simplify virtual timer and improve compatibility

Previous version of virtual timer used minimal modifications.
This one instead implements poll() directly, simplifying execution.
It also enforces one timer each poll, which makes it compatible with
one network even per poll cycle when network events are replaced by
timers for simulation.

Signed-off-by: Csaba Kiraly <csaba.kiraly@gmail.com>

chronos/asyncloop.nim

@@ -769,6 +769,36 @@ elif unixPlatform:
     var curTime = Moment.now()
     var curTimeout = 0
 
+    when asyncTimer == "virtual":
+      # Execute all queued callbacks and at most one timer event at a time.
+      # This is the closest to the one network event at a time semantics we have in other modalities.
+      # Enforcing one timer each poll is useful to make it compatible with original semantics when
+      # network events are replaced by timers for simulation.
+
+      # Execute queued callbacks in order.
+      while loop.callbacks.len > 0:
+        let cb = loop.callbacks.popFirst()
+        if not isSentinel(cb):
+          cb.function(cb.udata)
+
+      # Fill callbacks as needed, consuming at most one timer.
+      if loop.timers.len > 0:
+        let
+          timer = loop.timers.pop()
+          finish = timer.finishAt
+          callable = timer.function
+
+        if not(isNil(callable.function)):
+          loop.callbacks.addFirst(callable)
+
+        # Advance virtual time in simulation until next timer.
+        Moment.advance(finish - Moment.now())
+
+      # Handle idlers here, closest we can get to original definition.
+      else:
+        loop.processIdlers()
+      return
+
     when ioselSupportedPlatform:
       let customSet = {Event.Timer, Event.Signal, Event.Process,
                        Event.Vnode}
@@ -781,14 +811,8 @@ elif unixPlatform:
     # Moving expired timers to `loop.callbacks` and calculate timeout.
     loop.processTimersGetTimeout(curTimeout)
 
-    let timeout =
-      when asyncTimer == "virtual":
-        0
-      else:
-        curTimeout
-
     # Processing IO descriptors and all hardware events.
-    let count = loop.selector.selectInto(timeout, loop.keys)
+    let count = loop.selector.selectInto(curTimeout, loop.keys)
     for i in 0..<count:
       let fd = loop.keys[i].fd
       let events = loop.keys[i].events
@@ -827,10 +851,6 @@ elif unixPlatform:
     # All callbacks done, skip `processCallbacks` at start.
     loop.callbacks.addFirst(SentinelCallback)
 
-    # Advance virtual time in simulation if nothing expected in curTimeout millisec
-    when asyncTimer == "virtual":
-      Moment.advance(curTimeout.milliseconds)
-
 else:
   proc initAPI() = discard
   proc globalInit() = discard