wallet: reuse cached height when set after refresh

Refreshing sets cached height, which is otherwise got by calling
get_info. Since get_info is called upon needing to display a prompt
after a command has finished, it can be used to determine how much
time a given command took to run if the cache timeout lapses while
the command runs. Refreshing caches the height as a side effect, so
get_info will never be called as a result of displaying a prompt
after refreshing (and potentially leaking how much time it took to
process a set of transactions, therefore leaking whether we got
some monero in them).

src/wallet/node_rpc_proxy.cpp

@@ -77,6 +77,7 @@ void NodeRPCProxy::invalidate()
   m_rpc_payment_seed_height = 0;
   m_rpc_payment_seed_hash = crypto::null_hash;
   m_rpc_payment_next_seed_hash = crypto::null_hash;
+  m_height_time = 0;
 }
 
 boost::optional<std::string> NodeRPCProxy::get_rpc_version(uint32_t &rpc_version)
@@ -101,6 +102,7 @@ boost::optional<std::string> NodeRPCProxy::get_rpc_version(uint32_t &rpc_version
 void NodeRPCProxy::set_height(uint64_t h)
 {
   m_height = h;
+  m_height_time = time(NULL);
 }
 
 boost::optional<std::string> NodeRPCProxy::get_info()
@@ -126,12 +128,20 @@ boost::optional<std::string> NodeRPCProxy::get_info()
     m_target_height = resp_t.target_height;
     m_block_weight_limit = resp_t.block_weight_limit ? resp_t.block_weight_limit : resp_t.block_size_limit;
     m_get_info_time = now;
+    m_height_time = now;
   }
   return boost::optional<std::string>();
 }
 
 boost::optional<std::string> NodeRPCProxy::get_height(uint64_t &height)
 {
+  const time_t now = time(NULL);
+  if (now < m_height_time + 30) // re-cache every 30 seconds
+  {
+    height = m_height;
+    return boost::optional<std::string>();
+  }
+
   auto res = get_info();
   if (res)
     return res;

src/wallet/node_rpc_proxy.h

@@ -97,6 +97,7 @@ private:
   crypto::hash m_rpc_payment_seed_hash;
   crypto::hash m_rpc_payment_next_seed_hash;
   uint32_t m_rpc_payment_cookie;
+  time_t m_height_time;
 };
 
 }