fuzz: Implement `connect_block` harness #35850

1. 
   marcofleon commented at 4:28 PM on July 30, 2026: contributor

   Adds a fuzz target that directly calls ConnectBlock with fJustCheck set to true, so it hits block/transaction validation without writing undo data or updating the chainstate.

   This PR is essentially #34651 with some minor tweaks and style cleanups. Additional validation harnesses (e.g. #34895) could build on this test's setup.

2. DrahtBot added the label Fuzzing on Jul 30, 2026
3. 
   DrahtBot commented at 4:28 PM on July 30, 2026: contributor

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

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   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35850.

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   Reviews

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   Type	Reviewers
   ACK	dergoegge
   Concept ACK	Crypt-iQ

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   Conflicts

   Reviewers, this pull request conflicts with the following ones:

   • #34895 (fuzz: Fuzzing harnesses for ActivateBestChainStep and ActivateBestChain by RobinDavid)

   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.

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4. 
   marcofleon commented at 4:30 PM on July 30, 2026: contributor

   Coverage: https://marcofleon.github.io/coverage/connect_block/

5. fuzz: Implement connect_block harness

   Co-authored-by: marcofleon <marleo23@proton.me>

   020bede7c2
6. marcofleon force-pushed on Jul 30, 2026
7. DrahtBot added the label CI failed on Jul 30, 2026
8. DrahtBot removed the label CI failed on Jul 30, 2026
9. dergoegge approved
10. 
    dergoegge commented at 10:58 AM on August 3, 2026: member

    utACK 020bede7c2d3e7a764ed51344925fcfe2300b3cf

    A follow up could be to investigate if we can make this harness find https://bitcoincore.org/en/2026/05/05/disclose-cve-2024-52911/ (perhaps the only thing missing here is to actually use the multi-threaded checkqueue).

11. sedited requested review from ismaelsadeeq on Aug 3, 2026
12. 
    Crypt-iQ commented at 6:29 PM on August 3, 2026: contributor

    Concept ACK, will review

Review Requested
ismaelsadeeq