evo: prohibit any ProTx with legacy BLS version after basic BLS upgrade

Even though only ProUpRegTx updates the operator key field, it is not
good policy to allow transactions that don't use the version that
defines the masternode's currently active key scheme or higher.

Author: kwvg

src/evo/deterministicmns.cpp

@@ -1292,6 +1292,30 @@ static std::optional<ProTx> GetValidatedPayload(const CTransaction& tx, gsl::not
     return opt_ptx;
 }
 
+/**
+ * Validates potential changes to masternode state version by ProTx transaction version
+ * @param[in]  pindexPrev    Previous block index to validate DEPLOYMENT_V23 activation
+ * @param[in]  state_version Current masternode state version
+ * @param[in]  tx_version    Proposed transaction version
+ * @param[out] state         This may be set to an Error state if any error occurred processing them
+ * @returns                  true if version change is valid or DEPLOYMENT_V23 is not active
+ */
+bool IsVersionChangeValid(gsl::not_null<const CBlockIndex*> pindexPrev, const uint16_t state_version, const uint16_t tx_version,
+                          TxValidationState& state)
+{
+    if (!DeploymentActiveAfter(pindexPrev, Params().GetConsensus(), Consensus::DEPLOYMENT_V23)) {
+        // New restrictions only apply after v23 deployment
+        return true;
+    }
+
+    if (state_version >= ProTxVersion::BasicBLS && tx_version == ProTxVersion::LegacyBLS) {
+        // Don't allow legacy scheme versioned transactions after upgrading to basic scheme
+        return state.Invalid(TxValidationResult::TX_CONSENSUS, "bad-protx-version-downgrade");
+    }
+
+    return true;
+}
+
 bool CheckProRegTx(CDeterministicMNManager& dmnman, const CTransaction& tx, gsl::not_null<const CBlockIndex*> pindexPrev, TxValidationState& state, const CCoinsViewCache& view, bool check_sigs)
 {
     const auto opt_ptx = GetValidatedPayload<CProRegTx>(tx, pindexPrev, state);
@@ -1439,6 +1463,11 @@ bool CheckProUpServTx(CDeterministicMNManager& dmnman, const CTransaction& tx, g
         return state.Invalid(TxValidationResult::TX_CONSENSUS, "bad-protx-hash");
     }
 
+    if (!IsVersionChangeValid(pindexPrev, dmn->pdmnState->nVersion, opt_ptx->nVersion, state)) {
+        // pass the state returned by the function above
+        return false;
+    }
+
     // don't allow updating to addresses already used by other MNs
     for (const NetInfoEntry& entry : opt_ptx->netInfo->GetEntries()) {
         if (const auto& service_opt{entry.GetAddrPort()}; service_opt.has_value()) {
@@ -1501,6 +1530,11 @@ bool CheckProUpRegTx(CDeterministicMNManager& dmnman, const CTransaction& tx, gs
         return state.Invalid(TxValidationResult::TX_CONSENSUS, "bad-protx-hash");
     }
 
+    if (!IsVersionChangeValid(pindexPrev, dmn->pdmnState->nVersion, opt_ptx->nVersion, state)) {
+        // pass the state returned by the function above
+        return false;
+    }
+
     // don't allow reuse of payee key for other keys (don't allow people to put the payee key onto an online server)
     if (payoutDest == CTxDestination(PKHash(dmn->pdmnState->keyIDOwner)) || payoutDest == CTxDestination(PKHash(opt_ptx->keyIDVoting))) {
         return state.Invalid(TxValidationResult::TX_BAD_SPECIAL, "bad-protx-payee-reuse");
@@ -1560,6 +1594,11 @@ bool CheckProUpRevTx(CDeterministicMNManager& dmnman, const CTransaction& tx, gs
         return state.Invalid(TxValidationResult::TX_CONSENSUS, "bad-protx-hash");
     }
 
+    if (!IsVersionChangeValid(pindexPrev, dmn->pdmnState->nVersion, opt_ptx->nVersion, state)) {
+        // pass the state returned by the function above
+        return false;
+    }
+
     if (!CheckInputsHash(tx, *opt_ptx, state)) {
         // pass the state returned by the function above
         return false;