New "Halfway RingCT" outputs for coinbase transactions

When RingCT is enabled, outputs from coinbase transactions
are created as a single output, and stored as RingCT output,
with a fake mask. Their amount is not hidden on the blockchain
itself, but they are then able to be used as fake inputs in
a RingCT ring. Since the output amounts are hidden, their
"dustiness" is not an obstacle anymore to mixing, and this
makes the coinbase transactions a lot smaller, as well as
helping the TXO set to grow more slowly.

Also add a new "Null" type of rct signature, which decreases
the size required when no signatures are to be stored, as
in a coinbase tx.

src/wallet/wallet2.cpp

@@ -489,10 +489,17 @@ void wallet2::process_new_transaction(const cryptonote::transaction& tx, const s
             {
               td.m_mask = mask[o];
               td.m_amount = amount[o];
+              td.m_rct = true;
+            }
+            else if (miner_tx && tx.version == 2)
+            {
+              td.m_mask = rct::identity();
+              td.m_rct = true;
             }
             else
             {
               td.m_mask = rct::identity();
+              td.m_rct = false;
             }
 	    set_unspent(td);
 	    m_key_images[td.m_key_image] = m_transfers.size()-1;
@@ -529,10 +536,17 @@ void wallet2::process_new_transaction(const cryptonote::transaction& tx, const s
             {
               td.m_mask = mask[o];
               td.m_amount = amount[o];
+              td.m_rct = true;
+            }
+            else if (miner_tx && tx.version == 2)
+            {
+              td.m_mask = rct::identity();
+              td.m_rct = true;
             }
             else
             {
               td.m_mask = rct::identity();
+              td.m_rct = false;
             }
             THROW_WALLET_EXCEPTION_IF(td.m_key_image != ki[o], error::wallet_internal_error, "Inconsistent key images");
 	    THROW_WALLET_EXCEPTION_IF(td.m_spent, error::wallet_internal_error, "Inconsistent spent status");
@@ -2847,6 +2861,7 @@ void wallet2::transfer_selected(const std::vector<cryptonote::tx_destination_ent
     cryptonote::tx_source_entry& src = sources.back();
     transfer_details& td = *it;
     src.amount = td.amount();
+    src.rct = td.is_rct();
     //paste keys (fake and real)
 
     for (size_t n = 0; n < fake_outputs_count + 1; ++n)
@@ -3037,6 +3052,7 @@ void wallet2::transfer_selected_rct(std::vector<cryptonote::tx_destination_entry
     cryptonote::tx_source_entry& src = sources.back();
     transfer_details& td = *it;
     src.amount = td.amount();
+    src.rct = td.is_rct();
     //paste mixin transaction
     if(it->is_rct())
     {
@@ -3732,6 +3748,7 @@ void wallet2::transfer_from(const std::vector<size_t> &outs, size_t num_outputs,
     cryptonote::tx_source_entry& src = sources.back();
     transfer_details& td = *it;
     src.amount = td.amount();
+    src.rct = td.is_rct();
 
     //paste real transaction to the random index
     auto it_to_insert = std::find_if(src.outputs.begin(), src.outputs.end(), [&](const tx_output_entry& a)

src/wallet/wallet2.h

@@ -108,8 +108,9 @@ namespace tools
       crypto::key_image m_key_image; //TODO: key_image stored twice :(
       rct::key m_mask;
       uint64_t m_amount;
+      bool m_rct;
 
-      bool is_rct() const { return m_tx.vout[m_internal_output_index].amount == 0; }
+      bool is_rct() const { return m_rct; }
       uint64_t amount() const { return m_amount; }
     };
 
@@ -502,7 +503,7 @@ namespace tools
   };
 }
 BOOST_CLASS_VERSION(tools::wallet2, 14)
-BOOST_CLASS_VERSION(tools::wallet2::transfer_details, 3)
+BOOST_CLASS_VERSION(tools::wallet2::transfer_details, 4)
 BOOST_CLASS_VERSION(tools::wallet2::payment_details, 1)
 BOOST_CLASS_VERSION(tools::wallet2::unconfirmed_transfer_details, 5)
 BOOST_CLASS_VERSION(tools::wallet2::confirmed_transfer_details, 2)
@@ -527,6 +528,10 @@ namespace boost
         {
           x.m_spent_height = 0;
         }
+        if (ver < 4)
+        {
+          x.m_rct = x.m_tx.vout[x.m_internal_output_index].amount == 0;
+        }
     }
 
     template <class Archive>
@@ -563,8 +568,17 @@ namespace boost
       }
       a & x.m_spent_height;
       if (ver < 3)
+      {
+        initialize_transfer_details(a, x, ver);
         return;
+      }
       a & x.m_txid;
+      if (ver < 4)
+      {
+        initialize_transfer_details(a, x, ver);
+        return;
+      }
+      a & x.m_rct;
     }
 
     template <class Archive>
@@ -770,6 +784,7 @@ namespace tools
       cryptonote::tx_source_entry& src = sources.back();
       transfer_details& td = *it;
       src.amount = td.amount();
+      src.rct = false;
       //paste mixin transaction
       if(daemon_resp.outs.size())
       {