validation: do not add the snapshot to candidates set of the background chainstate #34786

1. 
   mzumsande commented at 11:53 am on March 10, 2026: contributor

   The snapshot block needs to be added to the candidates set of the assumed-valid chain because it will be the tip of that chainstate right after snapshot activation.

   However, adding it also to the background chainstate is not necessary for anything. Before, the index would be in the set without being connectable. It will be eventually added to the set as part of the normal block download - no extra logic is necessary here.

   This simplifies a unit test which had a comment that having the block in the set is “not intended”. This was suggested here and here in #34521

   Note that adding the snapshot block was harmless, since FindMostWorkChain() lazily removes blocks without data from the set, so this does not fix a bug but just simplifies some code.

2. validation: do not add the snapshot block to candidates of bg chainstate

   The snapshot block needs to be added to the candidates set of the
   assumed-valid chain because it will be the tip of that chainstate
   right after snapshot activation.
   However, adding it also to the background chainstate is not necessary
   for anything. Before, the index would linger in the set without being
   connectable. It will be eventually added to the set as part of the
   normal block download - no extra logic is necessary here.

   69baddc910
3. DrahtBot added the label Validation on Mar 10, 2026
4. 
   DrahtBot commented at 11:54 am on March 10, 2026: contributor

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   ACK	sedited, fjahr, stratospher, achow101
   Concept ACK	Bortlesboat

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5. 
   sedited commented at 4:55 pm on March 10, 2026: contributor

   Concept ACK, thanks for the follow up.
6. 
   Bortlesboat commented at 4:09 am on March 15, 2026: none

   Concept ACK 69baddc91033d2e42947260cde0f6780959f16b2. Removing TargetBlock() correctly limits the snapshot-block special-case to cs2 where it’s actually needed — the test’s own “not intended” comment was the tell.
7. sedited added this to the milestone 32.0 on Mar 20, 2026
8. sedited approved
9. 
   sedited commented at 8:33 am on March 24, 2026: contributor

   ACK 69baddc91033d2e42947260cde0f6780959f16b2
10. DrahtBot requested review from Bortlesboat on Mar 24, 2026
11. sedited removed review request from Bortlesboat on Mar 24, 2026
12. sedited requested review from fjahr on Mar 24, 2026
13. in src/validation.cpp:4924 in 69baddc910

    4920@@ -4921,7 +4921,7 @@ void Chainstate::PopulateBlockIndexCandidates()
    4921         // With assumeutxo, the snapshot block is a candidate for the tip, but it
    4922         // may not have BLOCK_VALID_TRANSACTIONS (e.g. if we haven't yet downloaded
    4923         // the block), so we special-case it here.
    4924-        if (pindex == SnapshotBase() || pindex == TargetBlock() ||
    4925+        if (pindex == SnapshotBase() ||
    

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    fjahr commented at 9:28 am on March 24, 2026:

    nit: Maybe biased because I just reviewed something else where this confused people but there could be a comment like this here added too: “Note that this only adds the tip to the snapshot chainstate and not the historical chainstate because SnapshotBase() is nullptr for the historical chainstate.” But it’s not a blocker because the docs of SnapshotBase() make it clear already.
14. 
    fjahr commented at 9:31 am on March 24, 2026: contributor

    ACK 69baddc91033d2e42947260cde0f6780959f16b2
15. DrahtBot requested review from Bortlesboat on Mar 24, 2026
16. 
    stratospher commented at 1:56 pm on March 24, 2026: contributor

    ACK 69baddc.
17. 
    achow101 commented at 9:51 pm on March 24, 2026: member

    ACK 69baddc91033d2e42947260cde0f6780959f16b2
18. achow101 merged this on Mar 24, 2026
19. achow101 closed this on Mar 24, 2026

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Contributors
mzumsande DrahtBot sedited Bortlesboat fjahr stratospher achow101

Review Requested
Bortlesboat

Labels
Validation

Milestone
32.0