Tick function is not working correctly inside fibers

[Alimadadi]

Description

The following code:

<?php

declare(ticks=1);

register_tick_function(function(){
  if(Fiber::getCurrent() !== null) {
    Fiber::suspend();
  }
});


for($i = 3; $i--;)
{
  $fibers[$i] = new fiber(function (){
    echo "1\n";
    echo "2\n";
    echo "3\n";
  });

  $fibers[$i]->start();
}

Resulted in this output:

1
1
2
3
1
2
3

But I expected this output instead:

1
1
1

Explanation:
If we are inside a fiber, after each tick, the tick function should call Fiber::suspend().
But currently this tick function only works inside first Fiber.

PHP Version

PHP 8.1.8

Operating System

macOS

[KapitanOczywisty]

Tick is working fine (https://3v4l.org/tGSha#v8.1.8), just by design Fiber cannot be stopped from outside the Fiber:

If this method is called from outside a fiber, a FiberError will be thrown.
-- https://www.php.net/manual/en/fiber.suspend.php

Only thing missing is that FiberError.

[Alimadadi]

Let's try another example to see how things work:

<?php

declare(ticks=1);

register_tick_function(function(){
  if(Fiber::getCurrent() === null)
  {
    echo "\tTICK - outside fiber\n";
    return;
  }

    echo "\tTICK - inside fiber\n";
    // Fiber::suspend();
    // echo "\tTICK - fiber suspended!\n";
});

echo "BEFORE:1\n";
echo "BEFORE:2\n";
echo "BEFORE:3\n";

foreach (range('A', 'C') as $v)
{
  $fibers[$v] = new fiber(function () use ($v){
    echo "FIBER {$v}:1\n";
    echo "FIBER {$v}:2\n";
    echo "FIBER {$v}:3\n";
  });

  $fibers[$v]->start();
}

echo "AFTER:1\n";
echo "AFTER:2\n";
echo "AFTER:3\n";

Here is the output:

	TICK - outside fiber
BEFORE:1
	TICK - outside fiber
BEFORE:2
	TICK - outside fiber
BEFORE:3
	TICK - outside fiber
	TICK - outside fiber
FIBER A:1
	TICK - inside fiber
FIBER A:2
	TICK - inside fiber
FIBER A:3
	TICK - inside fiber
	TICK - outside fiber
	TICK - outside fiber
FIBER B:1
	TICK - inside fiber
FIBER B:2
	TICK - inside fiber
FIBER B:3
	TICK - inside fiber
	TICK - outside fiber
	TICK - outside fiber
FIBER C:1
	TICK - inside fiber
FIBER C:2
	TICK - inside fiber
FIBER C:3
	TICK - inside fiber
	TICK - outside fiber
	TICK - outside fiber
AFTER:1
	TICK - outside fiber
AFTER:2
	TICK - outside fiber
AFTER:3
	TICK - outside fiber

Let's see what is happening here:

• Expected: Tick function is called after every tick, no matter if it is inside a fiber or outside of fibers.
• Expected: When we are inside a fiber, tick function is also called inside that fiber. In another word, Fiber::getCurrent() does not return null. So, we should also be able to call Fiber::suspend() too, right?

Now let's uncomment the // Fiber::suspend(); and // echo "\tTICK - fiber suspended!\n"; lines of this example's tick function, and see what is the results.

	TICK - outside fiber
BEFORE:1
	TICK - outside fiber
BEFORE:2
	TICK - outside fiber
BEFORE:3
	TICK - outside fiber
	TICK - outside fiber
FIBER A:1
	TICK - inside fiber
FIBER B:1
FIBER B:2
FIBER B:3
FIBER C:1
FIBER C:2
FIBER C:3
AFTER:1
AFTER:2
AFTER:3

Let's see what happened after uncommenting the // Fiber::suspend(); and // echo "\tTICK - fiber suspended!\n"; lines of the tick function:

• Expected: Tick function is called on every tick, just as normal.
• Expected: When PHP interpreter tries to execute code inside a fiber, Fiber::suspend() suspends the fiber. We no longer see FIBER A:2 and FIBER A:3 lines in the output.
• Unexpected: All instructions inside tick function after Fiber::suspend() no longer executes. We should see \tTICK - fiber suspended!\n in the output.
• Unexpected: Tick function no longer is called. No matter if we are inside another fiber or outside of fibers.

Clearly, this is a bug that somehow unregisters (and also returns) the tick function when Fiber::suspend() is called inside a tick function.
And no, FiberError should not be thrown, because tick function is called inside fiber, which is the expected behavior.

[Alimadadi]

I just found out what is exactly happening here.

