Merge bitcoin/bitcoin#25943: rpc: Add a parameter to sendrawtransaction which sets a maximum value for unspendable outputs.

7013da07fbcddb04abae9759767a9419ab90444c Add release note for PR#25943 (David Gumberg)
04f270b4358417fc2827b9f91717816062b1864e Add test for unspendable transactions and parameter 'maxburnamount' to sendrawtransaction. (David Gumberg)

Pull request description:

  This PR adds a user configurable, zero by default parameter — `maxburnamount` — to `sendrawtransaction`. This PR makes bitcoin core reject transactions that contain unspendable outputs which exceed `maxburnamount`.  closes #25899.

  As a result of this PR, `sendrawtransaction` will by default block 3 kinds of transactions:

  1. Those that begin with `OP_RETURN` - (datacarriers)
  2. Those whose lengths exceed the script limit.
  3. Those that contain invalid opcodes.

  The user is able to configure a `maxburnamount` that will override this check and allow a user to send a potentially unspendable output into the mempool.

  I see two legitimate use cases for this override:
  1. Users that deliberately use `OP_RETURN` for datacarrier transactions that embed data into the blockchain.
  2.  Users that refuse to update, or are unable to update their bitcoin core client would be able to make use of new opcodes that their client doesn't know about.

ACKs for top commit:
  glozow:
    reACK 7013da07fbcddb04abae9759767a9419ab90444c
  achow101:
    re-ACK 7013da07fbcddb04abae9759767a9419ab90444c

Tree-SHA512: f786a796fb71a587d30313c96717fdf47e1106ab4ee0c16d713695e6c31ed6f6732dff6cbc91ca9841d66232166eb058f96028028e75c1507324426309ee4525

2 files changed, 16 insertions, 1 deletions

diff --git a/src/rpc/client.cpp b/src/rpc/client.cpp
index 5fe914f0a1..eb91a151b5 100644
--- a/src/rpc/client.cpp
+++ b/src/rpc/client.cpp
@@ -114,6 +114,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "signrawtransactionwithkey", 2, "prevtxs" },
     { "signrawtransactionwithwallet", 1, "prevtxs" },
     { "sendrawtransaction", 1, "maxfeerate" },
+    { "sendrawtransaction", 2, "maxburnamount" },
     { "testmempoolaccept", 0, "rawtxs" },
     { "testmempoolaccept", 1, "maxfeerate" },
     { "submitpackage", 0, "package" },
diff --git a/src/rpc/mempool.cpp b/src/rpc/mempool.cpp
index 3143434633..3a69e2d8a2 100644
--- a/src/rpc/mempool.cpp
+++ b/src/rpc/mempool.cpp
@@ -18,6 +18,7 @@
 #include <rpc/server.h>
 #include <rpc/server_util.h>
 #include <rpc/util.h>
+#include <script/standard.h>
 #include <txmempool.h>
 #include <univalue.h>
 #include <util/moneystr.h>
@@ -44,7 +45,11 @@ static RPCHelpMan sendrawtransaction()
             {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The hex string of the raw transaction"},
             {"maxfeerate", RPCArg::Type::AMOUNT, RPCArg::Default{FormatMoney(DEFAULT_MAX_RAW_TX_FEE_RATE.GetFeePerK())},
              "Reject transactions whose fee rate is higher than the specified value, expressed in " + CURRENCY_UNIT +
-                 "/kvB.\nSet to 0 to accept any fee rate.\n"},
+                 "/kvB.\nSet to 0 to accept any fee rate."},
+            {"maxburnamount", RPCArg::Type::AMOUNT, RPCArg::Default{FormatMoney(0)},
+             "Reject transactions with provably unspendable outputs (e.g. 'datacarrier' outputs that use the OP_RETURN opcode) greater than the specified value, expressed in " + CURRENCY_UNIT + ".\n"
+             "If burning funds through unspendable outputs is desired, increase this value.\n"
+             "This check is based on heuristics and does not guarantee spendability of outputs.\n"},
         },
         RPCResult{
             RPCResult::Type::STR_HEX, "", "The transaction hash in hex"
@@ -61,10 +66,19 @@ static RPCHelpMan sendrawtransaction()
                 },
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
         {
+            const CAmount max_burn_amount = request.params[2].isNull() ? 0 : AmountFromValue(request.params[2]);
+
             CMutableTransaction mtx;
             if (!DecodeHexTx(mtx, request.params[0].get_str())) {
                 throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed. Make sure the tx has at least one input.");
             }
+
+            for (const auto& out : mtx.vout) {
+                if((out.scriptPubKey.IsUnspendable() || !out.scriptPubKey.HasValidOps()) && out.nValue > max_burn_amount) {
+                    throw JSONRPCTransactionError(TransactionError::MAX_BURN_EXCEEDED);
+                }
+            }
+
             CTransactionRef tx(MakeTransactionRef(std::move(mtx)));
 
             const CFeeRate max_raw_tx_fee_rate = request.params[1].isNull() ?