[PR pending] SystemLayerImplSelect: libev: avoid timers firing slight…

…ly too early (project-chip#28434)

This fixes a ReportEngine problem that was caused by libev-based timers
firing slightly (in the range of 20mS) too early, because libev by
default uses the time when events started processing as the "now"
reference for relative timers for efficiency reasons.

To ensure timers cannot fire early, timer setup must compensate for
any difference between ev_now() which is the time events started
processing and ev_time(), which is the actual current time (but is
a bit less efficient to obtain).

# Conflicts:
#	src/system/SystemLayerImplSelect.cpp

src/system/SystemLayerImplSelect.cpp

@@ -184,7 +184,12 @@ CHIP_ERROR LayerImplSelect::StartTimer(Clock::Timeout delay, TimerCompleteCallba
  ev_timer_init(&timer->mLibEvTimer, &LayerImplSelect::HandleLibEvTimer, 1, 0);
  timer->mLibEvTimer.data = timer;
  auto t = Clock::Milliseconds64(delay).count();
- ev_timer_set(&timer->mLibEvTimer, static_cast<double>(t) / 1E3, 0.);
+ // Note: libev uses the time when events started processing as the "now" reference for relative timers,
+ // for efficiency reasons. This point in time is represented by ev_now().
+ // The real time is represented by ev_time().
+ // As some chip code relies on timers to fire NEVER even only a bit early, we must increase the
+ // relative value passed to ev_timer_set().
+ ev_timer_set(&timer->mLibEvTimer, (static_cast<double>(t) / 1E3) + ev_time() - ev_now(mLibEvLoopP), 0.);
  (void) mTimerList.Add(timer);
  ev_timer_start(mLibEvLoopP, &timer->mLibEvTimer);
  return CHIP_NO_ERROR;