Fix timers firing up to 1ms early

libuv resamples its internal clock as the floor of the current
monotonic time truncated to whole milliseconds, then schedules a
timer for loop->time + timeout. Since the fractional millisecond
already elapsed at the moment of sampling is discarded, the timer can
fire up to ~1ms before the requested delay has actually passed. This
made asyncio.sleep(n) occasionally return slightly under n, as
reported in #739.

Pad the timeout handed to uv_timer_start() by 1ms to compensate, while
leaving the reported deadline (get_when()/TimerHandle.when()) as the
originally requested value.

Author: pctablet505

tests/test_base.py

@@ -220,6 +220,24 @@ def cb():
             finished = int(round(self.loop.time() * 1000))
             self.assertGreaterEqual(finished - started, 69)
 
+    def test_call_later_not_early(self):
+        # Refs #739: asyncio.sleep()/call_later() must never fire before
+        # the requested delay has actually elapsed, as measured by a
+        # wall-clock source independent of the loop's own (millisecond-
+        # truncated) internal clock. The underlying bug only manifests
+        # when the loop's internal clock happens to be sampled close to
+        # a millisecond boundary, so run enough iterations to make a
+        # regression here reliably observable.
+        async def main():
+            delay = 0.01
+            for _ in range(300):
+                started = time.monotonic()
+                await asyncio.sleep(delay)
+                elapsed = time.monotonic() - started
+                self.assertGreaterEqual(elapsed, delay)
+
+        self.loop.run_until_complete(main())
+
     def test_call_at(self):
         if (os.environ.get('TRAVIS_OS_NAME')
                 or os.environ.get('GITHUB_WORKFLOW')):

uvloop/handles/timer.pyx

@@ -50,9 +50,19 @@ cdef class UVTimer(UVHandle):
             uv.uv_update_time(self._loop.uvloop)  # void
             self.start_t = uv.uv_now(self._loop.uvloop)
 
+            # libuv's internal clock (loop->time, just refreshed above) is
+            # the current monotonic time truncated ("floored") to whole
+            # milliseconds, discarding up to just under 1ms. uv_timer_start()
+            # schedules the callback for loop->time + timeout, so without
+            # compensation the timer can fire up to ~1ms sooner than
+            # `self.timeout` milliseconds of actual wall-clock time have
+            # elapsed. Pad the timeout given to libuv by 1ms so the timer
+            # never fires earlier than requested; self.timeout (and thus
+            # get_when()/TimerHandle.when()) is left untouched so it keeps
+            # reporting the originally requested deadline.
             err = uv.uv_timer_start(<uv.uv_timer_t*>self._handle,
                                     __uvtimer_callback,
-                                    self.timeout, 0)
+                                    self.timeout + 1, 0)
             if err < 0:
                 exc = convert_error(err)
                 self._fatal_error(exc, True)