Re: [PATCH RFC 2/4] build: Download and build extnernal blktap2.5 repo

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-03-31 at 12:10 +0100, George Dunlap wrote:
> On 03/31/2015 11:40 AM, Stefano Stabellini wrote:
> > On Tue, 31 Mar 2015, George Dunlap wrote:
> >> On Mon, Mar 30, 2015 at 4:38 PM, Wei Liu <wei.liu2@citrix.com> wrote:
> >>>>> IMHO we need to support --with-system-blktap= in configure in case
> >>>>> distro wants to package blktap separately. Not sure if in practice this
> >>>>> makes sense since AIUI blktap is only used by Xen.
> >>>>
> >>>> I agree that would be ideal; however, it's not so simple, because at the
> >>>> moment libxl links directly against libblktap.  This would mean:
> >>>>
> >>>> 1) Changing libxl so that it could dynamically detect the presence of
> >>>> the proper version of libblktap at runtime and use the stubbed-out
> >>>> defaults if not available.
> >>>>
> >>>
> >>> This should be done in ./configure too, not during libxl build /
> >>> runtime.  If libblktap is not present during ./configure  then libxl
> >>> just use stubs.
> >>
> >> It sounds like you're talking about introducing a hard dependency,
> >> such that packages that use a libxl built this way won't function
> >> without blktap installed.  Yeah, that's simple enough.
> >>
> >> I'm not super experienced in the distro packaging mindset, but since
> >> (AFAIK) no other programs or projects use blktap, is there much point
> >> to having a separate repo if you can't "opt-out" of installing it?
> > 
> > It is not an hard dependency: as long as one doesn't use VHDs, ones
> > doesn't see any difference whether blktap is installed on not.
> 
> I'm talking about a hard dependency *for the resulting package*.
> 
> If libxl built linked against libblktap, and libblktap is not installed
> on the system, then won't running the binary at all result in "Can't
> find shared library" errors?
> 
> In any case, I'm pretty sure that if you build an RPM and link against a
> particular library, that the RPM will then refuse to install if that
> library isn't available.

Correct (for Debian too), but this is more about the RPM maintainers
freedom to choose to enable it or not. Once they chosen their users will
just end up with whatever they chose.

For arch and gentoo folks I suppose this would be a USE var or whatever
they call it.

Ian.