A partial upgrade of libatlas3-base may render the library broken and
libblas.so.3 may be gone. This situation does normally not arise,
because apt usually upgrades all binary packages from the same source
package together, but it can still be encountered in complex upgrades.

mmdebstrap bookworm /dev/null http://deb.debian.org/debian --variant=apt --include=libatlas3-base,libblas3 --chrooted-customize-hook='sed -i -e s/bookworm/trixie/ /etc/apt/sources.list && apt update && apt-get -o APT::Get::Upgrade-By-Source-Package=False install -y libatlas3-base && ls -la /usr/lib/x86_64-linux-gnu/libblas.so.3'

This reproducer lifts the lock-step upgrade to simulate upgrading to the
transitional package only. Once doing so, the alternative is a dangling
symbolic link and libblas.so.3 is gone. Upgrading libblas3 resolves the
problems, so most systems will not practically experience this symptom.

The solution to this problem is good practice: Transitional packages
should issue a versioned dependency. libatlas3-base should really
"Depends: libblas3 (= ${binary:Version})". Another good practice is for
them to be of the same architecture and Multi-Arch value as their target
package, so libatlas3-base should switch from Arch:all to Arch:any and
become M-A:same. Doing otherwise breaks multiarch-enabled upgrades.

I recommend trying to fix this for trixie.
--- a/debian/control
+++ b/debian/control
@@ -315,8 +315,9 @@
  This package contains a shared version of the library.

 Package: libatlas3-base
-Depends: libblas3, ${misc:Depends}
-Architecture: all
+Depends: libblas3 (= ${binary:Version}), ${misc:Depends}
+Architecture: any
+Multi-Arch: same
 Section: oldlibs
 Description: transitional package
  This is a transitional package. It can safely be removed.

Helmut

We believe that the bug you reported is fixed in the latest version of
lapack, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1110359@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Mo Zhou <lumin@debian.org> (supplier of updated lapack package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Tue, 05 Aug 2025 16:53:57 -0400
Source: lapack
Architecture: source
Version: 3.12.1-5
Distribution: unstable
Urgency: medium
Maintainer: Debian Science Team <debian-science-maintainers@lists.alioth.debian.org>
Changed-By: Mo Zhou <lumin@debian.org>
Closes: 1110359
Changes:
 lapack (3.12.1-5) unstable; urgency=medium
 .
   [ Helmut Grohne ]
   * Fix libatlas3-base: insufficient dependency on libblas3 (Closes: #1110359)
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...
...

Should this be (>= ${binary:Version}) instead of (=)?

In other packages that migrated to another name via a transitional
package (gdk-pixbuf, pango1.0) we've seen that sometimes users of stable
releases have to keep the transitional package installed for a long
time, even after it has been eliminated from Debian (hopefully during
forky in this case), because third-party packages outside Debian still
have a dependency on the old name.

After upgrading the transitional libatlas3-base to its Debian 13 version
(for simplicity let's pretend that it's version 13) there are three
possible scenarios:

1. libatlas3-base (= 13), libblas3 (= 13): good.
    This is the only one allowed by the proposed dependency.

2. libatlas3-base (= 13), libblas3 (<< 13):
    bad, for the reasons Helmut gave on #1110466

3. libatlas3-base (= 13), libblas3 (>> 13):
    should be fine, we can assume that libblas3 (>> 13) is
    "better than" libblas3 (= 13)

So I think the best-practice for empty transitional packages is that
they should retain their previous Architecture and Multi-Arch, and have:

     Depends: the-new-package (>= some suitable version)

Thanks,
     smcv

Hi Simon,

Thank you. I concur. Is this something we might reasonable get into
Debian policy or the developers reference? It's not the first time that
someone (myself included) gets this wrong.

Helmut

Hi Simon,

Thanks for the suggestion. I revised the fix and uploaded
a new version to unstable. The debdiff can be found below:

So we will need to unblock lapack/3.12.1-6 after buildd
finishes their jobs.

```
diff --git a/debian/changelog b/debian/changelog
index bfa08538..adb4b650 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,10 @@
+lapack (3.12.1-6) unstable; urgency=medium
+
+  * Revise the deps for libatlas3-base (transitional) as suggested in #1110359
+    Thanks to Simon McVittie for the suggestion.
+
+ -- Mo Zhou <lumin@debian.org>  Thu, 07 Aug 2025 13:26:25 -0400
+
 lapack (3.12.1-5) unstable; urgency=medium

   [ Helmut Grohne ]
diff --git a/debian/control b/debian/control
index 1c61bc90..b57e0f62 100644
--- a/debian/control
+++ b/debian/control
@@ -315,7 +315,7 @@ Description: Basic Linear Algebra Reference implementations, shared library
  This package contains a shared version of the library.

 Package: libatlas3-base
-Depends: libblas3 (= ${binary:Version}), ${misc:Depends}
+Depends: libblas3 (>= ${binary:Version}), ${misc:Depends}
 Architecture: any
 Multi-Arch: same
 Section: oldlibs
```