ASP-based solver: fix reusing externals on linux (#44316)

We need to tell clingo the libc compatibility of external nodes
in buildcaches or stores, to allow reuse.

lib/spack/spack/solver/asp.py

@@ -1939,6 +1939,11 @@ def _spec_clauses(
             for virtual in virtuals:
                 clauses.append(fn.attr("virtual_on_incoming_edges", spec.name, virtual))
 
+        # If the spec is external and concrete, we allow all the libcs on the system
+        if spec.external and spec.concrete and using_libc_compatibility():
+            for libc in self.libcs:
+                clauses.append(fn.attr("compatible_libc", spec.name, libc.name, libc.version))
+
         # add all clauses from dependencies
         if transitive:
             # TODO: Eventually distinguish 2 deps on the same pkg (build and link)

lib/spack/spack/solver/libc_compatibility.lp

@@ -10,7 +10,8 @@
 %=============================================================================
 
 % A package cannot be reused if the libc is not compatible with it
-:- provider(node(X, LibcPackage), node(0, "libc")),
+error(100, "Cannot reuse {0} since we cannot determine libc compatibility", ReusedPackage)
+  :- provider(node(X, LibcPackage), node(0, "libc")),
      attr("version", node(X, LibcPackage), LibcVersion),
      attr("hash", node(R, ReusedPackage), Hash),
      % Libc packages can be reused without the "compatible_libc" attribute

lib/spack/spack/test/concretize.py

@@ -2546,6 +2546,30 @@ def test_include_specs_from_externals_and_libcs(
 
         assert result["deprecated-versions"].satisfies("@1.0.0")
 
+    @pytest.mark.regression("44085")
+    @pytest.mark.only_clingo("Use case not supported by the original concretizer")
+    def test_can_reuse_concrete_externals_for_dependents(self, mutable_config, tmp_path):
+        """Test that external specs that are in the DB can be reused. This means they are
+        preferred to concretizing another external from packages.yaml
+        """
+        packages_yaml = {
+            "externaltool": {"externals": [{"spec": "externaltool@2.0", "prefix": "/fake/path"}]}
+        }
+        mutable_config.set("packages", packages_yaml)
+        # Concretize with gcc@9 to get a suboptimal spec, since we have gcc@10 available
+        external_spec = Spec("externaltool@2 %gcc@9").concretized()
+        assert external_spec.external
+
+        root_specs = [Spec("sombrero")]
+        with spack.config.override("concretizer:reuse", True):
+            solver = spack.solver.asp.Solver()
+            setup = spack.solver.asp.SpackSolverSetup()
+            result, _, _ = solver.driver.solve(setup, root_specs, reuse=[external_spec])
+
+        assert len(result.specs) == 1
+        sombrero = result.specs[0]
+        assert sombrero["externaltool"].dag_hash() == external_spec.dag_hash()
+
 
 @pytest.fixture()
 def duplicates_test_repository():

var/spack/repos/builtin.mock/packages/sombrero/package.py

@@ -0,0 +1,16 @@
+# Copyright 2013-2024 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack.package import *
+
+
+class Sombrero(Package):
+    """Simple package with a dependency on an external spec."""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/b-1.0.tar.gz"
+
+    version("1.0", md5="0123456789abcdef0123456789abcdef")
+    depends_on("externaltool")