entries are volatile and automatic
cleanup is provided.
To commit a staging entry, you would eventually use
FsCache.store() and rename the entire data directory into the
non-volatile entry. If the staging area and store are on
different file-systems, or if the data is to be retained for
further operations, then the data directory needs to be copied.
Additionally, the cache maintains a size limit and discards any entries
if the limit is exceeded. Future extensions will implement cache pruning
if a configured watermark is reached, based on last-recently-used
logics.
Many more cache extensions are possible. This module introduces a first
draft of the most basic cache and hopefully lays ground for a new cache
infrastructure.
Lastly, note that this only introduces the utility helper. Further work
is required to hook it up with osbuild/objectstore.py.
4df05b8509