fix: off-by-one in the way we use v19 activation helpers (#5431)

Some conditions won't trigger when reorging exactly from the forkpoint

pls see individual commits, tl;dr: you can't get correct results with
`GetAncestor` cause the answer is in the future

reorg to 850000 and back on testnet
```
invalidateblock 0000003eddb94218e7a3f41b2ac6e26143f8a748b50cd26e86bdbbab9ebe50aa
reconsiderblock 0000003eddb94218e7a3f41b2ac6e26143f8a748b50cd26e86bdbbab9ebe50aa
```
this fails on develop and work with this patch

n/a

- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

src/evo/deterministicmns.cpp

@@ -643,13 +643,8 @@ bool CDeterministicMNManager::ProcessBlock(const CBlock& block, const CBlockInde
         oldList = GetListForBlock(pindex->pprev);
         diff = oldList.BuildDiff(newList);
 
-        // NOTE: The block next to the activation is the one that is using new rules.
-        // If the fork was activated at pindex->prev block then current one is the first one
-        // using new rules. Save mn list snapsot for this block.
-        bool v19_just_activated = pindex->pprev == llmq::utils::V19ActivationIndex(pindex);
-
         m_evoDb.Write(std::make_pair(DB_LIST_DIFF, newList.GetBlockHash()), diff);
-        if ((nHeight % DISK_SNAPSHOT_PERIOD) == 0 || oldList.GetHeight() == -1 || v19_just_activated) {
+        if ((nHeight % DISK_SNAPSHOT_PERIOD) == 0 || oldList.GetHeight() == -1) {
             m_evoDb.Write(std::make_pair(DB_LIST_SNAPSHOT, newList.GetBlockHash()), newList);
             mnListsCache.emplace(newList.GetBlockHash(), newList);
             LogPrintf("CDeterministicMNManager::%s -- Wrote snapshot. nHeight=%d, mapCurMNs.allMNsCount=%d\n",

src/evo/specialtxman.cpp

@@ -154,7 +154,7 @@ bool ProcessSpecialTxsInBlock(const CBlock& block, const CBlockIndex* pindex, ll
         nTimeMerkle += nTime5 - nTime4;
         LogPrint(BCLog::BENCHMARK, "        - CheckCbTxMerkleRoots: %.2fms [%.2fs]\n", 0.001 * (nTime5 - nTime4), nTimeMerkle * 0.000001);
 
-        if (llmq::utils::V19ActivationIndex(pindex) == pindex) {
+        if (llmq::utils::V19ActivationHeight(pindex) == pindex->nHeight + 1) {
             // NOTE: The block next to the activation is the one that is using new rules.
             // V19 activated just activated, so we must switch to the new rules here.
             bls::bls_legacy_scheme.store(false);
@@ -175,7 +175,7 @@ bool UndoSpecialTxsInBlock(const CBlock& block, const CBlockIndex* pindex, llmq:
     auto bls_legacy_scheme = bls::bls_legacy_scheme.load();
 
     try {
-        if (llmq::utils::V19ActivationIndex(pindex) == pindex) {
+        if (llmq::utils::V19ActivationHeight(pindex) == pindex->nHeight + 1) {
             // NOTE: The block next to the activation is the one that is using new rules.
             // Removing the activation block here, so we must switch back to the old rules.
             bls::bls_legacy_scheme.store(true);

src/governance/object.cpp

@@ -316,7 +316,7 @@ bool CGovernanceObject::CheckSignature(const CBLSPublicKey& pubKey) const
 {
     CBLSSignature sig;
     const auto pindex = llmq::utils::V19ActivationIndex(::ChainActive().Tip());
-    bool is_bls_legacy_scheme = pindex == nullptr || nTime < pindex->nTime;
+    bool is_bls_legacy_scheme = pindex == nullptr || nTime < pindex->pprev->nTime;
     sig.SetByteVector(vchSig, is_bls_legacy_scheme);
     if (!sig.VerifyInsecure(pubKey, GetSignatureHash(), is_bls_legacy_scheme)) {
         LogPrintf("CGovernanceObject::CheckSignature -- VerifyInsecure() failed\n");

src/governance/vote.cpp

@@ -238,7 +238,7 @@ bool CGovernanceVote::CheckSignature(const CBLSPublicKey& pubKey) const
 {
     CBLSSignature sig;
     const auto pindex = llmq::utils::V19ActivationIndex(::ChainActive().Tip());
-    bool is_bls_legacy_scheme = pindex == nullptr || nTime < pindex->nTime;
+    bool is_bls_legacy_scheme = pindex == nullptr || nTime < pindex->pprev->nTime;
     sig.SetByteVector(vchSig, is_bls_legacy_scheme);
     if (!sig.VerifyInsecure(pubKey, GetSignatureHash(), is_bls_legacy_scheme)) {
         LogPrintf("CGovernanceVote::CheckSignature -- VerifyInsecure() failed\n");

src/llmq/utils.cpp

@@ -659,16 +659,21 @@ bool IsV19Active(const CBlockIndex* pindex)
     return VersionBitsState(pindex, Params().GetConsensus(), Consensus::DEPLOYMENT_V19, llmq_versionbitscache) == ThresholdState::ACTIVE;
 }
 
-const CBlockIndex* V19ActivationIndex(const CBlockIndex* pindex)
+const int V19ActivationHeight(const CBlockIndex* pindex)
 {
     assert(pindex);
 
    LOCK(cs_llmq_vbc);
    if (VersionBitsState(pindex, Params().GetConsensus(), Consensus::DEPLOYMENT_V19, llmq_versionbitscache) != ThresholdState::ACTIVE) {
-        return nullptr;
+       return -1;
    }
-    int nHeight = VersionBitsStateSinceHeight(pindex, Params().GetConsensus(), Consensus::DEPLOYMENT_V19, llmq_versionbitscache);
+   return VersionBitsStateSinceHeight(pindex, Params().GetConsensus(), Consensus::DEPLOYMENT_V19, llmq_versionbitscache);
-    return pindex->GetAncestor(nHeight);
+}
+
+const CBlockIndex* V19ActivationIndex(const CBlockIndex* pindex)
+{
+    assert(pindex);
+    return pindex->GetAncestor(V19ActivationHeight(pindex));
 }
 
 bool IsInstantSendLLMQTypeShared()

src/llmq/utils.h

@@ -84,6 +84,7 @@ Consensus::LLMQType GetInstantSendLLMQType(const CQuorumManager& qman, const CBl
 Consensus::LLMQType GetInstantSendLLMQType(bool deterministic);
 bool IsDIP0024Active(const CBlockIndex* pindex);
 bool IsV19Active(const CBlockIndex* pindex);
+const int V19ActivationHeight(const CBlockIndex* pindex);
 const CBlockIndex* V19ActivationIndex(const CBlockIndex* pindex);
 
 /// Returns the state of `-llmq-data-recovery`