Bug#614807: debian-policy: Please document autobuilder-imposed build-dependency alternative restrictions

[Simon McVittie <smcv@debian.org>]

On Sat, 26 Feb 2011 at 14:21:13 +0100, Sean Finney wrote:
> The Debian autobuilders only make use of the first alternative
> in a set of alternatives, in order to guarantee consistent,
> reproducible builds.  This does not include architecture
> restrictions, because architecture reduction takes place before
> alternative removal.  Alternatives are therefore allowed, and
> hence useful for backports and other distributions, but are not
> used by default.
> ---
>  policy.sgml |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
> 
> diff --git a/policy.sgml b/policy.sgml
> index 642f672..33a3a8a 100644
> --- a/policy.sgml
> +++ b/policy.sgml
> @@ -4446,6 +4446,19 @@ Checksums-Sha256:
>            by vertical bar (pipe) symbols <tt>|</tt>.  In such a case,
>            if any one of the alternative packages is installed, that
>            part of the dependency is considered to be satisfied.
> +          <footnote>
> +            While <tt>Build-Depends</tt> and <tt>Build-Depends-Indep</tt>
> +            permit the use of alternative dependencies, these are
> +            not normally used by the Debian autobuilders.  To avoid
> +            inconsistency between repeated builds of a package, the
> +            autobuilders will default to selecting the first alternative,
> +            after reducing any architecture-specific restrictions for
> +            the build architecture in question.  While this may limit
> +            the usefulness of alternatives in a single release, they can
> +            still be used to provide flexibility in building the same
> +            package across multiple distributions or releases, where a
> +            particular dependency is met by differently named packages.
> +          </footnote>
>  	</p>

6½ years later, ideally this would mention Build-Depends-Arch too.

Other than that, seconded. I'm not sure whether this is necessarily how
the autobuilders *should* work, but there's value in documenting how
the autobuilders *do* work.

Regards,
    smcv