preparing for GCC 5, especially libstdc++6

[Matthias Klose]

On 06/17/2015 09:42 PM, Lisandro Damián Nicanor Pérez Meyer wrote:
> On Tuesday 16 June 2015 23:37:41 Matthias Klose wrote:
>> Hi,
>>
>> it's time to prepare for GCC 5 as the default compiler in unstable. 
>> Compared to earlier version bumps, the switch to GCC 5 is a bit more
>> complicated because libstdc++6 sees a few ABI incompatibilities, partially
>> depending on the C++ standard version used for the builds.  libstdc++6 will
>> support two ABI's, the classic cxx98 ABI (currently in testing/unstable)
>> and the new cxx11 ABI (currently enabled in experimental as the default
>> ABI).
> 
> Hi Matthias!
> 
> [snip]
> 
>> My goal is to make the GCC version bump in early July, and use the time
>> until then to prepare libstdc++6 depending packages to get ready for GCC 5,
>> and avoiding version bumps for C++ libraries until this time.
> 
> As you already know we the Qt/KDE team, in order to avoid an ABI break, need 
> to either:
> 
> a) Push Qt 5.4.2 to unstable before gcc5 becomes the default compiler. This is 
> currently not an option due to #787689.
> 
> b) Push Qt 5.4.2 to unstable at the same time as gcc5 becomes the default 
> (well, one or two dinstalls later maybe). If some package gets compiled with 
> gcc5 and Qt < 5.4.2 in the meantime we can binNMU it.
> 
> c) Push Qt 5.4.2 to unstable a couple of days before gcc5 becomes the default. 
> Once gcc5 becomes the default ask for a give back in armhf.
> 
> d) Push Qt 5.4.2 to unstable either by forcing gcc5 as default at build time 
> or working around the bug. I would definitely would like to avoid this option 
> as:
>   - The current Qt stack is comprised of 25 source packages. I don't think me
>     or my teammates will have the time to hack all of them before beginnings
>     of July.
>   - I don't know if some other lib/app that build depends on qt5 will still
>     have the issue on armhf if we workaround it.
>   - I don't know what happens if Qt5 gets built against gcc5 but apps against
>     gcc4.9. Possibly and hopefully nothing, but I just don't know.
> 
> My current preference would be acb (totally discarding d), but I'm open to 
> suggestions too.

I would prefer b), and preparing the packages to build with the gcc-5 from
experimental (not unstable).  Does Qt 5.4.2 change sonames? If not, please
prepare to change the library package names, if new cxx11 symbols are introduced.

Matthias