(timeout in ms vs. XSyncValueSubtract) Frozen client, found cause, need advise for fix

[walter harms]

Am 20.02.2017 10:55, schrieb Daniel Martin:
> Hi everyone,
> 
> I've got a Qt5 client, which receives mouse input only and freezes
> soon and often. It gets stuck waiting for a reply from the server.
> Pressing a none-modifier key releases the lock and the client
> continues as normal until the next freeze.
> 
> I've traced the bug down to a regression implicitly introduced by:
>     e6636b4 os: Compute timeout in milliseconds instead of struct timeval
> Which states "simplifies the code without affecting functionality at all". ;-)
> 
> What happens is:
> - ServertimeBlockHandler() forwards a _big_ unsigned timeout to
> AdjustWaitForDelay()
> - AdjustWaitForDelay() takes this _big_ unsigned as int and it becomes
> a negative value
> - the negative value ends up as timeout in ospoll_wait() as parameter
> to epoll_wait()
>> epoll_wait() blocks due to the negative timeout
> 
> XSyncValueSubtract() doesn't handle unsigned wraps. That's why we get
> a big unsigned from ServertimeBlockHandler(). Just imagine two
> XSyncValues:
>     a = {.hi=1, .lo=1} and b = {.hi=0, lo=.2}.
> The result of XSyncValueSubtract(a - b) is
>     presult = {.hi=0, .lo=4294967295},
> where I would expect
>    presult = {.hi=0, .lo=9}.
> 
> If .lo=4294967295 is wrong and .lo=9 is what we want, could anyone
> provide a fix, please? I'm to dumb for the (binary?) arithmetic atm.
> (And if it is wrong, did I just uncovered an ancient bug?)
> 
	
XSyncValueSubtract is doing as expected,
XSyncValue is the simulation of 64bitvalues on 32bit.
see this in hex:
 100000001
-000000002
 0FFFFFFFF = 4294967295 in .lo


> To paper over the (mis)behaviour of XSyncValueSubtract() I would
> suggest to make sure that AdjustWaitForDelay() doesn't return negative
> values. We can't check for negatives in ospoll_wait() as the input
> thread calls it with -1.

no idea, i suspect a mix of unit somewhere seconds vs mseconds or so.

re,
 wh


> 
> Other thoughts: Why does the cursor move, but neither the movement or
> clicks, nor pressing a modifier unlock the client?
> 
> 
> Cheers,
>     Daniel Martin
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