api: execute lua directly from the remote api

src/nvim/api/vim.c

@@ -15,6 +15,7 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/api/buffer.h"
 #include "nvim/msgpack_rpc/channel.h"
+#include "nvim/lua/executor.h"
 #include "nvim/vim.h"
 #include "nvim/buffer.h"
 #include "nvim/file_search.h"
@@ -254,6 +255,25 @@ free_vim_args:
   return rv;
 }
 
+/// Execute lua code. Parameters might be passed, they are available inside
+/// the chunk as `...`. The chunk can return a value.
+///
+/// To evaluate an expression, it must be prefixed with "return ". For
+/// instance, to call a lua function with arguments sent in and get its
+/// return value back, use the code "return my_function(...)".
+///
+/// @param code       lua code to execute
+/// @param args       Arguments to the code
+/// @param[out] err   Details of an error encountered while parsing
+///                   or executing the lua code.
+///
+/// @return           Return value of lua code if present or NIL.
+Object nvim_execute_lua(String code, Array args, Error *err)
+  FUNC_API_SINCE(3) FUNC_API_REMOTE_ONLY
+{
+  return executor_exec_lua_api(code, args, err);
+}
+
 /// Calculates the number of display cells occupied by `text`.
 /// <Tab> counts as one cell.
 ///

src/nvim/lua/executor.c

@@ -373,6 +373,46 @@ static int nlua_eval_lua_string(lua_State *const lstate)
   return 0;
 }
 
+/// Evaluate lua string
+///
+/// Expects four values on the stack: string to evaluate, pointer to args array,
+/// and locations where result and error are saved, respectively. Always
+/// returns nothing (from the lua point of view).
+static int nlua_exec_lua_string_api(lua_State *const lstate)
+    FUNC_ATTR_NONNULL_ALL
+{
+  const String *str = (const String *)lua_touserdata(lstate, 1);
+  const Array *args = (const Array *)lua_touserdata(lstate, 2);
+  Object *retval = (Object *)lua_touserdata(lstate, 3);
+  Error *err = (Error *)lua_touserdata(lstate, 4);
+
+  lua_pop(lstate, 4);
+
+  if (luaL_loadbuffer(lstate, str->data, str->size, "<nvim>")) {
+    size_t len;
+    const char *str = lua_tolstring(lstate, -1, &len);
+    api_set_error(err, kErrorTypeValidation,
+                  "Error loading lua: %.*s", (int)len, str);
+    return 0;
+  }
+
+  for (size_t i = 0; i < args->size; i++) {
+    nlua_push_Object(lstate, args->items[i]);
+  }
+
+  if (lua_pcall(lstate, (int)args->size, 1, 0)) {
+    size_t len;
+    const char *str = lua_tolstring(lstate, -1, &len);
+    api_set_error(err, kErrorTypeException,
+                  "Error executing lua: %.*s", (int)len, str);
+    return 0;
+  }
+
+  *retval = nlua_pop_Object(lstate, err);
+
+  return 0;
+}
+
 /// Print as a Vim message
 ///
 /// @param  lstate  Lua interpreter state.
@@ -516,6 +556,28 @@ void executor_eval_lua(const String str, typval_T *const arg,
                          (void *)&str, arg, ret_tv);
 }
 
+/// Execute lua string
+///
+/// Used for nvim_execute_lua().
+///
+/// @param[in]  str  String to execute.
+/// @param[in]  args array of ... args
+/// @param[out]  err  Location where error will be saved.
+///
+/// @return Return value of the execution.
+Object executor_exec_lua_api(const String str, const Array args, Error *err)
+{
+  if (global_lstate == NULL) {
+    global_lstate = init_lua();
+  }
+
+  Object retval = NIL;
+  NLUA_CALL_C_FUNCTION_4(global_lstate, nlua_exec_lua_string_api, 0,
+                         (void *)&str, (void *)&args, &retval, err);
+  return retval;
+}
+
+
 /// Run lua string
 ///
 /// Used for :lua.

test/functional/api/vim_spec.lua

@@ -81,6 +81,36 @@ describe('api', function()
     end)
   end)
 
+  describe('nvim_execute_lua', function()
+    it('works', function()
+      meths.execute_lua('vim.api.nvim_set_var("test", 3)', {})
+      eq(3, meths.get_var('test'))
+
+      eq(17, meths.execute_lua('a, b = ...\nreturn a + b', {10,7}))
+
+      eq(NIL, meths.execute_lua('function xx(a,b)\nreturn a..b\nend',{}))
+      eq("xy", meths.execute_lua('return xx(...)', {'x','y'}))
+    end)
+
+    it('reports errors', function()
+      eq({false, 'Error loading lua: [string "<nvim>"]:1: '..
+                 "'=' expected near '+'"},
+         meth_pcall(meths.execute_lua, 'a+*b', {}))
+
+      eq({false, 'Error loading lua: [string "<nvim>"]:1: '..
+                 "unexpected symbol near '1'"},
+         meth_pcall(meths.execute_lua, '1+2', {}))
+
+      eq({false, 'Error loading lua: [string "<nvim>"]:1: '..
+                 "unexpected symbol"},
+         meth_pcall(meths.execute_lua, 'aa=bb\0', {}))
+
+      eq({false, 'Error executing lua: [string "<nvim>"]:1: '..
+                 "attempt to call global 'bork' (a nil value)"},
+         meth_pcall(meths.execute_lua, 'bork()', {}))
+    end)
+  end)
+
   describe('nvim_input', function()
     it("VimL error: does NOT fail, updates v:errmsg", function()
       local status, _ = pcall(nvim, "input", ":call bogus_fn()<CR>")