lua: new function luaT_return_error()

Currently, if we have to return errors using the format
'return nil, error', we must do it manually. Since this error
return method is described in our Lua coding style, it makes sense
to create a function that will return an error in this format.
This patch creates a function.

Needed for #4390

src/box/lua/session.c

@@ -395,14 +395,10 @@ lbox_session_push(struct lua_State *L)
 	}
 	struct port port;
 	port_lua_create(&port, L);
-	if (session_push(session, sync, &port) != 0) {
-		lua_pushnil(L);
-		luaT_pusherror(L, box_error_last());
-		return 2;
-	} else {
-		lua_pushboolean(L, true);
-		return 1;
-	}
+	if (session_push(session, sync, &port) != 0)
+		return luaT_return_error(L);
+	lua_pushboolean(L, true);
+	return 1;
 }
 
 /**

src/lua/error.c

@@ -104,6 +104,16 @@ luaT_error(lua_State *L)
 	return 0;
 }
 
+int
+luaT_return_error(lua_State *L)
+{
+	struct error *e = diag_last_error(&fiber()->diag);
+	assert(e != NULL);
+	lua_pushnil(L);
+	luaT_pusherror(L, e);
+	return 2;
+}
+
 void
 tarantool_lua_error_init(struct lua_State *L)
 {

src/lua/error.h

@@ -49,6 +49,15 @@ struct error;
 LUA_API int
 luaT_error(lua_State *L);
 
+/**
+ * Return nil as the first return value and an error as the
+ * second. The error is received using box_error_last().
+ *
+ * @param L Lua stack.
+ */
+LUA_API int
+luaT_return_error(lua_State *L);
+
 void
 luaT_pusherror(struct lua_State *L, struct error *e);
 /** \endcond public */

src/lua/fio.c

@@ -47,11 +47,9 @@
 #include "lua/utils.h"
 #include "coio_file.h"
 
-static inline void
-lbox_fio_pushsyserror(struct lua_State *L)
-{
-	diag_set(SystemError, "fio: %s", strerror(errno));
-	luaT_pusherror(L, diag_get()->last);
+#define lbox_fio_pushsyserror(L) {				\
+	diag_set(SystemError, "fio: %s", strerror(errno));	\
+	return luaT_return_error(L);				\
 }
 
 static int
@@ -69,11 +67,8 @@ lbox_fio_open(struct lua_State *L)
 	int mode = lua_tointeger(L, 3);
 
 	int fh = coio_file_open(pathname, flags, mode);
-	if (fh < 0) {
-		lua_pushnil(L);
+	if (fh < 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_pushinteger(L, fh);
 	return 1;
 }
@@ -90,11 +85,8 @@ lbox_fio_pwrite(struct lua_State *L)
 	size_t offset = lua_tonumber(L, 4);
 
 	int res = coio_pwrite(fh, buf, len, offset);
-	if (res < 0) {
-		lua_pushnil(L);
+	if (res < 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_pushinteger(L, res);
 	return 1;
 }
@@ -115,11 +107,8 @@ lbox_fio_pread(struct lua_State *L)
 
 	int res = coio_pread(fh, buf, len, offset);
 
-	if (res < 0) {
-		lua_pushnil(L);
+	if (res < 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_pushinteger(L, res);
 	return 1;
 }
@@ -129,7 +118,10 @@ lbox_fio_pushbool(struct lua_State *L, bool res)
 {
 	lua_pushboolean(L, res);
 	if (!res) {
-		lbox_fio_pushsyserror(L);
+		diag_set(SystemError, "fio: %s", strerror(errno));
+		struct error *e = diag_last_error(diag_get());
+		assert(e != NULL);
+		luaT_pusherror(L, e);
 		return 2;
 	}
 	return 1;
@@ -211,11 +203,8 @@ lbox_fio_write(struct lua_State *L)
 	size_t len = lua_tonumber(L, 3);
 
 	int res = coio_write(fh, buf, len);
-	if (res < 0) {
-		lua_pushnil(L);
+	if (res < 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_pushinteger(L, res);
 	return 1;
 }
@@ -298,11 +287,8 @@ lbox_fio_read(struct lua_State *L)
 
 	int res = coio_read(fh, buf, len);
 
-	if (res < 0) {
-		lua_pushnil(L);
+	if (res < 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_pushinteger(L, res);
 	return 1;
 }
@@ -371,11 +357,8 @@ DEF_STAT_METHOD(is_sock, S_ISSOCK);
 static int
 lbox_fio_pushstat(struct lua_State *L, int res, const struct stat *stat)
 {
-	if (res < 0) {
-		lua_pushnil(L);
+	if (res < 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_newtable(L);
 
 	PUSHTABLE("dev", lua_pushinteger, stat->st_dev);
@@ -518,11 +501,8 @@ lbox_fio_listdir(struct lua_State *L)
 	}
 	pathname = lua_tostring(L, 1);
 	char *buf;
-	if (coio_readdir(pathname, &buf) < 0) {
-		lua_pushnil(L);
+	if (coio_readdir(pathname, &buf) < 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_pushstring(L, buf);
 	free(buf);
 	return 1;
@@ -613,11 +593,8 @@ lbox_fio_readlink(struct lua_State *L)
 		goto usage;
 	char *path = (char *)lua_newuserdata(L, PATH_MAX);
 	int res = coio_readlink(pathname, path, PATH_MAX);
-	if (res < 0) {
-		lua_pushnil(L);
+	if (res < 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_pushlstring(L, path, res);
 	lua_remove(L, -2);
 	return 1;
@@ -629,16 +606,11 @@ lbox_fio_tempdir(struct lua_State *L)
 	char *buf = (char *)lua_newuserdata(L, PATH_MAX);
 	if (!buf) {
 		errno = ENOMEM;
-		lua_pushnil(L);
 		lbox_fio_pushsyserror(L);
-		return 2;
 	}
 
-	if (coio_tempdir(buf, PATH_MAX) != 0) {
-		lua_pushnil(L);
+	if (coio_tempdir(buf, PATH_MAX) != 0)
 		lbox_fio_pushsyserror(L);
-		return 2;
-	}
 	lua_pushstring(L, buf);
 	lua_remove(L, -2);
 	return 1;
@@ -650,20 +622,12 @@ lbox_fio_cwd(struct lua_State *L)
 	char *buf = (char *)lua_newuserdata(L, PATH_MAX);
 	if (!buf) {
 		errno = ENOMEM;
-		lua_pushnil(L);
 		lbox_fio_pushsyserror(L);
-		return 2;
 	}
-
-
-	if (getcwd(buf, PATH_MAX)) {
-		lua_pushstring(L, buf);
-		lua_remove(L, -2);
-	} else {
+	if (getcwd(buf, PATH_MAX) == NULL)
 		lbox_fio_pushsyserror(L);
-		lua_pushnil(L);
-		return 2;
-	}
+	lua_pushstring(L, buf);
+	lua_remove(L, -2);
 	return 1;
 }
 

src/lua/swim.c

@@ -69,11 +69,10 @@ lua_swim_new(struct lua_State *L)
 {
 	uint64_t generation = luaL_checkuint64(L, 1);
 	struct swim *s = swim_new(generation);
+	if (s == NULL)
+		return luaT_return_error(L);
 	*(struct swim **) luaL_pushcdata(L, ctid_swim_ptr) = s;
-	if (s != NULL)
-		return 1;
-	luaT_pusherror(L, diag_last_error(diag_get()));
-	return 2;
+	return 1;
 }
 
 /**