Cache VM stats

[ben-grande]

TTL cache to avoid multiple clients or requests from the same client of blocking CPU cycles from qubesd.

[ben-grande]

Marek said: #827 (comment)

The change to use self.stats_cache_time instead of the explicit
value from the parameter is not great. If nobody is updating
(requesting) stats for some time, next one will return average of
a long time pretending it's the "current" value.

Let's assume the previous moment the stats were collected was a long
time ago.

• Empty previous, stats never called:

  • Without this PR: 0%
  • With PR: 0%
  • Resolution: Unaffected

• Empty previous, but stats generated at least once:

  • Without this PR: 0%
  • With PR: 0%, calculate_cpu_usage_raw() is not called as
    domid is not in previous, as there is no previous
  • Resolution: Unaffected

• Non-empty previous:

  • Without PR: avg considering the previous time
  • With PR: avg considering the previous time
  • Resolution: It already happened if the client is not using the
    API loop and calling get_vm_stats() directly

• Empty previous, but stats generated at least once:
  ...
  • With PR: 0%, calculate_cpu_usage_raw() is not called as
    domid is not in previous, as there is no previous

But let's assume that I did an average of a long time by assigning previous to the last cache if previous=None:

if not previous and self.stats_cache_data:
    previous = self.stats_cache_data

I don't think it would be wrong to have an average of a long time compared to reporting 0%.

[codecov]

Codecov Report

❌ Patch coverage is 59.57447% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.49%. Comparing base (e231e51) to head (3aa3201).

Files with missing lines	Patch %	Lines
qubes/app.py	58.06%	13 Missing ⚠️
qubes/api/admin.py	62.50%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #859      +/-   ##
==========================================
- Coverage   70.51%   70.49%   -0.02%     
==========================================
  Files          61       61              
  Lines       14371    14375       +4     
==========================================
  Hits        10133    10133              
- Misses       4238     4242       +4     

Flag	Coverage Δ
unittests	70.49% <59.57%> (-0.02%)	⬇️

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[ben-grande]

If you don't like the _get_vm_stats checking for the cache. I could make it default to ignore so the behavior doesn't change (even though it is a protected version), and then, if calling via admin.vm.Stats, always use the cache.