Fix: time calculations in IOCP and Monitor thread (#16583)

* Fix: invalid absolute to relative time conversions in IOCP event loop
* Fix: invalid remaining time calculation in monitor thread

Author: ysbaddaden

src/crystal/event_loop/iocp.cr

@@ -100,11 +100,9 @@ class Crystal::EventLoop::IOCP < Crystal::EventLoop
       timeout = blocking ? LibC::INFINITE : 0_i64
     elsif blocking
       if time = @timers_mutex.synchronize { @timers.next_ready? }
-        # convert absolute time of next timer to relative time, expressed in
-        # milliseconds, rounded up
-        # Cannot use `time.elapsed` here because it calls `::Time.instant` which
-        # could be mocked.
-        relative = Crystal::System::Time.instant.duration_since(time)
+        # convert absolute time to relative time, expressed in milliseconds,
+        # rounded up; cannot use `::Time.instant` that could be mocked
+        relative = time.duration_since(Crystal::System::Time.instant)
         timeout = (relative.to_i * 1000 + (relative.nanoseconds + 999_999) // 1_000_000)
       else
         timeout = LibC::INFINITE

src/crystal/system/win32/waitable_timer.cr

@@ -14,10 +14,8 @@ class Crystal::System::WaitableTimer
 
   def set(time : ::Time::Instant) : Nil
     # convert absolute time to relative time, expressed in 100ns interval,
-    # rounded up
-    # Cannot use `time.elapsed` here because it calls `::Time.instant` which
-    # could be mocked.
-    relative = Crystal::System::Time.instant.duration_since(time)
+    # rounded up; cannot use `::Time.instant` that could be mocked
+    relative = time.duration_since(Crystal::System::Time.instant)
     ticks = (relative.to_i * 10_000_000 + (relative.nanoseconds + 99) // 100)
 
     # negative duration means relative time (positive would mean absolute

src/fiber/execution_context/monitor.cr

@@ -47,11 +47,14 @@ module Fiber::ExecutionContext
 
       loop do
         Thread.sleep(remaining)
-        now = Crystal::System::Time.instant
-        yield(now)
-        # Cannot use `now.elapsed` here because it calls `::Time.instant` which
-        # could be mocked.
-        remaining = Crystal::System::Time.instant.duration_since(now) + @every
+
+        start = Crystal::System::Time.instant
+        yield(start)
+        stop = Crystal::System::Time.instant
+
+        # calculate remaining time for more steady wakeups (minimize exponential
+        # delays)
+        remaining = (start + @every - stop).clamp(Time::Span.zero..)
       rescue exception
         Crystal.print_error_buffered("BUG: %s#every crashed", self.class.name, exception: exception)
       end