Issue Details

Number

31512

Title

validation: CheckBlockIndex can fail in combination with pruning

Description

Found this situation while fuzzing validation / `CheckBlockIndex()`: 1) Full block C (child) is received, so `nTx`, but not `m_chain_tx_count` will be set because we only know the header of the previous block P (parent). 2) Block C gets pruned 3) We request / receive full data for parent P. 4) Error in `CheckBlockIndex()` - multiple checks in `CheckBlockIndex()` fail, for example we expect `m_chain_tx_count` to be set for C. There is a simple functional test to trigger this in https://github.com/mzumsande/bitcoin/commit/b75577544c8f9e7e2b90d269de2a6d9bec438433. This is mostly a theoretical issue: - we don't usually request parents of already pruned blocks like in step 3, unless we do `getblockfrompeer` or similar - so it can't be triggered remotely - also, situations where we have full data for a child block but no data for the parent tend to not persist for long. - `CheckBlockIndex` is not active by default on mainnet However I think it's good to understand the possible constellations in validation in depth and formulate our expectations (i.e. `CheckBlockIndex`) accordingly.

URL

https://github.com/bitcoin/bitcoin/issue/31512

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Kill Factor (1-10)

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