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

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@@ -3383,10 +3383,30 @@ bool CChainState::AcceptBlockHeader(const CBlockHeader& block, CValidationState&
if (!ContextualCheckBlockHeader(block, state, chainparams, pindexPrev, GetAdjustedTime()))
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
// has been found invalid (m_failed_blocks), then mark pindexPrev and any blocks
// between them as failed.
/* Determine if this block descends from any block which has been found
* invalid (m_failed_blocks), then mark pindexPrev and any blocks between
* 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).

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@jamesob

jamesob Aug 10, 2018

Member

Is this true because ConnectTip was never called on D3, meaning it was never added to m_failed_blocks by way of InvalidBlockFound? If that is the case, it may be worth making a more detailed note of that on this line; took me a few minutes of digging in the code to evaluate this claim.

*
* In any case D3 will also be marked as BLOCK_FAILED_CHILD at restart
* in LoadBlockIndex.
*/
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) {
if (pindexPrev->GetAncestor(failedit->nHeight) == failedit) {
assert(failedit->nStatus & BLOCK_FAILED_VALID);
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