Track net value entering and exiting the Sapling circuit

src/chain.h

@@ -17,6 +17,7 @@
 #include <boost/foreach.hpp>
 
 static const int SPROUT_VALUE_VERSION = 1001400;
+static const int SAPLING_VALUE_VERSION = 1010100;
 
 struct CDiskBlockPos
 {
@@ -166,6 +167,15 @@ public:
     //! Will be boost::none if nChainTx is zero.
     boost::optional<CAmount> nChainSproutValue;
 
+    //! Change in value held by the Sapling circuit over this block.
+    //! Not a boost::optional because this was added before Sapling activated, so we can
+    //! rely on the invariant that every block before this was added had nSaplingValue = 0.
+    CAmount nSaplingValue;
+
+    //! (memory only) Total value held by the Sapling circuit up to and including this block.
+    //! Will be boost::none if nChainTx is zero.
+    boost::optional<CAmount> nChainSaplingValue;
+
     //! block header
     int nVersion;
     uint256 hashMerkleRoot;
@@ -197,6 +207,8 @@ public:
         nSequenceId = 0;
         nSproutValue = boost::none;
         nChainSproutValue = boost::none;
+        nSaplingValue = 0;
+        nChainSaplingValue = boost::none;
 
         nVersion       = 0;
         hashMerkleRoot = uint256();
@@ -383,6 +395,12 @@ public:
         if ((s.GetType() & SER_DISK) && (nVersion >= SPROUT_VALUE_VERSION)) {
             READWRITE(nSproutValue);
         }
+
+        // Only read/write nSaplingValue if the client version used to create
+        // this index was storing them.
+        if ((s.GetType() & SER_DISK) && (nVersion >= SAPLING_VALUE_VERSION)) {
+            READWRITE(nSaplingValue);
+        }
     }
 
     uint256 GetBlockHash() const

src/main.cpp

@@ -3162,7 +3162,14 @@ bool ReceivedBlockTransactions(const CBlock &block, CValidationState& state, CBl
     pindexNew->nTx = block.vtx.size();
     pindexNew->nChainTx = 0;
     CAmount sproutValue = 0;
+    CAmount saplingValue = 0;
     for (auto tx : block.vtx) {
+        // Negative valueBalance "takes" money from the transparent value pool
+        // and adds it to the Sapling value pool. Positive valueBalance "gives"
+        // money to the transparent value pool, removing from the Sapling value
+        // pool. So we invert the sign here.
+        saplingValue += -tx.valueBalance;
+
         for (auto js : tx.vjoinsplit) {
             sproutValue += js.vpub_old;
             sproutValue -= js.vpub_new;
@@ -3170,6 +3177,8 @@ bool ReceivedBlockTransactions(const CBlock &block, CValidationState& state, CBl
     }
     pindexNew->nSproutValue = sproutValue;
     pindexNew->nChainSproutValue = boost::none;
+    pindexNew->nSaplingValue = saplingValue;
+    pindexNew->nChainSaplingValue = boost::none;
     pindexNew->nFile = pos.nFile;
     pindexNew->nDataPos = pos.nPos;
     pindexNew->nUndoPos = 0;
@@ -3193,8 +3202,14 @@ bool ReceivedBlockTransactions(const CBlock &block, CValidationState& state, CBl
                 } else {
                     pindex->nChainSproutValue = boost::none;
                 }
+                if (pindex->pprev->nChainSaplingValue) {
+                    pindex->nChainSaplingValue = *pindex->pprev->nChainSaplingValue + pindex->nSaplingValue;
+                } else {
+                    pindex->nChainSaplingValue = boost::none;
+                }
             } else {
                 pindex->nChainSproutValue = pindex->nSproutValue;
+                pindex->nChainSaplingValue = pindex->nSaplingValue;
             }
             {
                 LOCK(cs_nBlockSequenceId);
@@ -3877,14 +3892,21 @@ bool static LoadBlockIndexDB()
                     } else {
                         pindex->nChainSproutValue = boost::none;
                     }
+                    if (pindex->pprev->nChainSaplingValue) {
+                        pindex->nChainSaplingValue = *pindex->pprev->nChainSaplingValue + pindex->nSaplingValue;
+                    } else {
+                        pindex->nChainSaplingValue = boost::none;
+                    }
                 } else {
                     pindex->nChainTx = 0;
                     pindex->nChainSproutValue = boost::none;
+                    pindex->nChainSaplingValue = boost::none;
                     mapBlocksUnlinked.insert(std::make_pair(pindex->pprev, pindex));
                 }
             } else {
                 pindex->nChainTx = pindex->nTx;
                 pindex->nChainSproutValue = pindex->nSproutValue;
+                pindex->nChainSaplingValue = pindex->nSaplingValue;
             }
         }
         // Construct in-memory chain of branch IDs.

src/rpcblockchain.cpp

@@ -159,6 +159,7 @@ UniValue blockToJSON(const CBlock& block, const CBlockIndex* blockindex, bool tx
 
     UniValue valuePools(UniValue::VARR);
     valuePools.push_back(ValuePoolDesc("sprout", blockindex->nChainSproutValue, blockindex->nSproutValue));
+    valuePools.push_back(ValuePoolDesc("sapling", blockindex->nChainSaplingValue, blockindex->nSaplingValue));
     result.push_back(Pair("valuePools", valuePools));
 
     if (blockindex->pprev)
@@ -776,6 +777,7 @@ UniValue getblockchaininfo(const UniValue& params, bool fHelp)
     CBlockIndex* tip = chainActive.Tip();
     UniValue valuePools(UniValue::VARR);
     valuePools.push_back(ValuePoolDesc("sprout", tip->nChainSproutValue, boost::none));
+    valuePools.push_back(ValuePoolDesc("sapling", tip->nChainSaplingValue, boost::none));
     obj.push_back(Pair("valuePools",            valuePools));
 
     const Consensus::Params& consensusParams = Params().GetConsensus();