refactor: reuse should_empty for chainstate flush condition #34125

1. 
   l0rinc commented at 12:14 pm on December 20, 2025: contributor

   minor follow-up to #33866 (included here). Drafted until that’s merged.
2. refactor: Let CCoinsViewCache::BatchWrite return void

   CCoinsViewCache::BatchWrite always returns true if called from a backed
   cache, so just return void instead. Also return void from ::Sync and
   ::Flush.
   
   This allows for dropping a FatalError condition and simplifying some
   dead error handling code a bit.
   
   Since we now no longer exercise the "error path" when returning from
   `CCoinsView::BatchWrite`, make the method clear the cache instead. This
   should only be exercised by tests and not change production behaviour.
   This might slightly improve the coins_view fuzz test's ability to
   generate better coverage.
   
   Co-authored-by: l0rinc <pap.lorinc@gmail.com>

   b70921edec
3. refactor: reuse `should_empty` for chainstate flush condition bd0c5cf622
4. DrahtBot added the label Refactoring on Dec 20, 2025
5. 
   DrahtBot commented at 12:14 pm on December 20, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34125.

   Reviews

   See the guideline for information on the review process. A summary of reviews will appear here.

   Conflicts

   Reviewers, this pull request conflicts with the following ones:

   • #34164 (validation: add reusable coins view for ConnectBlock by andrewtoth)
   • #34124 (refactor: make CCoinsView a purely virtual abstract base class by l0rinc)
   • #33866 (refactor: Let CCoinsViewCache::BatchWrite return void by sedited)
   • #33680 (validation: do not wipe utxo cache for stats/scans/snapshots by l0rinc)
   • #33512 (coins: use number of dirty cache entries in flush warnings/logs by l0rinc)
   • #31132 (validation: fetch block inputs on parallel threads 3x faster IBD by andrewtoth)

   If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Contributors
l0rinc DrahtBot

Labels
Refactoring