refactor mixin condition

src/cryptonote_core/blockchain.cpp

@@ -2613,7 +2613,7 @@ bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc,
   {
     size_t n_unmixable = 0, n_mixable = 0;
     size_t mixin = std::numeric_limits<size_t>::max();
-    const size_t min_mixin = hf_version >= HF_VERSION_MIN_MIXIN_7 ? 7 : hf_version >= HF_VERSION_MIN_MIXIN_4 ? 4 : 2;
+    const size_t min_mixin = hf_version >= HF_VERSION_MIN_MIXIN_9 ? 9 : hf_version >= HF_VERSION_MIN_MIXIN_7 ? 7 : hf_version >= HF_VERSION_MIN_MIXIN_4 ? 4 : 2;
     for (const auto& txin : tx.vin)
     {
       // non txin_to_key inputs will be rejected below
@@ -2642,9 +2642,6 @@ bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc,
       }
     }
 
-  // from v9, mixin is static
-  if (hf_version < HF_VERSION_MIN_MIXIN_9)
-  {
     if (mixin < min_mixin)
     {
       if (n_unmixable == 0)
@@ -2660,18 +2657,18 @@ bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc,
         return false;
       }
     }
-    else 
+
+    // from v9, all transactions must have the same ring size
+    if (hf_version >= HF_VERSION_MIN_MIXIN_9)
     {
-      const size_t mixin = DEFAULT_MIXIN;
-      const size_t min_mixin = DEFAULT_MIXIN;
       if (mixin != min_mixin)
       {
         MERROR_VER("Tx " << get_transaction_hash(tx) << " has invalid ring size");
+        tvc.m_low_mixin = true;
         return false;
       }
     }
-  } 
+
-  
     // min/max tx version based on HF, and we accept v1 txes if having a non mixable
     const size_t max_tx_version = (hf_version <= 3) ? 1 : 2;
     if (tx.version > max_tx_version)