Bug#950490: transition: gnat-9, Ada component of GCC-9

[Nicolas Boulenguez <nicolas@debian.org>]

Package: release.debian.org
Severity: normal
User: release.debian.org@packages.debian.org
Usertags: transition

Hello.

The gcc-$BV source package builds an Ada compiler named gnat-$BV,
and the companion libraries (libgnat-$BV, libgnatvsn$BV).
In unstable, gnat-8 and gnat-9 coexist.

The default Ada compiler in Debian is selected by the gnat empty
package, which only Depends on a specific gnat-$BV.
In unstable,     gnat/8.0.1  Depends: gnat-8.
In experimental, gnat/9~exp1 Depends: gnat-9.

In this language,
* A package using the default compiler Build-Depends: gnat.
* In addition, a library Build-Depends: gnat-$BV.
* Each change in the source of a library or its recursive dependencies
  requires a renaming of the -dev package.  All reverse dependencies
  must then update their Build-Depends.
A gnat transition requires a renaming of all -dev packages, in
addition to the traditional change of Shared Object Version.  In order
to reduce the number of such slow transitions, many unrelated changes
(like new upstream releases) have been introduced with a single new
name for the -dev package.

Ben file:

title = "gnat-9";
is_affected = .build-depends ~ "gnat";
is_good = .depends ~ "libgnat-9";
is_bad = .depends ~ /libgnat-[^1-8]/;

The only release-critical bug in reverse dependencies is #936869
(libgnatcoll-bindings build-depends on python2). The reupload to
experimental will declare an explicit dependency on python2 instead of
python, and lower the severity of this issue.

Apart from that, these packages are ready in experimental.

adabrowse
adacgi
adacontrol
ahven
anet
asis
dbusada
dh-ada-library
gnat
gnat-gps
gprbuild
libalog
libaunit
libaws
libflorist
libgmpada
libgnatcoll
libgnatcoll-bindings
libgnatcoll-db
libgtkada
liblog4ada
libncursesada
libtemplates-parser
libtexttools
libxmlada
libxmlezout
pcscada
plplot

When would a reupload to unstable be appropriate?

These are not libraries and will only need a rebuild in due time.
dw music123_16.4-4 .ANY . -m 'gnat (>= 9)'
dw    topal_80-1   .ANY . -m 'gnat (>= 9)'

The ada-reference-manual Build-Depends: gnat, but will not be affected
(it only builds an intermediate documentation formatter).

The ghdl package Build-Depends: gnat-8, but will not be affected
(it does not Build-Depend: gnat).

Thanks.