bpo-34819: Use a monotonic clock to compute timeouts in concurrent.futures (GH-9599)

Use a monotonic clock to compute timeouts in :meth:`Executor.map` and :func:`as_completed`, in order to prevent timeouts from deviating when the system clock is adjusted.

This may not be sufficient on all systems. On POSIX for example, the actual waiting (e.g. in ``sem_timedwait``) is specified to rely on the CLOCK_REALTIME clock.

Lib/concurrent/futures/_base.py

@@ -216,7 +216,7 @@ def as_completed(fs, timeout=None):
             before the given timeout.
     """
     if timeout is not None:
-        end_time = timeout + time.time()
+        end_time = timeout + time.monotonic()
 
     fs = set(fs)
     total_futures = len(fs)
@@ -235,7 +235,7 @@ def as_completed(fs, timeout=None):
             if timeout is None:
                 wait_timeout = None
             else:
-                wait_timeout = end_time - time.time()
+                wait_timeout = end_time - time.monotonic()
                 if wait_timeout < 0:
                     raise TimeoutError(
                             '%d (of %d) futures unfinished' % (
@@ -578,7 +578,7 @@ class Executor(object):
             Exception: If fn(*args) raises for any values.
         """
         if timeout is not None:
-            end_time = timeout + time.time()
+            end_time = timeout + time.monotonic()
 
         fs = [self.submit(fn, *args) for args in zip(*iterables)]
 
@@ -593,7 +593,7 @@ class Executor(object):
                     if timeout is None:
                         yield fs.pop().result()
                     else:
-                        yield fs.pop().result(end_time - time.time())
+                        yield fs.pop().result(end_time - time.monotonic())
             finally:
                 for future in fs:
                     future.cancel()

Misc/NEWS.d/next/Library/2018-09-27-09-45-00.bpo-34819.9ZaFyO.rst

@@ -0,0 +1 @@
+Use a monotonic clock to compute timeouts in :meth:`Executor.map` and :func:`as_completed`, in order to prevent timeouts from deviating when the system clock is adjusted.