Merge #18772: rpc: calculate fees in getblock using BlockUndo data

66d012ad7f9381bacfc9b8388fa2ebf82cb86c9e test: RPC: getblock fee calculations (Elliott Jin)
bf7d6e31b1062ab5f90e14e83c56309f499fa2e9 RPC: getblock: tx fee calculation for verbosity 2 via Undo data (Elliott Jin)

Pull request description:

  This change is progress towards #18771 .  It adapts the fee calculation part of #16083 and addresses some feedback.  The additional "verbosity level 3" features are planned for a future PR.

  **Original PR description:**

  > Using block undo data (like in #14802) we can now show fee information for each transaction in a block without the need for additional -txindex and/or a ton of costly lookups. For a start we'll add transaction fee information to getblock verbosity level 2. This comes at a negligible speed penalty (<1%).

ACKs for top commit:
  luke-jr:
    tACK 66d012ad7f9381bacfc9b8388fa2ebf82cb86c9e
  fjahr:
    tACK 66d012ad7f9381bacfc9b8388fa2ebf82cb86c9e
  MarcoFalke:
    review ACK 66d012ad7f9381bacfc9b8388fa2ebf82cb86c9e 🗜

Tree-SHA512: be1fe4b866946a8dc36427f7dc72a20e10860e320a28fa49bc85bd2a93a0d699768179be29fa52e18b2ed8505d3ec272e586753ef2239b4230e0aefd233acaa2

3 files changed, 41 insertions, 11 deletions

diff --git a/src/core_io.h b/src/core_io.h
index aaee9c445d..fa6900a42a 100644
--- a/src/core_io.h
+++ b/src/core_io.h
@@ -18,6 +18,7 @@ class CTransaction;
 struct CMutableTransaction;
 class uint256;
 class UniValue;
+class CTxUndo;
 
 // core_read.cpp
 CScript ParseScript(const std::string& s);
@@ -45,6 +46,6 @@ std::string EncodeHexTx(const CTransaction& tx, const int serializeFlags = 0);
 std::string SighashToStr(unsigned char sighash_type);
 void ScriptPubKeyToUniv(const CScript& scriptPubKey, UniValue& out, bool fIncludeHex);
 void ScriptToUniv(const CScript& script, UniValue& out, bool include_address);
-void TxToUniv(const CTransaction& tx, const uint256& hashBlock, UniValue& entry, bool include_hex = true, int serialize_flags = 0);
+void TxToUniv(const CTransaction& tx, const uint256& hashBlock, UniValue& entry, bool include_hex = true, int serialize_flags = 0, const CTxUndo* txundo = nullptr);
 
 #endif // BITCOIN_CORE_IO_H
diff --git a/src/core_write.cpp b/src/core_write.cpp
index 3980d8cb2e..675864200b 100644
--- a/src/core_write.cpp
+++ b/src/core_write.cpp
@@ -11,7 +11,9 @@
 #include <script/standard.h>
 #include <serialize.h>
 #include <streams.h>
+#include <undo.h>
 #include <univalue.h>
+#include <util/check.h>
 #include <util/system.h>
 #include <util/strencodings.h>
 
@@ -177,7 +179,7 @@ void ScriptPubKeyToUniv(const CScript& scriptPubKey,
     out.pushKV("addresses", a);
 }
 
-void TxToUniv(const CTransaction& tx, const uint256& hashBlock, UniValue& entry, bool include_hex, int serialize_flags)
+void TxToUniv(const CTransaction& tx, const uint256& hashBlock, UniValue& entry, bool include_hex, int serialize_flags, const CTxUndo* txundo)
 {
     entry.pushKV("txid", tx.GetHash().GetHex());
     entry.pushKV("hash", tx.GetWitnessHash().GetHex());
@@ -189,13 +191,20 @@ void TxToUniv(const CTransaction& tx, const uint256& hashBlock, UniValue& entry,
     entry.pushKV("weight", GetTransactionWeight(tx));
     entry.pushKV("locktime", (int64_t)tx.nLockTime);
 
