indexes: set prune lock to genesis before first block #36150

1. 
   andrewtoth commented at 4:47 PM on September 2, 2026: contributor

   When setting both a new index and prune size and restarting an unpruned node, the node will prune the block store first and then the index will fail to start syncing.

   Fix this by setting the prune lock to 0 if the index does not yet have a best block.

2. indexes: set prune lock to genesis before first block

   When setting both a new index and prune size and restarting
   an unpruned node, the node will prune the block store first
   and then the index will fail to start syncing.
   
   Fix this by setting the prune lock to 0 if the index does not
   yet have a best block.

   39e4a6bb41
3. DrahtBot added the label UTXO Db and Indexes on Sep 2, 2026
4. 
   DrahtBot commented at 4:47 PM on September 2, 2026: contributor

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   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36150.

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   Concept ACK	l0rinc, mzumsande

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5. 
   l0rinc commented at 5:47 PM on September 2, 2026: contributor

   Concept ACK! Since #34897 the locator can lag the in-memory tip while SetBestBlockIndex() still advances the lock on every update, so should we maybe derive the lock from the committed locator instead (initialize in Init(), advance in Commit(), cap at the chain height on reorg, flush and drain before pruneblockchain) and cover it with a prune-mode index test?

6. 
   andrewtoth commented at 5:51 PM on September 2, 2026: contributor

   Since #34897 the locator can lag the in-memory tip while SetBestBlockIndex() still advances the lock on every update, so should we maybe derive the lock from the committed locator instead (initialize in Init(), advance in Commit(), cap at the chain height on reorg, flush and drain before pruneblockchain) and cover it with a prune-mode index test?

   That seems out of scope for what this is fixing. If you have an unpruned node, and then add both a new index and a prune value to the config and restart, it will break on indexing because it will prune first before syncing the index. This is not about a lagging locator, but startup failing to lock the prune height for an empty index.

7. 
   mzumsande commented at 3:47 PM on September 3, 2026: contributor

   Concept ACK

8. sedited approved
9. 
   sedited commented at 7:05 PM on September 3, 2026: contributor

   ACK 39e4a6bb41749547c983456ce05281ca9eae16fe

10. DrahtBot requested review from mzumsande on Sep 3, 2026
11. DrahtBot requested review from l0rinc on Sep 3, 2026

Review Requested
l0rincmzumsande