[wallet] Add GetLegacyBalance method to simplify getbalance RPC

This adds a simpler new implementation of getbalance logic along with asserts
to confirm it behaves identically to the old logic. The old logic is removed in
the next commit.

src/wallet/rpcwallet.cpp

@@ -729,6 +729,8 @@ UniValue getbalance(const JSONRPCRequest& request)
     if (request.params.size() == 0)
         return  ValueFromAmount(pwallet->GetBalance());
 
+    const std::string* account = request.params[0].get_str() != "*" ? &request.params[0].get_str() : nullptr;
+
     int nMinDepth = 1;
     if (request.params.size() > 1)
         nMinDepth = request.params[1].get_int();
@@ -737,6 +739,8 @@ UniValue getbalance(const JSONRPCRequest& request)
         if(request.params[2].get_bool())
             filter = filter | ISMINE_WATCH_ONLY;
 
+    CAmount legacyBalance = pwallet->GetLegacyBalance(filter, nMinDepth, account);
+
     if (request.params[0].get_str() == "*") {
         // Calculate total balance in a very different way from GetBalance().
         // The biggest difference is that GetBalance() sums up all unspent
@@ -764,6 +768,7 @@ UniValue getbalance(const JSONRPCRequest& request)
                 nBalance -= s.amount;
             nBalance -= allFee;
         }
+        assert(nBalance == legacyBalance);
         return  ValueFromAmount(nBalance);
     }
 
@@ -771,6 +776,7 @@ UniValue getbalance(const JSONRPCRequest& request)
 
     CAmount nBalance = pwallet->GetAccountBalance(strAccount, nMinDepth, filter);
 
+    assert(nBalance == legacyBalance);
     return ValueFromAmount(nBalance);
 }
 
@@ -902,6 +908,8 @@ UniValue sendfrom(const JSONRPCRequest& request)
 
     // Check funds
     CAmount nBalance = pwallet->GetAccountBalance(strAccount, nMinDepth, ISMINE_SPENDABLE);
+    CAmount legacyBalance = pwallet->GetLegacyBalance(ISMINE_SPENDABLE, nMinDepth, &strAccount);
+    assert(nBalance == legacyBalance);
     if (nAmount > nBalance)
         throw JSONRPCError(RPC_WALLET_INSUFFICIENT_FUNDS, "Account has insufficient funds");
 
@@ -1011,6 +1019,8 @@ UniValue sendmany(const JSONRPCRequest& request)
 
     // Check funds
     CAmount nBalance = pwallet->GetAccountBalance(strAccount, nMinDepth, ISMINE_SPENDABLE);
+    CAmount legacyBalance = pwallet->GetLegacyBalance(ISMINE_SPENDABLE, nMinDepth, &strAccount);
+    assert(nBalance == legacyBalance);
     if (totalAmount > nBalance)
         throw JSONRPCError(RPC_WALLET_INSUFFICIENT_FUNDS, "Account has insufficient funds");
 

src/wallet/wallet.cpp

@@ -1975,6 +1975,49 @@ CAmount CWallet::GetImmatureWatchOnlyBalance() const
     return nTotal;
 }
 
+// Calculate total balance in a different way from GetBalance. The biggest
+// difference is that GetBalance sums up all unspent TxOuts paying to the
+// wallet, while this sums up both spent and unspent TxOuts paying to the
+// wallet, and then subtracts the values of TxIns spending from the wallet. This
+// also has fewer restrictions on which unconfirmed transactions are considered
+// trusted.
+CAmount CWallet::GetLegacyBalance(const isminefilter& filter, int minDepth, const std::string* account) const
+{
+    LOCK2(cs_main, cs_wallet);
+
+    CAmount balance = 0;
+    for (const auto& entry : mapWallet) {
+        const CWalletTx& wtx = entry.second;
+        const int depth = wtx.GetDepthInMainChain();
+        if (depth < 0 || !CheckFinalTx(*wtx.tx) || wtx.GetBlocksToMaturity() > 0) {
+            continue;
+        }
+
+        // Loop through tx outputs and add incoming payments. For outgoing txs,
+        // treat change outputs specially, as part of the amount debited.
+        CAmount debit = wtx.GetDebit(filter);
+        const bool outgoing = debit > 0;
+        for (const CTxOut& out : wtx.tx->vout) {
+            if (outgoing && IsChange(out)) {
+                debit -= out.nValue;
+            } else if (IsMine(out) & filter && depth >= minDepth && (!account || *account == GetAccountName(out.scriptPubKey))) {
+                balance += out.nValue;
+            }
+        }
+
+        // For outgoing txs, subtract amount debited.
+        if (outgoing && (!account || *account == wtx.strFromAccount)) {
+            balance -= debit;
+        }
+    }
+
+    if (account) {
+        balance += CWalletDB(*dbw).GetAccountCreditDebit(*account);
+    }
+
+    return balance;
+}
+
 void CWallet::AvailableCoins(std::vector<COutput>& vCoins, bool fOnlySafe, const CCoinControl *coinControl, bool fIncludeZeroValue) const
 {
     vCoins.clear();
@@ -2911,6 +2954,21 @@ bool CWallet::DelAddressBook(const CTxDestination& address)
     return CWalletDB(*dbw).EraseName(CBitcoinAddress(address).ToString());
 }
 
+const std::string& CWallet::GetAccountName(const CScript& scriptPubKey) const
+{
+    CTxDestination address;
+    if (ExtractDestination(scriptPubKey, address) && !scriptPubKey.IsUnspendable()) {
+        auto mi = mapAddressBook.find(address);
+        if (mi != mapAddressBook.end()) {
+            return mi->second.name;
+        }
+    }
+    // A scriptPubKey that doesn't have an entry in the address book is
+    // associated with the default account ("").
+    const static std::string DEFAULT_ACCOUNT_NAME;
+    return DEFAULT_ACCOUNT_NAME;
+}
+
 bool CWallet::SetDefaultKey(const CPubKey &vchPubKey)
 {
     if (!CWalletDB(*dbw).WriteDefaultKey(vchPubKey))

src/wallet/wallet.h

@@ -918,6 +918,7 @@ public:
     CAmount GetWatchOnlyBalance() const;
     CAmount GetUnconfirmedWatchOnlyBalance() const;
     CAmount GetImmatureWatchOnlyBalance() const;
+    CAmount GetLegacyBalance(const isminefilter& filter, int minDepth, const std::string* account) const;
 
     /**
      * Insert additional inputs into the transaction by
@@ -1004,6 +1005,8 @@ public:
 
     bool DelAddressBook(const CTxDestination& address);
 
+    const std::string& GetAccountName(const CScript& scriptPubKey) const;
+
     void Inventory(const uint256 &hash) override
     {
         {