New malloc breaks old libpthread

[Scott Long]

Daniel Eischen wrote:
> On Mon, 16 Jan 2006, Doug White wrote:
> 
> 
>>Got this trying to run an old Xorg binary on a -CURRENT machine I don't
>>update very frequently:
>>
>>/libexec/ld-elf.so.1: /usr/lib/libpthread.so.1: Undefined symbol "__malloc_lock"
>>
>>The libpthread.so.1 was from June 2005, prior to the libpthread version
>>bump. Unfortunately this means that RELENG_6 compatibility is broken in
>>-HEAD since the new libc.so.6 is not compatible with libraries built
>>against it prior to the merge date of the new user malloc.
> 
> 
> Don't do that.  We don't guarantee -current libraries built on
> (vastly) different dates will run nicely together.
> 
> 
>>I guess its time libc's version number.  We haven't yet for post-RELENG_6,
>>and this it the usual case calling for the bump.
> 
> 
> No.  There's no need -- this is -current, and we have symbol versioning
> now anyways.
> 
> 

So.......

How exactly does symbol versioning help this case?  It obviously doesn't
magically make all problems go away, so is there a set procude that
one must follow when introducing incompatibilities?  If there is a
procedure, is it published anywhere?

Scott