policy: Count data in a dead conditional branch

doc/release-notes-dead-branch-data.md

@@ -0,0 +1,12 @@
+Policy
+------
+
+- Data in a conditional part that constants make unreachable now counts as
+  data carrier bytes, so under the default settings a transaction carrying it
+  is rejected with `txn-datacarrier-nonstandard`. This already applied to the
+  `OP_FALSE OP_IF ... OP_ENDIF` inscription envelope; it now also covers
+  `OP_1 OP_NOTIF ... OP_ENDIF`, guards computed from constants, and the dead
+  part of an `OP_ELSE`. Scripts that branch on witness data or a signature
+  check, which is how spendable scripts branch, are unaffected. The new
+  `-rejectdeadbranches` option (default: 1, reset by `-corepolicy`) turns the
+  wider rule off and leaves only the envelope counted.

src/init.cpp

@@ -721,6 +721,7 @@ void SetupServerArgs(ArgsManager& argsman, bool can_listen_ipc)
     argsman.AddArg("-bytespersigopstrict", strprintf("Minimum bytes per sigop in transactions we relay and mine (default: %u)", DEFAULT_BYTES_PER_SIGOP_STRICT), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarrier", strprintf("Relay and mine data carrier transactions (default: %u)", DEFAULT_ACCEPT_DATACARRIER), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarriercost", strprintf("Treat extra data in transactions as at least N vbytes per actual byte (default: %s)", DEFAULT_WEIGHT_PER_DATA_BYTE / 4.0), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
+    argsman.AddArg("-rejectdeadbranches", strprintf("Count data in a conditional branch that a pushed constant makes unreachable as data carrier bytes; the OP_FALSE OP_IF envelope is counted either way (default: %u)", DEFAULT_REJECT_DEAD_BRANCHES), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarrierfullcount", strprintf("Apply datacarriersize limit to all known datacarrier methods (default: %u)", DEFAULT_DATACARRIER_FULLCOUNT), ArgsManager::ALLOW_ANY | (DEFAULT_DATACARRIER_FULLCOUNT ? uint32_t{ArgsManager::DEBUG_ONLY} : 0), OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarriersize",
                    strprintf("Maximum size of data in data carrier transactions we relay and mine, in bytes (default: %u)",
@@ -873,6 +874,7 @@ void InitParameterInteraction(ArgsManager& args)
         args.SoftSetArg("-subdustfeepenalty", "0");
         args.SoftSetArg("-datacarriercost", "0.25");
         args.SoftSetArg("-datacarrierfullcount", "0");
+        args.SoftSetArg("-rejectdeadbranches", "0");
         args.SoftSetArg("-maxtxlegacysigops", strprintf("%s", std::numeric_limits<unsigned int>::max()));
         args.SoftSetArg("-maxscriptsize", strprintf("%s", std::numeric_limits<unsigned int>::max()));
         args.SoftSetArg("-mempooltruc", "enforce");
@@ -1213,6 +1215,8 @@ bool AppInitParameterInteraction(const ArgsManager& args)
 
     g_script_size_policy_limit = args.GetIntArg("-maxscriptsize", g_script_size_policy_limit);
 
+    g_reject_dead_branches = args.GetBoolArg("-rejectdeadbranches", g_reject_dead_branches);
+
     nBytesPerSigOp = args.GetIntArg("-bytespersigop", nBytesPerSigOp);
     nBytesPerSigOpStrict = args.GetIntArg("-bytespersigopstrict", nBytesPerSigOpStrict);
 

src/policy/policy.cpp

@@ -33,6 +33,7 @@
 #include <vector>
 
 unsigned int g_script_size_policy_limit{DEFAULT_SCRIPT_SIZE_POLICY_LIMIT};
+bool g_reject_dead_branches{DEFAULT_REJECT_DEAD_BRANCHES};
 
 CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
 {
@@ -633,14 +634,24 @@ std::pair<CScript, unsigned int> GetScriptForTransactionInput(CScript prevScript
     return std::make_pair(CScript(), 0);
 }
 
+/**
+ * Whether to look for dead branches in this input. A witness over the script size
+ * limit fails IsWitnessStandard right after, so following its constants would be
+ * work spent on a transaction that is rejected anyway.
+ */
+static bool FollowDeadBranches(const CTxIn& txin)
+{
+    return ::g_reject_dead_branches && GetSerializeSize(txin.scriptWitness.stack) <= g_script_size_policy_limit;
+}
+
 std::pair<size_t, size_t> DatacarrierBytes(const CTransaction& tx, const CCoinsViewCache& view)
 {
     std::pair<size_t, size_t> ret{0, 0};
 
     for (const CTxIn& txin : tx.vin) {
         const CTxOut &utxo = view.AccessCoin(txin.prevout).out;
         auto[script, consensus_weight_per_byte] = GetScriptForTransactionInput(utxo.scriptPubKey, txin);
-        const auto dcb = script.DatacarrierBytes(0, &txin.scriptWitness);
+        const auto dcb = script.DatacarrierBytes(0, &txin.scriptWitness, FollowDeadBranches(txin));
         ret.first += dcb.first;
         ret.second += dcb.second;
     }
@@ -664,7 +675,7 @@ int32_t CalculateExtraTxWeight(const CTransaction& tx, const CCoinsViewCache& vi
             const CTxOut &utxo = view.AccessCoin(txin.prevout).out;
             auto[script, consensus_weight_per_byte] = GetScriptForTransactionInput(utxo.scriptPubKey, txin);
             if (weight_per_data_byte > consensus_weight_per_byte) {
-                const auto dcb = script.DatacarrierBytes(0, &txin.scriptWitness);
+                const auto dcb = script.DatacarrierBytes(0, &txin.scriptWitness, FollowDeadBranches(txin));
                 mod_weight += int64_t(dcb.first + dcb.second) * (weight_per_data_byte - consensus_weight_per_byte);
             }
         }

src/policy/policy.h

@@ -64,6 +64,8 @@ static constexpr unsigned int DEFAULT_BYTES_PER_SIGOP{20};
 static constexpr unsigned int DEFAULT_BYTES_PER_SIGOP_STRICT{20};
 /** Default for -datacarriercost (multiplied by WITNESS_SCALE_FACTOR) */
 static constexpr unsigned int DEFAULT_WEIGHT_PER_DATA_BYTE{4};
+/** Default for -rejectdeadbranches */
+static constexpr bool DEFAULT_REJECT_DEAD_BRANCHES{true};
 /** Default for -rejecttokens */
 static constexpr bool DEFAULT_REJECT_TOKENS{true};
 /** Default for -subdustfeepenalty */

src/policy/settings.h

@@ -10,5 +10,6 @@ extern unsigned int g_script_size_policy_limit;
 extern unsigned int nBytesPerSigOp;
 extern unsigned int nBytesPerSigOpStrict;
 extern unsigned int g_weight_per_data_byte;
+extern bool g_reject_dead_branches;
 
 #endif // BITCOIN_POLICY_SETTINGS_H