[Cleanup] Deprecate SPORK 16 (zerocoin maintenance)

SPORK_16 value is still checked in:
- AcceptToMemoryPool (but there's a static check at lines 418-421)
- ConnectBlock (but there's a height-based check at lines 1562-1564, and
an additional height-based check inside CheckBlock)

Author: random-zebra

src/net_processing.cpp

@@ -1229,7 +1229,6 @@ bool static ProcessMessage(CNode* pfrom, std::string strCommand, CDataStream& vR
         // TODO: Move this to an instant broadcast of the sporks.
         bool fMissingSporks = !pSporkDB->SporkExists(SPORK_14_NEW_PROTOCOL_ENFORCEMENT) ||
                               !pSporkDB->SporkExists(SPORK_15_NEW_PROTOCOL_ENFORCEMENT_2) ||
-                              !pSporkDB->SporkExists(SPORK_16_ZEROCOIN_MAINTENANCE_MODE) ||
                               !pSporkDB->SporkExists(SPORK_19_COLDSTAKING_MAINTENANCE) ||
                               !pSporkDB->SporkExists(SPORK_20_SAPLING_MAINTENANCE);
 

src/spork.cpp

@@ -23,7 +23,6 @@ std::vector<CSporkDef> sporkDefs = {
     MAKE_SPORK_DEF(SPORK_13_ENABLE_SUPERBLOCKS,             4070908800ULL), // OFF
     MAKE_SPORK_DEF(SPORK_14_NEW_PROTOCOL_ENFORCEMENT,       4070908800ULL), // OFF
     MAKE_SPORK_DEF(SPORK_15_NEW_PROTOCOL_ENFORCEMENT_2,     4070908800ULL), // OFF
-    MAKE_SPORK_DEF(SPORK_16_ZEROCOIN_MAINTENANCE_MODE,      4070908800ULL), // OFF
     MAKE_SPORK_DEF(SPORK_19_COLDSTAKING_MAINTENANCE,        4070908800ULL), // OFF
     MAKE_SPORK_DEF(SPORK_20_SAPLING_MAINTENANCE,            4070908800ULL), // OFF
     MAKE_SPORK_DEF(SPORK_21_LEGACY_MNS_MAX_HEIGHT,          4070908800ULL), // OFF

src/sporkid.h

@@ -20,7 +20,7 @@ enum SporkId : int32_t {
     SPORK_13_ENABLE_SUPERBLOCKS                 = 10012,
     SPORK_14_NEW_PROTOCOL_ENFORCEMENT           = 10013,
     SPORK_15_NEW_PROTOCOL_ENFORCEMENT_2         = 10014,
-    SPORK_16_ZEROCOIN_MAINTENANCE_MODE          = 10015,
+    SPORK_16_ZEROCOIN_MAINTENANCE_MODE          = 10015,       // Deprecated in 5.2.99
     SPORK_17_COLDSTAKING_ENFORCEMENT            = 10017,       // Deprecated in 4.3.99
     SPORK_18_ZEROCOIN_PUBLICSPEND_V4            = 10018,       // Deprecated in 5.2.99
     SPORK_19_COLDSTAKING_MAINTENANCE            = 10019,

src/validation.cpp

@@ -402,10 +402,6 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
         *pfMissingInputs = false;
 
     // Check maintenance mode
-    bool hasTxZerocoins = tx.ContainsZerocoins();
-    if (sporkManager.IsSporkActive(SPORK_16_ZEROCOIN_MAINTENANCE_MODE) && hasTxZerocoins)
-        return state.DoS(10, error("%s : Zerocoin transactions are temporarily disabled for maintenance",
-                __func__), REJECT_INVALID, "bad-tx-zerocoin-maintenance");
     if (sporkManager.IsSporkActive(SPORK_20_SAPLING_MAINTENANCE) && tx.IsShieldedTx())
         return state.DoS(10, error("%s : Shielded transactions are temporarily disabled for maintenance",
                 __func__), REJECT_INVALID, "bad-tx-sapling-maintenance");
@@ -415,7 +411,7 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
     int chainHeight = chainActive.Height();
 
     // Zerocoin txes are not longer accepted in the mempool.
-    if (hasTxZerocoins) {
+    if (tx.ContainsZerocoins()) {
         return state.DoS(100, error("%s : v5 upgrade enforced, zerocoin disabled", __func__),
                          REJECT_INVALID, "bad-tx-with-zc");
     }
@@ -1539,7 +1535,6 @@ static bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockInd
     std::vector<PrecomputedTransactionData> precomTxData;
     precomTxData.reserve(block.vtx.size()); // Required so that pointers to individual precomTxData don't get invalidated
     bool fInitialBlockDownload = IsInitialBlockDownload();
-    bool fZerocoinMaintenance =  (block.nTime > sporkManager.GetSporkValue(SPORK_16_ZEROCOIN_MAINTENANCE_MODE));
     bool fSaplingMaintenance =  (block.nTime > sporkManager.GetSporkValue(SPORK_20_SAPLING_MAINTENANCE));
     for (unsigned int i = 0; i < block.vtx.size(); i++) {
         const CTransaction& tx = *block.vtx[i];
@@ -1550,11 +1545,8 @@ static bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockInd
             return state.DoS(100, error("ConnectBlock() : too many sigops"), REJECT_INVALID, "bad-blk-sigops");
 
         // Check maintenance mode
-        if (!fInitialBlockDownload) {
-            if (fZerocoinMaintenance && tx.ContainsZerocoins())
-                return state.DoS(100, error("%s : zerocoin transactions are currently in maintenance mode", __func__));
-            if (fSaplingMaintenance && tx.IsShieldedTx())
-                return state.DoS(100, error("%s : shielded transactions are currently in maintenance mode", __func__));
+        if (!fInitialBlockDownload && fSaplingMaintenance && tx.IsShieldedTx()) {
+            return state.DoS(100, error("%s : shielded transactions are currently in maintenance mode", __func__));
         }
 
         // If v5 is active, bye bye zerocoin