[ python-Bugs-1508864 ] threading.Timer/timeouts break on change of win32 local time

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Bugs item #1508864, was opened at 2006-06-19 14:53
Message generated for change (Comment added) made by qopit
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Category: Python Library
Group: Platform-specific
Status: Open
Resolution: None
Priority: 5
Submitted By: Russell Warren (qopit)
Assigned to: Nobody/Anonymous (nobody)
Summary: threading.Timer/timeouts break on change of win32 local time

Initial Comment:
THE ISSUE...
---
threading.py imports time.time as _time.

On win32 systems, time.time() periodically reads the
system time to figure out when to fire an Event.

System time can change while waiting for an Event!

eg:  If the system time is changed while a
threading.Timer is pending, the execution time is
affected by the time change.  eg: set a pending Timer
and then change the clock back an hour - this causes
your Timer to fire an hour later.  This is clearly not
desirable.


A FIX...
---
A fix for this is to use time.clock() on win32 systems
instead.  Once I found the problem, I currently just
fix it by overriding threading._time to be time.clock.
 Right now I do this in every file that uses
threading.Timer.


COMMENTS...
---
The penalty for this is that there will be a rollover
problem eventaully... when the 64-bit performance
counter rolls over in 30+ years of continuous pc
operation.  I'd much rather have this near impossible
event than the very likely system time change.

This is a general problem I find with the time module
and I often have to switch between time() and clock()
dependent on operating system, but I only work with
win32 and Linux.  The issue is that if you want a high
resolution and extended rollover counter, it is a
different call on each system.



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>Comment By: Russell Warren (qopit)
Date: 2006-06-27 15:01

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No - just stating that clock() is definitely a better
solution for win32.

As you say, any system should just use a time-indpendent
uptime counter that is as high resolution as possible.  I
don't know how to get this on linux, but I do seem to recall
that, on linux, time() is higher resolution than clock() for
some reason.

If linux has no performance counter equivalent (isn't it a
hardware thing anyway?) I have no clue which is worse... low
resolution, or local time change issues.  The first is
limiting all the time, the second results in wacky and
sporadic errors that people might not expect.


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Comment By: Jim Jewett (jimjjewett)
Date: 2006-06-27 10:04

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Logically, these calls should always use clock, since they 
don't care about the actual time; they only want a baseline 
for computing elapsed time.

Are you saying that they should still use clock-time on 
linux anyway, because of resolution issues?


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Comment By: Russell Warren (qopit)
Date: 2006-06-26 16:59

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This is an issue for anything that uses threading.py's _time
function.

This includes _Condition.wait(), which screws up both
Conditions and Events, which then screw up (for example) the
core Queue.Queue timeout implementation.

threading.Thread.join also uses the _time call, so it could
also get screwed by changes to the local time on Win32.



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