wallet: fix multisig key memory leak

Multisig keys per-transfer were being wiped, but not erased, which lead to a ginormous
quadratic bloat the more transfers and exports you performed with the wallet.

src/wallet/wallet2.cpp

@@ -6163,6 +6163,20 @@ void wallet2::load(const std::string& wallet_, const epee::wipeable_string& pass
       error::wallet_files_doesnt_correspond, m_keys_file, m_wallet_file);
   }
 
+  // Wallets used to wipe, but not erase, old unused multisig key info, which lead to huge memory leaks.
+  // Here we erase these multisig keys if they're zero'd out to free up space.
+  for (auto &td : m_transfers)
+  {
+    auto mk_it = td.m_multisig_k.begin();
+    while (mk_it != td.m_multisig_k.end())
+    {
+      if (*mk_it == rct::zero())
+        mk_it = td.m_multisig_k.erase(mk_it);
+      else
+        ++mk_it;
+    }
+  }
+
   cryptonote::block genesis;
   generate_genesis(genesis);
   crypto::hash genesis_hash = get_block_hash(genesis);
@@ -7036,7 +7050,10 @@ void wallet2::commit_tx(pending_tx& ptx)
 
   // tx generated, get rid of used k values
   for (size_t idx: ptx.selected_transfers)
+  {
     memwipe(m_transfers[idx].m_multisig_k.data(), m_transfers[idx].m_multisig_k.size() * sizeof(m_transfers[idx].m_multisig_k[0]));
+    m_transfers[idx].m_multisig_k.clear();
+  }
 
   //fee includes dust if dust policy specified it.
   LOG_PRINT_L1("Transaction successfully sent. <" << txid << ">" << ENDL
@@ -7540,7 +7557,10 @@ std::string wallet2::save_multisig_tx(multisig_tx_set txs)
   // txes generated, get rid of used k values
   for (size_t n = 0; n < txs.m_ptx.size(); ++n)
     for (size_t idx: txs.m_ptx[n].construction_data.selected_transfers)
+    {
       memwipe(m_transfers[idx].m_multisig_k.data(), m_transfers[idx].m_multisig_k.size() * sizeof(m_transfers[idx].m_multisig_k[0]));
+      m_transfers[idx].m_multisig_k.clear();
+    }
 
   // zero out some data we don't want to share
   for (auto &ptx: txs.m_ptx)
@@ -7864,7 +7884,10 @@ bool wallet2::sign_multisig_tx(multisig_tx_set &exported_txs, std::vector<crypto
   //   inputs in the transactions worked on here)
   for (size_t n = 0; n < exported_txs.m_ptx.size(); ++n)
     for (size_t idx: exported_txs.m_ptx[n].construction_data.selected_transfers)
+    {
       memwipe(m_transfers[idx].m_multisig_k.data(), m_transfers[idx].m_multisig_k.size() * sizeof(m_transfers[idx].m_multisig_k[0]));
+      m_transfers[idx].m_multisig_k.clear();
+    }
 
   exported_txs.m_signers.insert(get_multisig_signer_public_key());
 
@@ -13511,7 +13534,10 @@ cryptonote::blobdata wallet2::export_multisig()
     transfer_details &td = m_transfers[n];
     crypto::key_image ki;
     if (td.m_multisig_k.size())
+    {
       memwipe(td.m_multisig_k.data(), td.m_multisig_k.size() * sizeof(td.m_multisig_k[0]));
+      td.m_multisig_k.clear();
+    }
     info[n].m_LR.clear();
     info[n].m_partial_key_images.clear();