AP_Scripting: add writestring for serial

Enables more efficient scripting.

libraries/AP_Scripting/docs/docs.lua

@@ -1242,6 +1242,13 @@ function AP_Scripting_SerialAccess_ud:begin(baud_rate) end
 ---@return uint32_t_ud -- 1 if success else 0
 function AP_Scripting_SerialAccess_ud:write(value) end
 
+-- Writes a string. The number of bytes actually written, i.e. the length of the
+-- written prefix of the string, is returned. It may be 0 up to the length of
+-- the string.
+---@param data string -- string of bytes to write
+---@return integer -- number of bytes actually written, which may be 0
+function AP_Scripting_SerialAccess_ud:writestring(data) end
+
 -- Reads a single byte from the serial port
 ---@return integer -- byte, -1 if error or none available
 function AP_Scripting_SerialAccess_ud:read() end

libraries/AP_Scripting/generator/description/bindings.desc

@@ -397,6 +397,7 @@ include AP_Scripting/AP_Scripting_SerialAccess.h
 userdata AP_Scripting_SerialAccess creation null -1
 userdata AP_Scripting_SerialAccess method begin void uint32_t 1U UINT32_MAX
 userdata AP_Scripting_SerialAccess method write uint32_t uint8_t'skip_check
+userdata AP_Scripting_SerialAccess manual writestring lua_serial_writestring 1 1
 userdata AP_Scripting_SerialAccess method read int16_t
 userdata AP_Scripting_SerialAccess manual readstring lua_serial_readstring 1 1
 userdata AP_Scripting_SerialAccess method available uint32_t

libraries/AP_Scripting/lua_bindings.cpp

@@ -743,6 +743,25 @@ int lua_serial_find_serial(lua_State *L) {
 }
 #endif // HAL_GCS_ENABLED
 
+int lua_serial_writestring(lua_State *L)
+{
+    binding_argcheck(L, 2);
+
+    AP_Scripting_SerialAccess * port = check_AP_Scripting_SerialAccess(L, 1);
+
+    // get the bytes the user wants to write, along with their length
+    size_t req_bytes;
+    const char *data = lua_tolstring(L, 2, &req_bytes);
+
+    // write up to that number of bytes
+    const uint32_t written_bytes = port->write((const uint8_t*)data, req_bytes);
+
+    // return the number of bytes that were actually written
+    lua_pushinteger(L, written_bytes);
+
+    return 1;
+}
+
 int lua_serial_readstring(lua_State *L) {
     binding_argcheck(L, 2);
 

libraries/AP_Scripting/lua_bindings.h

@@ -11,6 +11,7 @@ int AP_HAL__I2CDevice_read_registers(lua_State *L);
 int lua_get_CAN_device(lua_State *L);
 int lua_get_CAN_device2(lua_State *L);
 int lua_serial_find_serial(lua_State *L);
+int lua_serial_writestring(lua_State *L);
 int lua_serial_readstring(lua_State *L);
 int lua_dirlist(lua_State *L);
 int lua_removefile(lua_State *L);