Let's add these lines just after the code in my last comment:

echo "Let's resume the Fiber A...\n";
$fibers['A']->resume();

echo "Let's resume the Fiber A, again...\n";
$fibers['A']->resume();

echo "Let's resume the Fiber A, again...\n";
$fibers['A']->resume();

echo "Done!\n";

Our code should look like this now:

<?php

declare(ticks=1);

register_tick_function(function(){
  if(Fiber::getCurrent() === null)
  {
    return print("\tTICK - outside fiber\n");
  }

    echo "\tTICK - inside fiber\n";
    Fiber::suspend();
    echo "\tTICK - fiber suspended!\n";
});

echo "BEFORE:1\n";
echo "BEFORE:2\n";
echo "BEFORE:3\n";

foreach (range('A', 'C') as $v)
{
  $fibers[$v] = new fiber(function () use ($v){
    echo "FIBER {$v}:1\n";
    echo "FIBER {$v}:2\n";
    echo "FIBER {$v}:3\n";
  });

  $fibers[$v]->start();
}

echo "AFTER:1\n";
echo "AFTER:2\n";
echo "AFTER:3\n";

echo "Let's resume the Fiber A...\n";
$fibers['A']->resume();

echo "Let's resume the Fiber A, again...\n";
$fibers['A']->resume();

echo "Let's resume the Fiber A, again...\n";
$fibers['A']->resume();

echo "Done!\n";

The output will be this:

	TICK - outside fiber
BEFORE:1
	TICK - outside fiber
BEFORE:2
	TICK - outside fiber
BEFORE:3
	TICK - outside fiber
	TICK - outside fiber
FIBER A:1
	TICK - inside fiber
FIBER B:1
FIBER B:2
FIBER B:3
FIBER C:1
FIBER C:2
FIBER C:3
AFTER:1
AFTER:2
AFTER:3
Let's resume the Fiber A...
	TICK - fiber suspended!
FIBER A:2
	TICK - inside fiber
Let's resume the Fiber A, again...
	TICK - fiber suspended!
FIBER A:3
	TICK - inside fiber
Let's resume the Fiber A, again...
	TICK - fiber suspended!
	TICK - outside fiber
Done!
	TICK - outside fiber

As you see, unlike what I said before, running Fiber::suspend() inside a tick function does not unregisters the tick function, nor it will not returns the tick function, but It just suspends both the fiber and the tick function. And when we resume the suspended fiber, it resumes from the tick function!

That's because the tick function is running inside the fiber, and when we suspend the fiber, the tick function suspends too.

Even though I believe that we should be able to call Fiber::suspend() inside a tick function (Which can open a world of opportunity to create flow control tools, such as coroutines), the tick function, by definition, should not stick to the fibers and suspend itself:

register_tick_function — Register a function for execution on each tick
https://www.php.net/manual/en/function.register-tick-function.php

[cmb69]

Fibers use cooperative multitasking; what you're trying to accomplish would be preemtive multitasking. It's pretty unlikely that this will be supported.

I think we need to document this limitation, and perhaps we should block switching fibers in tick functions in the first place. @trowski, thoughts?

[Alimadadi]

@cmb69 Is there any reason or technical limitation to not allow calling of Fiber::suspend() in tick function?
Just trying to understand why "It's pretty unlikely that this will be supported" and why "we should block switching fibers in tick functions in the first place".

[cmb69]

I don't think there is a hard technical limitation; it's rather something that we may not really want to support.

[Alimadadi]

@cmb69
Ok, I see.
Using tick functions to schedule fibers somehow complicate things, I believe.

Ummm... what about adding a tiny but useful feature to Fiber class?

Just adding two $maxTicks parameters to Fiber class's methods can achieve the same goal, but is much cleaner and easier:

public Fiber::__construct(callable $callback, int $maxTicks = null)
public Fiber::resume(mixed $value = null, int $maxTicks = null)

Default value should be set to null, which will work as normal (like current version of PHP).

A $maxTicks = 0 in constructor will suspend the fiber immediately after start(), any other positive value will run the fiber for exact same number of ticks and then suspends the fiber, unless suspend() is called sooner inside that fiber.

Constructor's $maxTicks parameter will be forced on next calls to resume() too, unless a new value is set during resume() call, which replaces that value for next resume() calls too.

resume() can accept $maxTicks = null, which will reset settings to default behavior (no tick limits). And maybe resume() should not accept $maxTicks = 0.

I understand that Fibers try to add cooperative multitasking to PHP's core, but this small feature can be very useful in many cases, and there will be no harm I can think of, including breaking the backward compatibility. I personally can't see any problem on Fibers supporting preemptive multitasking too.

I don't know if I should submit a RFC or what to request this feature.

[cmb69]

I don't know if I should submit a RFC or what to request this feature.

Well, at least some discussion on the internals list seems to be in order.

[trowski]

@cmb69 The most sensible thing to do IMO is block switching fibers in the tick function. Should that target 8.1 or master? I can look a providing a PR.

The tick function may be able to be made re-entrant, but preemptive scheduling was not the design goal of Fibers, so is not something I'm interested in implementing. Allowing arbitrary suspension of a fiber even further complicates design logic. Fibers do not exist completely independently like OS processes, so arbitrary suspension can lead to unexpected state changes at any time, not just during a function call.

[cmb69]

@trowski, I agree. I'd target "master" only.

[kelunik]

As you see, unlike what I said before, running Fiber::suspend() inside a tick function does not unregisters the tick function, nor it will not returns the tick function, but It just suspends both the fiber and the tick function. And when we resume the suspended fiber, it resumes from the tick function!

That's because the tick function is running inside the fiber, and when we suspend the fiber, the tick function suspends too.

I haven't really followed your examples closely, but given this reasoning, everything seems to be working as expected to me? You should be able to implement these $maxTicks yourself using a tick function, no?