Re: [Qemu-devel] QEMU produces invalid JSON due to locale-dependent code

[Markus Armbruster]

"Daniel P. Berrange" <address@hidden> writes:

> On Wed, Aug 26, 2015 at 11:13:00AM +0200, Alberto Garcia wrote:
>> On Tue 25 Aug 2015 09:54:42 AM CEST, Markus Armbruster wrote:
>> 
>> >> (D) Run in a controlled mixed locale
>> >>    GTK runs completely in the locale determined by setlocale() (since it
>> >> never has to display raw JSON)
>> >>    We fix our JSON output code to use thread-specific locale
>> >> manipulations to (temporarily) switch to C locale before printing JSON
>> >>
>> >> that way, GTK still shows full German formatting for any localized
>> >> message in the GUI that happens to print numbers, but the JSON output
>> >> (which is independent of the GUI) also behaves correctly as a C-only
>> >> entity.
>> >
>> > Switching back to C locale whenever some unwanted locale-dependency
>> > breaks the code is problematic, because it involves finding all the
>> > places that break, iteratively (euphemism for "we debug one breakage
>> > after the other, adding temporary locale switches as we go).
>> 
>> For some cases we probably don't even need to switch the locale. For the
>> JSON case cannot we just easily convert the QFloat into a string
>> ourselves without using printf's "%f" ?
>
> FWIW, libvirt had this exact same problem with formatting doubles for
> JSON while non-C locales are in effect. We used the glibc thread-local
> locale support, and fallback to some sick string munging in non-glibc
> case:
[...]

For QEMU, I doubt sick string munging just to support GUI localization
on losing systems is worthwhile.

>> That doesn't invalidate however your point.
>> 
>> > I'd feel much better about confining GTK in its own thread, and
>> > setting only that thread's locale.
>> 
>> I haven't digged much into that part of the QEMU code, but if my
>> assumptions are true I think we would need at least:
>> 
>> - A separate GMainContext with its own main loop
>> - Making sure that all the code that touches the UI runs from that
>>   thread.
>> 
>> This is in principle possible, but I fear that we might be opening a can
>> of worms.
>
> Agreed, my experiance is that you should always run GTK in the main
> thread and never try todo anything more clever with threads + GTK.
> It has always ended in tears for me - even if you think you have it
> working on your system, you'll inevitably find a different version
> of GTK where it has broken. It just isn't worth the pain to try to
> run anything GTK related in a non-main thread.

If we can't move GTK out of the main thread, we need to move everything
else out.

One more option: instead of switching back to the C locale around
identified troublemakers (and then chase them down one by one, bug by
bug), switch away from it just around the GUI.  Thread-locally of
course.  Callbacks may require extra care.

We should've stayed out of the GUI business.