Make setTimeout and setImmediate timers run consecutively

[bnoordhuis]

Continuing from libuv/libuv#1035 (comment):

Node.js uses a uv_idle_t handle to implement setImmediate() timers but I think it could - and should - hang them off the big setTimeout() timer.

Right now you have this weird situation where:

a) setTimeout timers run,
b) some other callbacks run (mostly close callbacks), then
c) setImmediate timers run.

[Fishrock123]

implement setImmediate() timers

What are these? Just Immediates or timers scheduled in Immediates?

Immediates should fire after I/O before Timeouts, right?

[bnoordhuis]

Just Immediates or timers scheduled in Immediates?

Immediates.

Immediates should fire after I/O before Timeouts, right?

That's right, the actual callbacks are made from a check handle.

I realize what tripped me up: we call uv_run(UV_RUN_ONCE) in a loop, causing the big setTimeout timer to run twice per loop tick sometimes.

The order of events is approximately: start tick -> timeouts -> I/O -> immediates -> close callbacks -> timeouts again -> end tick.

[Trott]

Should this stay open?

If so, @Fishrock123: Are you likely to get to this soon? If not, should we unassign you and/or add mentor available and help wanted labels?

[Fishrock123]

I realize what tripped me up: we call uv_run(UV_RUN_ONCE) in a loop, causing the big setTimeout timer to run twice per loop tick sometimes.

The uv_run(UV_RUN_ONCE) part was changed in 9e08695 ... @bnoordhuis any idea if the ordering issues are still present? (Although we may want to make the change regardless.)

[bnoordhuis]

9e08695 should have fixed the ordering, yes. The issue of timer callbacks and immediate callbacks getting interleaved with other callbacks still exists.

[j0t3x]

Is this taken? I would like to work on this one.

[apapirovski]

Hi @j0t3x, unfortunately I don't know if there's a clear direction here even... It's a strong possibility that there might be code out there relying on immediates running after I/O.

(For reference http://docs.libuv.org/en/v1.x/design.html)

Also, the fact that setImmediate will run within the same iteration of the loop for everything but close callbacks is IMO a good thing.

[j0t3x]

The issue of timer callbacks and immediate callbacks getting interleaved with other callbacks still exists.
@apapirovski how can I help investigate this one?