caucase.utils: Fix support for host computer suspension in "until".

Break requested sleep period into smaller chunks, to try to compensate for
extended suspension periods.
The chosen values seem to be a reasonable trade-off between accuracy and
number of wake-ups.

caucase/utils.py

@@ -717,10 +717,32 @@ def until(deadline): # pragma: no cover
   """
   Call interruptibleSleep until deadline is reached.
   """
+  try:
+    while True:
       now = datetime.datetime.utcnow()
-  while now < deadline:
-    interruptibleSleep((deadline - now).total_seconds())
-    now = datetime.datetime.utcnow()
+      remaining_time = (deadline - now).total_seconds()
+      if remaining_time <= 0:
+        break
+      # Wake up half-way to the deadline, to re-evaluate the time left: the
+      # computer may have been suspended, or had its time changed.
+      # ...but do not sleep less than 10 minutes.
+      # ...but do not sleep past the deadline.
+      # Simulating a computer suspended 75% of the time (continuously, which is
+      # a big approximation of the actual discrete nature of suspend periods)
+      # shows this approach should lead to 5 wakeups for a deadline initially
+      # 7 days away: first wakeup after 6.1 days, second 18 hours later, then
+      # 2 hours, then 17 minutes and finally the few remaining seconds.
+      interruptibleSleep(
+        max(
+          remaining_time / 2,
+          min(
+            remaining_time,
+            600,
+          ),
+        ),
+      )
+  except KeyboardInterrupt:
+    raise SleepInterrupt
   return now
 
 def log_exception(error_file, exc_info, client_address):