Coins Cache Cleanup checklist #34280

The coins cache (the CCoinsView* stack, CCoinsViewCache, and friends) is a correctness- and performance-critical part of validation, IBD, and assumeutxo. Over time it has accumulated subtle invariants and legacy states (around cache entry flags, spent coin handling, and error catching), and we have been chipping away at it through focused refactors, bugfixes, tests, and performance work.

This issue tracks the related PRs to help reviewers see how they fit together. The overall aim is fewer error cases, cleaner cache layering, and parallel validation.

PRs needing review

Testing

• #34866 - fuzz: target concurrent leveldb reads

Remove invalid cache states

• #34864 - remove unreachable FRESH-but-not-DIRTY state from code and tests.

Simplify view hierarchy

• #35180
• #35078
• #34320 - replace HaveCoin() with GetCoin().
• #34132 - coins: drop error catcher, centralize fatal read handling.

Parallel coin fetching

• #31132 - async input prefetch during ConnectBlock.

Other

• #34641 - node: scale default -dbcache with system RAM
• #33854 - fix assumevalid ignored during reindex (see #31494).
• #32317 - move UTXO access to SpendBlock (kernel API, Utreexo, SwiftSync).

Recently merged

• #31449 - getblockstats RPC UTXO overhead fix
• #34124 - make CCoinsView purely virtual, add explicit CCoinsViewEmpty.
• #34692 - increase the default dbcache value from 450MiB to 1024MiB
• #34576 - threadpool: add ranged Submit overload
• #34655 - fuzz: keep coins_view fuzzers within caller contracts
• #34165 - add non-mutating PeekCoin() to avoid polluting the main cache.
• #33512 - track dirty entries so flush progress reflects actual writes.
• #33680 - split FORCE_SYNC/FORCE_FLUSH, keep cache warm for scantxoutset/gettxoutsetinfo/snapshots.
• #34164 - add Reset() and reusable view to discard failed connects cheaply.
• #34207 - ensure GetCoin() only returns unspent coins, align tests with production.
• #34253 - validation: cache tip recency for lock-free IsInitialBlockDownload()
• #33866 - remove always-true return from BatchWrite.
• #33657 - allow reading partial block data (useful for bindex).
• #33602 - reduce map lookups in BatchWrite.
• #33333 - warn when -dbcache exceeds RAM cap.

IRC context: https://bitcoin-irc.chaincode.com/bitcoin-core-dev/2026-01-08

The issue seems a bit overladen - I don't think listing all these PRs is useful. Maybe distill this down to just the coins-related PRs? The benefit I would expect from this issue is that I can connect the various PRs touching the coins and see how they relate to each other. Maybe to add to the benefits in the opening paragraph a bit: Fewer error cases, more ergonomic cache layering, and preparation for asynchronous operations.

Thanks for the suggestions @sedited and @andrewtoth. Updated the tracker and grouped the PRs - hopefully this helps reviewers see the structure (though many conflict or overlap depending on merge order). I put cleanups first since they'd let us avoid ugly hacks in the larger PRs, culminating in the parallel input fetcher. Let me know what else to adjust.

GitHub doesn't allow me to edit the issue anymore, wanted to try the close/reopen dance here, but apparently I can't reopen. Can someone please reopen and check if they can edit the issue?