ASP-based solver: discard unknown packages from reuse (#30357)

* ASP-based solver: discard unknown packages from reuse

This is an add-on to #28259 that cover for the case of
a single package.py being removed from a repository,
rather than an entire custom repository being removed.

* Add unit test

lib/spack/spack/solver/asp.py

@@ -1661,8 +1661,8 @@ def _facts_from_concrete_spec(self, spec, possible):
         if spec.name in possible and h not in self.seen_hashes:
             try:
                 # Only consider installed packages for repo we know
-                spack.repo.path.repo_for_pkg(spec)
-            except spack.repo.UnknownNamespaceError:
+                spack.repo.path.get(spec)
+            except (spack.repo.UnknownNamespaceError, spack.repo.UnknownPackageError):
                 return
 
             # this indicates that there is a spec like this installed
@@ -2053,6 +2053,8 @@ def build_specs(self, function_tuples):
         # namespace assignment is done after the fact, as it is not
         # currently part of the solve
         for spec in self._specs.values():
+            if spec.namespace:
+                continue
             repo = spack.repo.path.repo_for_pkg(spec)
             spec.namespace = repo.namespace
 
@@ -2062,7 +2064,7 @@ def build_specs(self, function_tuples):
         # inject patches -- note that we' can't use set() to unique the
         # roots here, because the specs aren't complete, and the hash
         # function will loop forever.
-        roots = [spec.root for spec in self._specs.values()]
+        roots = [spec.root for spec in self._specs.values() if not spec.root.installed]
         roots = dict((id(r), r) for r in roots)
         for root in roots.values():
             spack.spec.Spec.inject_patches_variant(root)

lib/spack/spack/test/concretize.py

@@ -3,6 +3,7 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 import os
+import shutil
 import sys
 
 import jinja2
@@ -1645,3 +1646,22 @@ def test_reuse_with_unknown_namespace_dont_raise(
         with spack.config.override("concretizer:reuse", True):
             s = Spec('c').concretized()
         assert s.namespace == 'builtin.mock'
+
+    @pytest.mark.regression('28259')
+    def test_reuse_with_unknown_package_dont_raise(
+            self, additional_repo_with_c, mutable_mock_repo, monkeypatch
+    ):
+        s = Spec('c').concretized()
+        assert s.namespace == 'myrepo'
+        s.package.do_install(fake=True, explicit=True)
+
+        # Here we delete the package.py instead of removing the repo and we
+        # make it such that "c" doesn't exist in myrepo
+        del sys.modules['spack.pkg.myrepo.c']
+        c_dir = os.path.join(additional_repo_with_c.root, 'packages', 'c')
+        shutil.rmtree(c_dir)
+        monkeypatch.setattr(additional_repo_with_c, 'exists', lambda x: False)
+
+        with spack.config.override("concretizer:reuse", True):
+            s = Spec('c').concretized()
+        assert s.namespace == 'builtin.mock'