VMA Fixes

[JSCU-CNI]

Two bugs related to VMA are covered in this PR:

• vma-extract had a seek to the start of the data inside an extent in the wrong place (within the loop over the blocks instead of before the loop)
• A logic-bug related to the fact that inside an extent, blocks of multiple devices could be stored. These blocks should be handled in the loop inside _iter_clusters as well, in order to return the right block_offset.

[Schamper]

Also small suggestion (can't put a comment there), can you change the @lru_cache(65536) at line 94 to a self._extent = lru_cache(65536)(self._extent) at the bottom of __init__?

[JSCU-CNI]

Also small suggestion (can't put a comment there), can you change the @lru_cache(65536) at line 94 to a self._extent = lru_cache(65536)(self._extent) at the bottom of __init__?

Done! Could you please explain your reasons for asking this? We don't see how this benefits the code, but we suspect you've got good reasons for doing so 😄

[Schamper]

Also small suggestion (can't put a comment there), can you change the @lru_cache(65536) at line 94 to a self._extent = lru_cache(65536)(self._extent) at the bottom of __init__?

Done! Could you please explain your reasons for asking this? We don't see how this benefits the code, but we suspect you've got good reasons for doing so smile

https://rednafi.github.io/python/lru_cache_on_methods/

TL;DR: @lru_cache on class methods is bad, as it can result in an unintended memory leak because self is cached. New code should use the change suggested, and old code fixed as we go.

[codecov]

Codecov Report

Merging #17 (93178b6) into main (072e025) will increase coverage by 0.02%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   60.96%   60.99%   +0.02%     
==========================================
  Files          29       29              
  Lines        2375     2374       -1     
==========================================
  Hits         1448     1448              
+ Misses        927      926       -1     

Flag	Coverage Δ
unittests	60.99% <72.72%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dissect/hypervisor/tools/vma.py	0.00% <0.00%> (ø)
dissect/hypervisor/backup/vma.py	81.72% <88.88%> (ø)

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