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

@@ -10,12 +10,13 @@
 %=============================================================================
 
 % A package cannot be reused if the libc is not compatible with it
-:- 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
-   ReusedPackage != LibcPackage,
-   not attr("compatible_libc", node(R, ReusedPackage), LibcPackage, LibcVersion).
+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
+     ReusedPackage != LibcPackage,
+     not attr("compatible_libc", node(R, ReusedPackage), LibcPackage, LibcVersion).
 
 % Check whether the DAG has any built package
 has_built_packages() :- build(X), not external(X).

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():