Add read locks to the Lua runner

Support requesting read locks from Lua contracts instead
of only write-locks.

Signed-off-by: Michael Maurer <maurer.mi@northeastern.edu>

scripts/gen_bytecode.lua

@@ -1,5 +1,10 @@
 function gen_bytecode()
     pay_contract = function(param)
+        -- 0 is a read lock, 1 is a write lock in C++ scope
+        Locks = {
+            READ = 0,
+            WRITE = 1,
+        }
         from, to, value, sequence, sig = string.unpack("c32 c32 I8 I8 c64", param)
 
         function get_account_key(name)
@@ -10,7 +15,8 @@ function gen_bytecode()
 
         function get_account(name)
             account_key = get_account_key(name)
-            account_data = coroutine.yield(account_key)
+
+            account_data = coroutine.yield(account_key, Locks.WRITE)
             if string.len(account_data) > 0 then
                 account_balance, account_sequence
                 = string.unpack("I8 I8", account_data)

src/parsec/agent/runners/interface.hpp

@@ -38,7 +38,7 @@ namespace cbdc::parsec::agent::runner {
             internal_error,
             /// Function yielded more than one key to lock.
             yield_count,
-            /// Function yielded a non-string key.
+            /// Function yielded a invalid datatype.
             yield_type,
             /// Error acquiring lock on key.
             lock_error,

src/parsec/agent/runners/lua/impl.cpp

@@ -138,25 +138,55 @@ namespace cbdc::parsec::agent::runner {
         return buf;
     }
 
+    auto lua_runner::get_stack_integer(int index) -> std::optional<int64_t> {
+        if(!lua_isinteger(m_state.get(), index)) {
+            return std::nullopt;
+        }
+        return lua_tointeger(m_state.get(), index);
+    }
+
     void lua_runner::schedule_contract() {
         int n_results{};
         auto resume_ret = lua_resume(m_state.get(), nullptr, 1, &n_results);
         if(resume_ret == LUA_YIELD) {
-            if(n_results != 1) {
-                m_log->error("Contract yielded more than one key");
+            if(n_results > 2) {
+                m_log->error("Contract yielded more than two keys");
+                m_result_callback(error_code::yield_count);
+                return;
+            } else if(n_results < 1) {
+                m_log->error("Contract yielded no keys");
                 m_result_callback(error_code::yield_count);
                 return;
             }
+
+            auto lock_level = broker::lock_type::write;
+            if(n_results == 2) {
+                auto lock_type = get_stack_integer(-1);
+                if(!lock_type.has_value()) {
+                    m_log->error("Contract yielded two keys, but the second "
+                                 "is not an integer");
+                    m_result_callback(error_code::yield_type);
+                    return;
+                }
+                lua_pop(m_state.get(), 1);
+
+                lock_level = (lock_type.value() == 0)
+                               ? broker::lock_type::read
+                               : broker::lock_type::write;
+            }
+
             auto key_buf = get_stack_string(-1);
             if(!key_buf.has_value()) {
                 m_log->error("Contract did not yield a string");
                 m_result_callback(error_code::yield_type);
                 return;
             }
-            lua_pop(m_state.get(), n_results);
+
+            lua_pop(m_state.get(), 1);
+
             auto success
                 = m_try_lock_callback(std::move(key_buf.value()),
-                                      broker::lock_type::write,
+                                      lock_level,
                                       [&](auto res) {
                                           handle_try_lock(std::move(res));
                                       });

src/parsec/agent/runners/lua/impl.hpp

@@ -18,6 +18,10 @@ namespace cbdc::parsec::agent::runner {
     /// function execution, signature checking and commiting execution results.
     /// Class cannot be re-used for different functions/transactions, manages
     /// the lifecycle of a single transaction.
+    /// NOTE: When writing contracts, to pass data between the Lua environment
+    /// and the C++ environment, use `coroutine.yield()`. To request a
+    /// read-lock use coroutine.yield(<data>, 0). To request a write-lock use
+    /// coroutine.yield(<data>, 1) or coroutine.yield(<data>).
     class lua_runner : public interface {
       public:
         /// \copydoc interface::interface()
@@ -47,6 +51,8 @@ namespace cbdc::parsec::agent::runner {
 
         auto get_stack_string(int index) -> std::optional<buffer>;
 
+        auto get_stack_integer(int index) -> std::optional<int64_t>;
+
         void schedule_contract();
 
         void