-    UniValue vin(UniValue::VARR);
+    UniValue vin{UniValue::VARR};
+
+    // If available, use Undo data to calculate the fee. Note that txundo == nullptr
+    // for coinbase transactions and for transactions where undo data is unavailable.
+    const bool calculate_fee = txundo != nullptr;
+    CAmount amt_total_in = 0;
+    CAmount amt_total_out = 0;
+
     for (unsigned int i = 0; i < tx.vin.size(); i++) {
         const CTxIn& txin = tx.vin[i];
         UniValue in(UniValue::VOBJ);
-        if (tx.IsCoinBase())
+        if (tx.IsCoinBase()) {
             in.pushKV("coinbase", HexStr(txin.scriptSig));
-        else {
+        } else {
             in.pushKV("txid", txin.prevout.hash.GetHex());
             in.pushKV("vout", (int64_t)txin.prevout.n);
             UniValue o(UniValue::VOBJ);
@@ -210,6 +219,10 @@ void TxToUniv(const CTransaction& tx, const uint256& hashBlock, UniValue& entry,
             }
             in.pushKV("txinwitness", txinwitness);
         }
+        if (calculate_fee) {
+            const CTxOut& prev_txout = txundo->vprevout[i].out;
+            amt_total_in += prev_txout.nValue;
+        }
         in.pushKV("sequence", (int64_t)txin.nSequence);
         vin.push_back(in);
     }
@@ -228,9 +241,19 @@ void TxToUniv(const CTransaction& tx, const uint256& hashBlock, UniValue& entry,
         ScriptPubKeyToUniv(txout.scriptPubKey, o, true);
         out.pushKV("scriptPubKey", o);
         vout.push_back(out);
+
+        if (calculate_fee) {
+            amt_total_out += txout.nValue;
+        }
     }
     entry.pushKV("vout", vout);
 
+    if (calculate_fee) {
+        const CAmount fee = amt_total_in - amt_total_out;
+        CHECK_NONFATAL(MoneyRange(fee));
+        entry.pushKV("fee", ValueFromAmount(fee));
+    }
+
     if (!hashBlock.IsNull())
         entry.pushKV("blockhash", hashBlock.GetHex());
 
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 57327e6004..9047492642 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -172,16 +172,21 @@ UniValue blockToJSON(const CBlock& block, const CBlockIndex* tip, const CBlockIn
     result.pushKV("versionHex", strprintf("%08x", block.nVersion));
     result.pushKV("merkleroot", block.hashMerkleRoot.GetHex());
     UniValue txs(UniValue::VARR);
-    for(const auto& tx : block.vtx)
-    {
-        if(txDetails)
-        {
+    if (txDetails) {
+        CBlockUndo blockUndo;
+        const bool have_undo = !IsBlockPruned(blockindex) && UndoReadFromDisk(blockUndo, blockindex);
+        for (size_t i = 0; i < block.vtx.size(); ++i) {
+            const CTransactionRef& tx = block.vtx.at(i);
+            // coinbase transaction (i == 0) doesn't have undo data
+            const CTxUndo* txundo = (have_undo && i) ? &blockUndo.vtxundo.at(i - 1) : nullptr;
             UniValue objTx(UniValue::VOBJ);
-            TxToUniv(*tx, uint256(), objTx, true, RPCSerializationFlags());
+            TxToUniv(*tx, uint256(), objTx, true, RPCSerializationFlags(), txundo);
             txs.push_back(objTx);
         }
-        else
+    } else {
+        for (const CTransactionRef& tx : block.vtx) {
             txs.push_back(tx->GetHash().GetHex());
+        }
     }
     result.pushKV("tx", txs);
     result.pushKV("time", block.GetBlockTime());
@@ -936,6 +941,7 @@ static RPCHelpMan getblock()
                         {RPCResult::Type::OBJ, "", "",
                         {
                             {RPCResult::Type::ELISION, "", "The transactions in the format of the getrawtransaction RPC. Different from verbosity = 1 \"tx\" result"},
+                            {RPCResult::Type::NUM, "fee", "The transaction fee in " + CURRENCY_UNIT + ", omitted if block undo data is not available"},
                         }},
                     }},
                 }},