Re: [courier-users] Ubuntu and Courier upgrade

[Sam V. <mr...@co...>]

Matus UHLAR - fantomas writes:

>> When you're talking about simple, basic packages – a couple of shell  
>> scripts, or standalone executables, no complex dependencies, maybe a few man  
>> pages – this is not complicated. dpkg will take care of installing the new  
>> files, remove any files that are no longer a part of the package, and the  
>> running executable will chug along until the shutdown script SIGTERMs it,  
>> and it cleans itself up gracefully.
>>
>> But when you get involving in complex system libraries and demons the  
>> situation quickly becomes not as clear-cut. I really don't want to be  
>> running do_release_upgrade when it replaces glibc and libstdc++ with a  
>> binary-incompatible version, and there's a bunch of stuff running that's  
>> dependent on a specific libstdc++ ABI.
>
> Exactly this problem is avoided by having different packages with different  
> ABI, e.g. libstdc++5 and libstdc++6.
>
> If the ABI changes, your package depends on older library version and will  
> use the old libraty. Restarting won't change anything here.

This is not really about having multiple versions installed currently.  
That's a known, solved issue.

This is about updating to a new ABI version, the old ABI gets removed,  
together with updating applications as a result of the ABI switch.

So, in addition to updating from libstdc++5 to libstdc++6 all applications  
that were built for libstdc++5 get updated and replaced with versions that  
use the libstdc++6 ABI, as part of the release upgrade.

If you have a goal of mitigating risks of a major update, as much as  
possible, you don't want to do something like that in the way Ubuntu does  
it: full speed ahead, and update a live system, in place.

How risky this is might be a matter for debate. But it's unquestionable that  
doing the whole in single-user mode just has fewer moving pieces that may  
potentially break.

> In the meantime, you can have packages depending on different library  
> versions installed in parallel. Proper Depends: and Breaks: statements  
> handle this.

This is not about parallel installation.