rpc: faster getblockstats using BlockUndo data

Using undo data for a block (rev?????.dat) we can retrieve value information about prevouts and calculate the final transaction fee (rate). This approach is about 80x faster, drops the requirement for -txindex, and works for all non-pruned blocks.

doc/release-notes-14802.md

@@ -0,0 +1,3 @@
+RPC changes
+-----------
+The `getblockstats` RPC is faster for fee calculation by using BlockUndo data. Also, `-txindex` is no longer required and `getblockstats` works for all non-pruned blocks.

src/rpc/blockchain.cpp

@@ -28,6 +28,7 @@
 #include <sync.h>
 #include <txdb.h>
 #include <txmempool.h>
+#include <undo.h>
 #include <util/strencodings.h>
 #include <util/system.h>
 #include <util/validation.h>
@@ -822,6 +823,20 @@ static CBlock GetBlockChecked(const CBlockIndex* pblockindex)
     return block;
 }
 
+static CBlockUndo GetUndoChecked(const CBlockIndex* pblockindex)
+{
+    CBlockUndo blockUndo;
+    if (IsBlockPruned(pblockindex)) {
+        throw JSONRPCError(RPC_MISC_ERROR, "Undo data not available (pruned data)");
+    }
+
+    if (!UndoReadFromDisk(blockUndo, pblockindex)) {
+        throw JSONRPCError(RPC_MISC_ERROR, "Can't read undo data from disk");
+    }
+
+    return blockUndo;
+}
+
 static UniValue getblock(const JSONRPCRequest& request)
 {
     if (request.fHelp || request.params.size() < 1 || request.params.size() > 2)
@@ -1783,8 +1798,7 @@ static UniValue getblockstats(const JSONRPCRequest& request)
 {
     const RPCHelpMan help{"getblockstats",
                 "\nCompute per block statistics for a given window. All amounts are in satoshis.\n"
-                "It won't work for some heights with pruning.\n"
-                "It won't work without -txindex for utxo_size_inc, *fee or *feerate stats.\n",
+                "It won't work for some heights with pruning.\n",
                 {
                     {"hash_or_height", RPCArg::Type::NUM, RPCArg::Optional::NO, "The block hash or height of the target block", "", {"", "string or numeric"}},
                     {"stats", RPCArg::Type::ARR, /* default */ "all values", "Values to plot (see result below)",
@@ -1879,6 +1893,7 @@ static UniValue getblockstats(const JSONRPCRequest& request)
     }
 
     const CBlock block = GetBlockChecked(pindex);
+    const CBlockUndo blockUndo = GetUndoChecked(pindex);
 
     const bool do_all = stats.size() == 0; // Calculate everything if nothing selected (default)
     const bool do_mediantxsize = do_all || stats.count("mediantxsize") != 0;
@@ -1892,10 +1907,6 @@ static UniValue getblockstats(const JSONRPCRequest& request)
     const bool do_calculate_weight = do_all || SetHasKeys(stats, "total_weight", "avgfeerate", "swtotal_weight", "avgfeerate", "feerate_percentiles", "minfeerate", "maxfeerate");
     const bool do_calculate_sw = do_all || SetHasKeys(stats, "swtxs", "swtotal_size", "swtotal_weight");
 
-    if (loop_inputs && !g_txindex) {
-        throw JSONRPCError(RPC_INVALID_PARAMETER, "One or more of the selected stats requires -txindex enabled");
-    }
-
     CAmount maxfee = 0;
     CAmount maxfeerate = 0;
     CAmount minfee = MAX_MONEY;
@@ -1916,7 +1927,8 @@ static UniValue getblockstats(const JSONRPCRequest& request)
     std::vector<std::pair<CAmount, int64_t>> feerate_array;
     std::vector<int64_t> txsize_array;
 
-    for (const auto& tx : block.vtx) {
+    for (size_t i = 0; i < block.vtx.size(); ++i) {
+        const auto& tx = block.vtx.at(i);
         outputs += tx->vout.size();
 
         CAmount tx_total_out = 0;
@@ -1960,14 +1972,9 @@ static UniValue getblockstats(const JSONRPCRequest& request)
 
         if (loop_inputs) {
             CAmount tx_total_in = 0;
-            for (const CTxIn& in : tx->vin) {
-                CTransactionRef tx_in;
-                uint256 hashBlock;
-                if (!GetTransaction(in.prevout.hash, tx_in, Params().GetConsensus(), hashBlock)) {
-                    throw JSONRPCError(RPC_INTERNAL_ERROR, std::string("Unexpected internal error (tx index seems corrupt)"));
-                }
-
-                CTxOut prevoutput = tx_in->vout[in.prevout.n];
+            const auto& txundo = blockUndo.vtxundo.at(i - 1);
+            for (const Coin& coin: txundo.vprevout) {
+                const CTxOut& prevoutput = coin.out;
 
                 tx_total_in += prevoutput.nValue;
                 utxo_size_inc -= GetSerializeSize(prevoutput, PROTOCOL_VERSION) + PER_UTXO_OVERHEAD;