core: from v12, require consistent ring size for mixable txes

We're supposed to have a fixed ring size now

Already checked by MLSAG verification, but here seems more intuitive

src/cryptonote_config.h

@@ -151,6 +151,7 @@
 #define HF_VERSION_LONG_TERM_BLOCK_WEIGHT       10
 #define HF_VERSION_MIN_2_OUTPUTS                12
 #define HF_VERSION_MIN_V2_COINBASE_TX           12
+#define HF_VERSION_SAME_MIXIN                   12
 
 #define PER_KB_FEE_QUANTIZATION_DECIMALS        8
 

src/cryptonote_core/blockchain.cpp

@@ -2860,7 +2860,8 @@ bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc,
   if (hf_version >= 2)
   {
     size_t n_unmixable = 0, n_mixable = 0;
-    size_t mixin = std::numeric_limits<size_t>::max();
+    size_t min_actual_mixin = std::numeric_limits<size_t>::max();
+    size_t max_actual_mixin = 0;
     const size_t min_mixin = hf_version >= HF_VERSION_MIN_MIXIN_10 ? 10 : hf_version >= HF_VERSION_MIN_MIXIN_6 ? 6 : hf_version >= HF_VERSION_MIN_MIXIN_4 ? 4 : 2;
     for (const auto& txin : tx.vin)
     {
@@ -2885,29 +2886,43 @@ bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc,
           else
             ++n_mixable;
         }
-        if (in_to_key.key_offsets.size() - 1 < mixin)
-          mixin = in_to_key.key_offsets.size() - 1;
+        size_t ring_mixin = in_to_key.key_offsets.size() - 1;
+        if (ring_mixin < min_actual_mixin)
+          min_actual_mixin = ring_mixin;
+        if (ring_mixin > max_actual_mixin)
+          max_actual_mixin = ring_mixin;
       }
     }
+    MDEBUG("Mixin: " << min_actual_mixin << "-" << max_actual_mixin);
 
-    if (((hf_version == HF_VERSION_MIN_MIXIN_10 || hf_version == HF_VERSION_MIN_MIXIN_10+1) && mixin != 10) || (hf_version >= HF_VERSION_MIN_MIXIN_10+2 && mixin > 10))
+    if (hf_version >= HF_VERSION_SAME_MIXIN)
     {
-      MERROR_VER("Tx " << get_transaction_hash(tx) << " has invalid ring size (" << (mixin + 1) << "), it should be 11");
+      if (min_actual_mixin != max_actual_mixin)
+      {
+        MERROR_VER("Tx " << get_transaction_hash(tx) << " has varying ring size (" << (min_actual_mixin + 1) << "-" << (max_actual_mixin + 1) << "), it should be constant");
+        tvc.m_low_mixin = true;
+        return false;
+      }
+    }
+
+    if (((hf_version == HF_VERSION_MIN_MIXIN_10 || hf_version == HF_VERSION_MIN_MIXIN_10+1) && min_actual_mixin != 10) || (hf_version >= HF_VERSION_MIN_MIXIN_10+2 && min_actual_mixin > 10))
+    {
+      MERROR_VER("Tx " << get_transaction_hash(tx) << " has invalid ring size (" << (min_actual_mixin + 1) << "), it should be 11");
       tvc.m_low_mixin = true;
       return false;
     }
 
-    if (mixin < min_mixin)
+    if (min_actual_mixin < min_mixin)
     {
       if (n_unmixable == 0)
       {
-        MERROR_VER("Tx " << get_transaction_hash(tx) << " has too low ring size (" << (mixin + 1) << "), and no unmixable inputs");
+        MERROR_VER("Tx " << get_transaction_hash(tx) << " has too low ring size (" << (min_actual_mixin + 1) << "), and no unmixable inputs");
         tvc.m_low_mixin = true;
         return false;
       }
       if (n_mixable > 1)
       {
-        MERROR_VER("Tx " << get_transaction_hash(tx) << " has too low ring size (" << (mixin + 1) << "), and more than one mixable input with unmixable inputs");
+        MERROR_VER("Tx " << get_transaction_hash(tx) << " has too low ring size (" << (min_actual_mixin + 1) << "), and more than one mixable input with unmixable inputs");
         tvc.m_low_mixin = true;
         return false;
       }