libbacktrace integration for _GLIBCXX_DEBUG mode

[François Dumont]

On 12/21/18 10:03 PM, Jonathan Wakely wrote:
> On 21/12/18 22:47 +0200, Ville Voutilainen wrote:
>> On Fri, 21 Dec 2018 at 22:35, Jonathan Wakely <jwakely@redhat.com> 
>> wrote:
>>> >Â Â Â  I also explcitely define BACKTRACE_SUPPORTED to 0 to make sure
>>> >libstdc++ has no libbacktrace dependency after usual build.
>>
>>> I'm concerned about the requirement to link to libbacktrace
>>> explicitly (which will break existing makefiles and build systems that
>>> currently use debug mode in testing).
>>
>> But see what Francois wrote, "I also explcitely define
>> BACKTRACE_SUPPORTED to 0 to make sure
>> libstdc++ has no libbacktrace dependency after usual build."
>
> Yes, but if you happen to install libbacktrace headers, the behaviour
> for users building their own code changes. I agree that if you install
> those headers, it's probably for a reason, but it might be a different
> reason to "so that libstdc++ prints better backtraces".
>
>>> Also, some of the glibc team pointed out to me that running *any*
>>> extra code after undefined behaviour has been detected is a potential
>>> risk. The less that you do between detecting UB and calling abort(),
>>> the better. Giving the users more information is helpful, but comes
>>> with some additional risk.
>>
>> Ditto. Having said those things, I think we need to figure out a good
>> way to provide this sensibly
>> as an opt-in. The backtrace support is bloody useful, and dovetails
>> into a possible Contracts-aware
>> implementation of our library, but I think we need to do some more
>> thought-work on this, thus I agree
>> that it's not stage3 material. I do think it's something that we need
>> to keep in mind, thanks
>> for working on it, Francois!
>
> Yes, I agree that making it available via a more explicit opt-in would
> be good. Maybe require users to define _GLIBCXX_DEBUG_BACKTRACE as well
> as _GLIBCXX_DEBUG, or something like that.
>
>
Yes, I considered that if you had install libbacktrace you wanted to use 
it. But I agree that an explicit activation might be better.

I'll prepare that.

François