Cannot concretize when specifying platform= in externals

[fgava90]

Steps to reproduce

This environment fails to concretize:

spack:
  specs:
  - hwloc
  view: false
  concretizer:
    unify: true
  packages:
    hwloc:
      externals:
      - spec: hwloc@2.2.0 platform=linux
        prefix: /usr
      buildable: False

The behavior regressed in #42253

Original report

I have a package.yaml defined like this (minimal reproducer):

packages:
  hwloc:
    externals:
      - spec: hwloc@2.2.0 # arch=None-None-None works as well
        prefix: /usr
    buildable: False

If i concretize:

$ spack spec hwloc
Input spec
--------------------------------
 -   hwloc

Concretized
--------------------------------
[e]  hwloc@2.2.0%gcc@12.3.0~cairo~cuda~gl~libudev+libxml2~netloc~nvml~oneapi-level-zero~opencl+pci~rocm build_system=autotools libs=shared,static arch=linux-rocky8-zen2

All good.
Now if I add anything specific to the arch= spec, like this for example:

packages:
  hwloc:
    externals:
      - spec: hwloc@2.2.0 arch=linux-None-None # added platform
        prefix: /usr
    buildable: False

And try to concretize:

$ spack spec hwloc
==> Error: concretization failed for the following reasons:

   1. Attempted to use external for 'hwloc' which does not satisfy any configured external spec version
   2. Attempted to use external for 'hwloc' which does not satisfy any configured external spec
   3. Attempted to build package hwloc which is not buildable and does not have a satisfying external
        attr('node_version_satisfies', 'hwloc', '=2.2.0') is an external constraint for hwloc which was not satisfied

The error message complains about the version but it's actually fake, as if i add more specific constraints it will complain about something else:

packages:
  hwloc:
    externals:
      - spec: hwloc@2.2.0 arch=linux-rocky8-None # added OS
        prefix: /usr
    buildable: False

$ spack spec hwloc
==> Error: concretization failed for the following reasons:

   1. Cannot select a single "node_os" for package "hwloc"
   2. Attempted to use external for 'hwloc' which does not satisfy any configured external spec
   3. Attempted to build package hwloc which is not buildable and does not have a satisfying external
        attr('node_os', 'hwloc', 'rocky8') is an external constraint for hwloc which was not satisfied

Specifying variants works fine.

I'm executing this with the latest develop branch but this error is present also in 0.21.2.
0.21.1 works fine.

Error message

==> Error: concretization failed for the following reasons:

   1. Attempted to use external for 'hwloc' which does not satisfy any configured external spec version
   2. Attempted to use external for 'hwloc' which does not satisfy any configured external spec
   3. Attempted to build package hwloc which is not buildable and does not have a satisfying external
        attr('node_version_satisfies', 'hwloc', '=2.2.0') is an external constraint for hwloc which was not satisfied

Information on your system

• Spack: 0.22.0.dev0 (32deca2)
• Python: 3.6.8
• Platform: linux-rocky8-zen2
• Concretizer: clingo

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output

[fgava90]

@becker33 @alalazo could it be this commit here?
9963e2a

[bvanessen]

@becker33 @scheibelp @alalazo
I believe that this is the same bug that I am hitting.

[alalazo]

The issue is specific to platform= specification. Minimal reproducer is:

spack:
  specs:
  - hwloc
  view: false
  concretizer:
    unify: true
  packages:
    hwloc:
      externals:
      - spec: hwloc@2.2.0 platform=linux
        prefix: /usr
      buildable: False

[alalazo]

While I look for a fix, note that the issue comes from the least useful part of the arch= (at least for an external). I would suggest, as a workaround and as a best practice, to always use the disjoint form and write os= and target= explicitly.

[alalazo]

Bisection says this regressed in #42253

[alalazo]

Edited the description, kept the original report in a <details> section.