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0465574c127907df9b764055a585e8281bae8d1d test: Fixes send_blocks_and_test docs (Sergi Delgado Segura) 09c95f21e71d196120e6c9d0b1d1923a4927408d test: Adds block tiebreak over restarts tests (Sergi Delgado Segura) 18524b072e6bdd590a9f6badd15d897b5ef5ce54 Make nSequenceId init value constants (Sergi Delgado Segura) 8b91883a23aac64a37d929eeae81325e221d177d Set the same best tip on restart if two candidates have the same work (Sergi Delgado Segura) 5370bed21e0b04feca6ec09738ecbe792095a338 test: add functional test for complex reorgs (Pieter Wuille) ab145cb3b471d07a2e8ee79edde46ec67f47d580 Updates CBlockIndexWorkComparator outdated comment (Sergi Delgado Segura) Pull request description: This PR grabs some interesting bits from https://github.com/bitcoin/bitcoin/pull/29284 and fixes some edge cases in how block tiebreaks are dealt with. ## Regarding #29284 The main functionality from the PR was dropped given it was not an issue anymore, however, reviewers pointed out some comments were outdated https://github.com/bitcoin/bitcoin/pull/29284#discussion_r1522023578 (which to my understanding may have led to thinking that there was still an issue) it also added test coverage for the aforementioned case which was already passing on master and is useful to keep. ## New functionality While reviewing the superseded PR, it was noticed that blocks that are loaded from disk may face a similar issue (check https://github.com/bitcoin/bitcoin/pull/29284#issuecomment-1994317785 for more context). The issue comes from how tiebreaks for equal work blocks are handled: if two blocks have the same amount of work, the one that is activatable first wins, that is, the one for which we have all its data (and all of its ancestors'). The variable that keeps track of this, within `CBlockIndex` is `nSequenceId`, which is not persisted over restarts. This means that when a node is restarted, all blocks loaded from disk are defaulted the same `nSequenceId`: 0. Now, when trying to decide what chain is best on loading blocks from disk, the previous tiebreaker rule is not decisive anymore, so the `CBlockIndexWorkComparator` has to default to its last rule: whatever block is loaded first (has a smaller memory address). This means that if multiple same work tip candidates were available before restarting the node, it could be the case that the selected chain tip after restarting does not match the one before. Therefore, the way `nSequenceId` is initialized is changed to: - 0 for blocks that belong to the previously known best chain - 1 to all other blocks loaded from disk ACKs for top commit: sipa: utACK 0465574c127907df9b764055a585e8281bae8d1d TheCharlatan: ACK 0465574c127907df9b764055a585e8281bae8d1d furszy: Tested ACK 0465574c127907df9b764055a585e8281bae8d1d. Tree-SHA512: 161da814da03ce10c34d27d79a315460a9c98d019b85ee35bc5daa991ed3b6a2e69a829e421fc70d093a83cf7a2e403763041e594df39ed1991445e54c16532a