feat(crypto) Provide a method to check whether server backup exists without hitting the server every time

[andybalaam]

Part of element-hq/element-meta#2638

Provide fast_exists_on_server so we can call it when figuring out whether a UTD is expected. (Updated 2024-12-13 to reflect final implementation.)

Modify exists_on_server to cache the last value we received. Provide fetch_exists_on_server that explicitly makes a server request to get the latest information.

exists_on_server does not always provide a fully-correct answer, but it is a best effort that is much cheaper than the full fetch_exists_on_server which always makes a request to the server. Since we only plan to call the new method when reporting UTDs, for now, the negative consequence of an incorrect answer is only to report the wrong kind of UTD.

If you have suggestions for better ways to implement this, I am interested. I wish we didn't need to have a lock on the cached value but I couldn't see a better way.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.19%. Comparing base (e402ed4) to head (c37b485).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/test_utils/mocks.rs	85.71%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4356      +/-   ##
==========================================
- Coverage   85.20%   85.19%   -0.02%     
==========================================
  Files         281      281              
  Lines       30926    30963      +37     
==========================================
+ Hits        26352    26379      +27     
- Misses       4574     4584      +10     

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[bnjbvr]

There are three things that are hard in computer science: invalidating caches, and off by ones. A few questions/suggestions below.

[bnjbvr]

Thanks! Slightly concerned that the cached value may be outdated because another client erased the backup, but if you're fine with it, I'm fine with it :) (Isn't there a way to listen to some sync or to-device events, and realize the backup has been removed elsewhere?)