Add vendors directive

For the time being this directive prevents the vendored package
to be in the same DAG as the one vendoring it.

lib/spack/spack/directives.py

@@ -69,6 +69,7 @@ class OpenMpi(Package):
     "resource",
     "build_system",
     "requires",
+    "vendors",
 ]
 
 #: These are variant names used by Spack internally; packages can't use them
@@ -916,6 +917,29 @@ def _execute_requires(pkg):
     return _execute_requires
 
 
+@directive("vendors")
+def vendors(spec, when=None):
+    """Declares that a package has an internal copy of another package.
+
+    Currently, the effect is to forbid having the two packages in the same
+    "unification set".
+
+    Args:
+        spec: spec being vendored
+        when: optional constraint that triggers vendoring
+    """
+
+    def _execute_vendors(pkg):
+        when_spec = make_when_spec(when)
+        if not when_spec:
+            return
+
+        when_spec_list = pkg.vendors.setdefault(spec, [])
+        when_spec_list.append(when_spec)
+
+    return _execute_vendors
+
+
 class DirectiveError(spack.error.SpackError):
     """This is raised when something is wrong with a package directive."""
 

lib/spack/spack/solver/asp.py

@@ -1040,6 +1040,15 @@ def conflict_rules(self, pkg):
                 )
                 self.gen.newline()
 
+    def vendor_rules(self, pkg):
+        """Facts about vendored packages."""
+        for vendored_spec_str, constraints in pkg.vendors.items():
+            vendored_spec = spack.spec.Spec(vendored_spec_str)
+            for constraint in constraints:
+                constraint_id = self.condition(constraint, name=pkg.name)
+                self.gen.fact(fn.pkg_fact(pkg.name, fn.vendors(constraint_id, vendored_spec.name)))
+                self.gen.newline()
+
     def compiler_facts(self):
         """Facts about available compilers."""
 
@@ -1189,6 +1198,9 @@ def pkg_rules(self, pkg, tests):
         # conflicts
         self.conflict_rules(pkg)
 
+        # vendoring
+        self.vendor_rules(pkg)
+
         # default compilers for this package
         self.package_compiler_defaults(pkg)
 

lib/spack/spack/solver/concretize.lp

@@ -449,6 +449,16 @@ error(1, Msg)
      not external(node(ID, Package)),  % ignore conflicts for externals
      not attr("hash", node(ID, Package), _).  % ignore conflicts for installed packages
 
+%-----------------------------------------------------------------------------
+% Vendoring
+%-----------------------------------------------------------------------------
+error(1, "{0} vendors an internal copy of {1}, so it cannot be in the same unification set as {1}", Package, VendoredPackage)
+  :- pkg_fact(Package, vendors(ConditionID, VendoredPackage)),
+     attr("node", node(ID, Package)),
+     condition_holds(ConditionID, node(ID, Package)),
+     unification_set(X, node(ID, Package)),
+     unification_set(X, node(_, VendoredPackage)).
+
 %-----------------------------------------------------------------------------
 % Virtual dependencies
 %-----------------------------------------------------------------------------

lib/spack/spack/test/env.py

@@ -551,3 +551,38 @@ def test_environment_config_scheme_used(tmp_path, unify_in_config):
     with spack.config.override("concretizer:unify", unify_in_config):
         with ev.Environment(manifest.parent) as e:
             assert e.unify == unify_in_config
+
+
+@pytest.mark.parametrize(
+    "spec_str,expected_raise,expected_spec",
+    [
+        # vendorsb vendors "b" only when @=1.1
+        ("vendorsb", False, "vendorsb@=1.0"),
+        ("vendorsb@=1.1", True, None),
+    ],
+)
+def test_vendors_directive(
+    spec_str, expected_raise, expected_spec, tmp_path, mock_packages, config
+):
+    """Tests that we cannot concretize two specs together, if one vendors the other."""
+    if spack.config.get("config:concretizer") == "original":
+        pytest.xfail("Known failure of the original concretizer")
+
+    manifest = tmp_path / "spack.yaml"
+    manifest.write_text(
+        f"""\
+spack:
+  specs:
+  - {spec_str}
+  - b
+  concretizer:
+    unify: true
+"""
+    )
+    with ev.Environment(manifest.parent) as e:
+        if expected_raise:
+            with pytest.raises(spack.solver.asp.UnsatisfiableSpecError):
+                e.concretize()
+        else:
+            e.concretize()
+            assert any(s.satisfies(expected_spec) for s in e.concrete_roots())