Merge #13185: Bugfix: the end of a reorged chain is invalid when connect fails

a2f678d Bugfix: the end of a reorged chain is invalid when connect fails (Pieter Wuille)

Pull request description:

  Introduced in 4e0eed8

  When an invalid block is found during a reorg, we know the last of the blocks in the was-to-be-connected chain is invalid, but not necessarily the first. As `vpIndexToConnect` is ordered in decreasing height, the end of the reorg is the front of the vector, and not the back.

  This only affected the warning system.

Tree-SHA512: ddf749f8a78083811a5a17152723f545c1463768d09dc9832ec3682e803a3c106fb768de9fa91c03aa95e644d4e41361a7e4ee791940fd7d51cdefea90de31fc

Author: laanwj

src/validation.cpp

@@ -2551,8 +2551,9 @@ bool CChainState::ActivateBestChainStep(CValidationState& state, const CChainPar
             if (!ConnectTip(state, chainparams, pindexConnect, pindexConnect == pindexMostWork ? pblock : std::shared_ptr<const CBlock>(), connectTrace, disconnectpool)) {
                 if (state.IsInvalid()) {
                     // The block violates a consensus rule.
-                    if (!state.CorruptionPossible())
-                        InvalidChainFound(vpindexToConnect.back());
+                    if (!state.CorruptionPossible()) {
+                        InvalidChainFound(vpindexToConnect.front());
+                    }
                     state = CValidationState();
                     fInvalidFound = true;
                     fContinue = false;