Hi!

The patch looks good for me but could you please add a small test case.
Some examples might be found in test/app/fiber.test.lua file.

On Friday, February 22, 2019 10:53:58 PM MSK Cyrill Gorcunov wrote:
> If I create a new fiber and the join to himself we get an abort:
>  | tarantool> f = require('fiber')
>  | ---
>  | ...
>  | 
>  | tarantool> f.join(f.self())
>  | tarantool: src/fiber.c:407: fiber_join: Assertion `fiber->flags &
>  | FIBER_IS_JOINABLE' failed. Aborted (core dumped)
> 
> we should better throw an error.
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
> ---
>  src/lua/fiber.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> Index: tarantool.git/src/lua/fiber.c
> ===================================================================
> --- tarantool.git.orig/src/lua/fiber.c
> +++ tarantool.git/src/lua/fiber.c
> @@ -676,10 +676,14 @@ lbox_fiber_join(struct lua_State *L)
>  {
>  	struct fiber *fiber = lbox_checkfiber(L, 1);
>  	struct lua_State *child_L = fiber->storage.lua.stack;
> -	fiber_join(fiber);
>  	struct error *e = NULL;
>  	int num_ret = 0;
>  	int coro_ref = 0;
> +
> +	if (!(fiber->flags & FIBER_IS_JOINABLE))
> +		luaL_error(L, "the fiber is not joinable");
> +	fiber_join(fiber);
> +
>  	if (child_L != NULL) {
>  		coro_ref = lua_tointeger(child_L, -1);
>  		lua_pop(child_L, 1);