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Date: Sat, 10 Feb 2018 17:56:30 +0800
Cc: Aaron Piotrowski <aaron@trowski.com>,
 PHP Internals <internals@lists.php.net>,
 Dmitry Stogov <dmitry@zend.com>
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To: Niklas Keller <me@kelunik.com>
Feedback-ID: bizesmtp:lvht.net:qybgforeign:qybgforeign4
Subject: Re: [PHP-DEV] [RFC] Fiber support (again)
From: i@lvht.net (Haitao Lv)

Hi, all,

I have updated the RFC https://wiki.php.net/rfc/fiber

changes list:

- introduce the `throw(Exception $exceptin)` API
- record issues discussed

> On Feb 9, 2018, at 08:12, Haitao Lv <i@lvht.net> wrote:
>=20
>>=20
>> On Feb 9, 2018, at 06:22, Niklas Keller <me@kelunik.com> wrote:
>>=20
>>>=20
>>>> - How do you determine when a fiber has returned? Looking at the =
source,
>>> it appears Fiber::status() must be used, comparing against =
constants.
>>> Separate methods similar to Generator would be better IMO. e.g.:
>>> Fiber::alive(), Fiber::suspended(), Fiber::running()
>>>=20
>>> Offering methods like Fiber::alive, Fiber::running makes no =
difference to
>>> check the Fiber::status() return value. This is just a style issue. =
And as
>>> a language feature,
>>> Fiber only offer the essential API and let other works to the user =
land.
>>=20
>>=20
>> The language should offer a sane API, not the absolute minimum =
required to
>> work for these things.
>=20
> The Ruby's Fiber do offer a live? method but does not have a getStatus =
method.
> The Lua's coroutine only offer a status method.
>=20
> So do we really need to offer three additional helper method? Or what =
is your
> advice about these API?
>=20
>>=20
>>>> - What about throwing exceptions into a fiber?
>>>=20
>>> Currently does not support throw exception into the fiber. User land =
code
>>> could check
>>> the value of Fiber::yield and throw exception themselves. The Ruby's =
Fiber
>>> and Lua's
>>> coroutine also does not support such api as well.
>>=20
>>=20
>> And throw the exception where? That means async code with fibers =
can't
>> really handle errors?
>=20
> Actually you can transfer any thing to Fiber by the resume method. And =
you can
> check the return value of Fiber::yield to handle error.
>=20
> Fiber is designed as a primitive, low level, and lightweight feature. =
User land
> code seldom not need to use them directly in your normal code.
> So the following is not a big problem,
>=20
> $a =3D Fiber::yield(...);
> if ($a =3D=3D=3D false) {
>   throw new Exception(...);
> }
>=20
> And both the Ruby and Lua does not offer such API as well.
>>=20
>>>=20
>>>>=20
>>>> - Using Fiber::resume() to initialize the fiber and resume feels
>>> awkward. Separate methods again would be better here, perhaps
>>> Fiber::init(...$args) and Fiber::resume($send).
>>>=20
>>> All Fiber created with a suspended status. So make resume to do both =
the
>>> init and resume
>>> do make sense.
>>>=20
>>=20
>> It does't make sense to me. Reading the example in the README and
>> understanding why the first resume() takes two arguments instead of =
one
>> took me quite some minutes.
>=20
> This Ruby's Fiber and Lua's coroutine using one resume API to init and =
resume
> the coroutine. I do not think a dedicate is really required.
>=20
> The generator cannot be init by it's send method. And if you want to =
implement
> coroutine feature(without stack) by it, you have to write code
>=20
> function run() {
>    if ($this->beforeFirstYield) {
>        $this->beforeFirstYield =3D false;
>        return $this->coroutine->current();
>    } else {
>        $retval =3D $this->coroutine->send($this->sendValue);
>        $this->sendValue =3D null;
>        return $retval;     =20
>    }  =20
> }
>=20
> It is verbose.
>=20
> See =
https://nikic.github.io/2012/12/22/Cooperative-multitasking-using-coroutin=
es-in-PHP.html
>=20
>>=20
>>> Please see Ruby Fiber API =
https://ruby-doc.org/core-2.2.0/Fiber.html.
>>>=20
>>>>=20
>>>> - What happens if the sub1() function in the RFC is invoked outside =
of a
>>> fiber?
>>>=20
>>> You will get a Fatal Error like
>>> Fatal error: Uncaught Error: Cannot call Fiber::yield out of Fiber
>>>=20
>>>> - I think a keyword here would be beneficial, even if it has a =
minor BC
>>> impact. Fibers could then be written like generators. `await` or =
`emit` as
>>> a keyword perhaps? This would be a less verbose API, feel less =
magical (a
>>> static method call that actually pauses execution feels out of =
place), and
>>> would allow Fibers to be returned from methods, named functions, etc =
with
>>> less boilerplate.
>>>=20
>>> Wishing this to be accepted by the community in the PHP 7.3, so no =
keyword
>>> is accepted.
>>> And if the community cannot accept, the Fiber can be still =
distributed as
>>> a standalone
>>> extension. So we cannot depend on a new keyword.
>>=20
>>=20
>> Right, if it's a standalone extension it can't use a new keyword, but =
as a
>> language feature it totally can.
>=20
> In my opinion, using a keyword or call a method is just a coding style =
problem.
> Introducing a new keyword does not offer any benefit by makes a minor =
BC.
>=20
> Both Ruby's Fiber and Lua's coroutine does not required a dedicate =
keyword.
>=20
>> Looking at the current README, there are two issues that must be =
completely
>> solved IMO before accepting this:
>>=20
>>> Each Fiber has a separate 4k stack. You can use the fiber.stack_size =
ini
>> option to change the default stack size. You can also use the second
>> argument of Fiber::__construct to set the stack size on fly.
>>=20
>> Resizing of the stack should happen automatically, just like =
generators
>> resize automatically.
>=20
> This size is the init stack size. It means when a Fiber created, it =
will get a
> dedicate stack of 4k size. When the fiber use all the stack space, =
zend vm will
> allocate additional space for feature call frame, automatically.
>=20
> The default size is 4k means that every fiber requires at least 4k =
memory to use
> as there own stack. But user can change this by php.ini and the =
construct argument
> to reduce the memory footprint.
>=20
>>=20
>>> Fiber::yield cannot be used in internal callback
>>=20
>> This also seems problematic and will make fibers quite less useful,
>> especially as these yields can happen anywhere down the stack.
>>=20
>=20
> This do be a problem. But not so much big problem.
>=20
> You cannot use Fiber::yield like
>=20
> $f =3D new Fiber(function () {
>  array_map(function ($i) {
>    Fiber::yield($i);
>  }, [1, 2, 3]);
> });
> $f->resume();
> $f->resume();
>=20
> Because when zend execute the array_map, it will push a new frame onto =
the c stack.
> When zend execute Fiber::yield, it only backup it's php stack, and =
it's c stack
> will be overwrites.
>=20
> However, you can use Fiber::yield like
>=20
> $f =3D new Fiber(function () {
>   Fiber::yield(1); // will cause by resume, by an internal callback
> });
>=20
> ExtEventLoop::onRead($fd, function () { // this is an internal call
>   $f->resume();
> });=20
>=20
>> Regards, Niklas
>=20
>=20
>=20
>=20
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