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util/fscache: introduce versioning

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Add a new field to the cache-information called `version`, which is a
simple integer that is incremented on any backward-incompatible change.

The cache-implementation is modified to avoid any access to the cache
except for `<cache>/staging/`. This means, changes to the staging area
must be backwards compatible at all cost. Furthermore, it means we can
always successfully run osbuild even on possibly incompatible caches,
because we can always just ignore the cache and fully rely on the
staging area being accessible.

The `load()` method will always return cache-misses. The `store()`
method simply discards the entry instead of storing it. Note that
`store()` needs to provide a context to the caller, hence this
implementation simply creates another staging-context to provide to the
caller and then discard. This is non-optimal, but keeps the API simple
and avoids raising an exception to the caller (but this can be changed
if it turns out to be problematic or unwanted).

Lastly, the `cache.info` field behaves as usual, since this is also the
field used to read the cache-version. However, this file is never
written to improve resiliency and allow blacklisting buggy versions from
the past.

Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>
This commit is contained in:
David Rheinsberg 2022-12-02 14:49:09 +01:00 • committed by Christian Kellner
parent fdd9e859dc
commit ef20b40faa
2 changed files with 41 additions and 5 deletions
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5
test/mod/test_util_fscache.py
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@ -4,6 +4,7 @@
# pylint: disable=protected-access
import json
import os
import tempfile
@ -194,7 +195,7 @@ def test_scaffolding(tmpdir):
assert len(list(os.scandir(os.path.join(tmpdir, cache._dirname_stage)))) == 0
with open(os.path.join(tmpdir, cache._filename_cache_info), "r", encoding="utf8") as f:
assert f.read() == "{}"
assert json.load(f) == {"version": 1}
with open(os.path.join(tmpdir, cache._filename_cache_lock), "r", encoding="utf8") as f:
assert f.read() == ""
with open(os.path.join(tmpdir, cache._filename_cache_size), "r", encoding="utf8") as f:
@ -210,7 +211,7 @@ def test_cache_info(tmpdir):
cache = fscache.FsCache("osbuild-test-appid", tmpdir)
with cache:
assert cache._info == fscache.FsCacheInfo()
assert cache._info == fscache.FsCacheInfo(version=1)
assert cache.info == cache._info
assert cache.info.maximum_size is None