Actually Check Simplicity when feature enabled

Author: roconnor-blockstream

src/script/interpreter.cpp

@@ -2026,6 +2026,18 @@ static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion,
                 execdata.m_validation_weight_left_init = true;
                 return ExecuteWitnessScript(stack, exec_script, flags, SigVersion::TAPSCRIPT, checker, execdata, serror);
             }
+            if ((flags & SCRIPT_VERIFY_SIMPLICITY) && (control[0] & TAPROOT_LEAF_MASK) == TAPROOT_LEAF_TAPSIMPLICITY) {
+                if (stack.size() != 2 || script.size() != 32) return set_error(serror, SCRIPT_ERR_SIMPLICITY_WRONG_LENGTH);
+                // Tapsimplicity (leaf version 0xbe)
+                const valtype& simplicity_program = SpanPopBack(stack);
+                const valtype& simplicity_witness = SpanPopBack(stack);
+                const int64_t budget = ::GetSerializeSize(witness.stack) + VALIDATION_WEIGHT_OFFSET;
+                rawBitcoinTapEnv simplicityRawTap;
+                simplicityRawTap.controlBlock = control.data();
+                simplicityRawTap.pathLen = (control.size() - TAPROOT_CONTROL_BASE_SIZE) / TAPROOT_CONTROL_NODE_SIZE;
+                simplicityRawTap.scriptCMR = script.data();
+                return checker.CheckSimplicity(simplicity_program, simplicity_witness, simplicityRawTap, budget, serror);
+            }
             if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_TAPROOT_VERSION) {
                 return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_TAPROOT_VERSION);
             }

src/script/interpreter.h

@@ -146,6 +146,9 @@ enum : uint32_t {
     // Making unknown public key versions (in BIP 342 scripts) non-standard
     SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_PUBKEYTYPE = (1U << 20),
 
+    // Support simplicity
+    SCRIPT_VERIFY_SIMPLICITY = (1U << 21),
+
     // Constants to point to the highest flag in use. Add new flags above this line.
     //
     SCRIPT_VERIFY_END_MARKER