fix(event cache): don't deadlock when event-focused cache and redecryption are invoked

[bnjbvr]

The following ABBA situation would happen before this fix:

• an event-focused cache starts pagination (holding onto the event-focused inner lock), and then takes the event cache room state lock (while saving the events it's received in the pagination).
• redecryption takes the event cache room state lock, to update internal room events; then it would try to go through all the event-focused caches to replace their UTDs (thus taking all the inner locks, in turn).

So, having an event-focused cache paginating while the redecryption happens would cause a deadlock. The fix consists in changing the order of operations when re-decrypting: instead of holding onto the event cache room state lock all the time, first perform all the replacements necessary in the room cache, and clone the event-focused / pinned-event locks; then, replace UTDs in those (after the event cache room state lock has been released).

• No public API changes.
• This PR was made with the help of AI. (Used Claude to find the reason for the deadlock, based on the repro steps from the issue.)

(Likely) fixes #6524.

cc @jmartinesp @poljar

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Signed-off-by: benjamin@bouvier.cc

[codecov]

Codecov Report

❌ Patch coverage is 98.68421% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.97%. Comparing base (2ec031b) to head (bba09c9).
⚠️ Report is 9 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/redecryptor.rs	98.68%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6542      +/-   ##
==========================================
+ Coverage   89.92%   89.97%   +0.04%     
==========================================
  Files         381      381              
  Lines      105914   105989      +75     
  Branches   105914   105989      +75     
==========================================
+ Hits        95243    95362     +119     
+ Misses       7029     6994      -35     
+ Partials     3642     3633       -9     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing bnjbvr:bnjbvr/fix-deadlock-with-ef-cache-and-redecryptor (bba09c9) with main (2ec031b)}

[jmartinesp]

Thanks, I'll test it on Element X Android, but your explanation matches what I saw when trying to reproduce the issue, so I have high hopes!

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I wasn't able to reproduce with my previous setup, which was almost guaranteed to make the issue happen, so I think it's fixed!

[jmartinesp]

LGTM, but a 2nd review from someone more familiar with r2d2 or the event caches would be appreciated!

[Hywan]

LGTM