Use higher precision time source in retry decorator/tests and avoid f…

…laky failures (#12839)

I've replaced time.monotonic() with time.perf_counter() in the
retry utility and test suite since it's effectively monotonic[^1] while
providing much higher resolution (esp. on Windows).

In addition, to avoid flaky failures (originally on Windows) I've added
a margin of error to the timed tests. I've also outright disabled the
timed tests on Windows as further testing revealed that they were simply
too unreliable to be useful (I once observed a deviation of 30%).

[^1]: It's not guaranteed to be monotonic in the Python docs, but it is
      _indeed_ monotonic on all platforms we care about.

src/pip/_internal/utils/retry.py

@@ -1,5 +1,5 @@
 import functools
-from time import monotonic, sleep
+from time import perf_counter, sleep
 from typing import Callable, TypeVar
 
 from pip._vendor.typing_extensions import ParamSpec
@@ -26,12 +26,14 @@ def wrapper(func: Callable[P, T]) -> Callable[P, T]:
 
         @functools.wraps(func)
         def retry_wrapped(*args: P.args, **kwargs: P.kwargs) -> T:
-            start_time = monotonic()
+            # The performance counter is monotonic on all platforms we care
+            # about and has much better resolution than time.monotonic().
+            start_time = perf_counter()
             while True:
                 try:
                     return func(*args, **kwargs)
                 except Exception:
-                    if monotonic() - start_time > stop_after_delay:
+                    if perf_counter() - start_time > stop_after_delay:
                         raise
                     sleep(wait)
 

tests/unit/test_utils_retry.py

@@ -1,5 +1,6 @@
 import random
-from time import monotonic, sleep
+import sys
+from time import perf_counter, sleep
 from typing import List, NoReturn, Tuple, Type
 from unittest.mock import Mock
 
@@ -65,39 +66,46 @@ def create_timestamped_callable(sleep_per_call: float = 0) -> Tuple[Mock, List[f
     timestamps = []
 
     def _raise_error() -> NoReturn:
-        timestamps.append(monotonic())
+        timestamps.append(perf_counter())
         if sleep_per_call:
             sleep(sleep_per_call)
         raise RuntimeError
 
     return Mock(wraps=_raise_error), timestamps
 
 
-# Use multiple of 15ms as Windows' sleep is only accurate to 15ms.
+@pytest.mark.skipif(
+    sys.platform == "win32", reason="Too flaky on Windows due to poor timer resolution"
+)
 @pytest.mark.parametrize("wait_duration", [0.015, 0.045, 0.15])
 def test_retry_wait(wait_duration: float) -> None:
     function, timestamps = create_timestamped_callable()
     # Only the first retry will be scheduled before the time limit is exceeded.
     wrapped = retry(wait=wait_duration, stop_after_delay=0.01)(function)
-    start_time = monotonic()
+    start_time = perf_counter()
     with pytest.raises(RuntimeError):
         wrapped()
     assert len(timestamps) == 2
-    assert timestamps[1] - start_time >= wait_duration
+    # Add a margin of 10% to permit for unavoidable variation.
+    assert timestamps[1] - start_time >= (wait_duration * 0.9)
 
 
+@pytest.mark.skipif(
+    sys.platform == "win32", reason="Too flaky on Windows due to poor timer resolution"
+)
 @pytest.mark.parametrize(
-    "call_duration, max_allowed_calls", [(0.01, 10), (0.04, 3), (0.15, 1)]
+    "call_duration, max_allowed_calls", [(0.01, 11), (0.04, 3), (0.15, 1)]
 )
 def test_retry_time_limit(call_duration: float, max_allowed_calls: int) -> None:
     function, timestamps = create_timestamped_callable(sleep_per_call=call_duration)
     wrapped = retry(wait=0, stop_after_delay=0.1)(function)
 
-    start_time = monotonic()
+    start_time = perf_counter()
     with pytest.raises(RuntimeError):
         wrapped()
     assert len(timestamps) <= max_allowed_calls
-    assert all(t - start_time <= 0.1 for t in timestamps)
+    # Add a margin of 10% to permit for unavoidable variation.
+    assert all(t - start_time <= (0.1 * 1.1) for t in timestamps)
 
 
 def test_retry_method() -> None: