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Merge #13930: doc: Better explain GetAncestor check for m_failed_blocks in AcceptBlockHeader

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66e15e8f97 Explain GetAncestor check for m_failed_blocks in AcceptBlockHeader (Sjors Provoost)

Pull request description:

  Salvaged (but slightly modified) from #12138, the comment there was really helpful to wrap my head around that part of the code.

  In addition, a naive reader like yours truly will first think `IsValid(BLOCK_VALID_SCRIPTS)` means the previous block was invalid. But IIUC that's not what  it means. Instead, it means the block hasn't been checked for validity at the `BLOCK_VALID_SCRIPTS` level yet. So in that case the existing text "previous block index isn't valid" is wrong.

Tree-SHA512: 442a319a83290d94697fdf51376463b70454e0f3909d4a45594ddc2e7c26cd19dc703808385a25e26d6d2dddab0aa35ca41722f2e65ee6fe57bbaf62652d3ec8
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MarcoFalke 2018-12-22 21:40:02 +01:00
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1 changed files with 23 additions and 3 deletions
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26
src/validation.cpp
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@ -3374,10 +3374,30 @@ bool CChainState::AcceptBlockHeader(const CBlockHeader& block, CValidationState&
if (!ContextualCheckBlockHeader(block, state, chainparams, pindexPrev, GetAdjustedTime())) if (!ContextualCheckBlockHeader(block, state, chainparams, pindexPrev, GetAdjustedTime()))
return error("%s: Consensus::ContextualCheckBlockHeader: %s, %s", __func__, hash.ToString(), FormatStateMessage(state)); return error("%s: Consensus::ContextualCheckBlockHeader: %s, %s", __func__, hash.ToString(), FormatStateMessage(state));
// If the previous block index isn't valid, determine if it descends from any block which /* Determine if this block descends from any block which has been found
// has been found invalid (m_failed_blocks), then mark pindexPrev and any blocks * invalid (m_failed_blocks), then mark pindexPrev and any blocks between
// between them as failed. * them as failed. For example:
*
* D3
* /
* B2 - C2
* / \
* A D2 - E2 - F2
* \
* B1 - C1 - D1 - E1
*
* In the case that we attempted to reorg from E1 to F2, only to find
* C2 to be invalid, we would mark D2, E2, and F2 as BLOCK_FAILED_CHILD
* but NOT D3 (it was not in any of our candidate sets at the time).
*
* In any case D3 will also be marked as BLOCK_FAILED_CHILD at restart
* in LoadBlockIndex.
*/
if (!pindexPrev->IsValid(BLOCK_VALID_SCRIPTS)) { if (!pindexPrev->IsValid(BLOCK_VALID_SCRIPTS)) {
// The above does not mean "invalid": it checks if the previous block
// hasn't been validated up to BLOCK_VALID_SCRIPTS. This is a performance
// optimization, in the common case of adding a new block to the tip,
// we don't need to iterate over the failed blocks list.
for (const CBlockIndex* failedit : m_failed_blocks) { for (const CBlockIndex* failedit : m_failed_blocks) {
if (pindexPrev->GetAncestor(failedit->nHeight) == failedit) { if (pindexPrev->GetAncestor(failedit->nHeight) == failedit) {
assert(failedit->nStatus & BLOCK_FAILED_VALID); assert(failedit->nStatus & BLOCK_FAILED_VALID);