package preferences: allow specs to be configured buildable when their virtuals are not (#18269)

* respect spec buildable that overrides virtual buildable

lib/spack/spack/package_prefs.py

@@ -195,23 +195,23 @@ def _package(maybe_abstract_spec):
 
 def is_spec_buildable(spec):
     """Return true if the spec is configured as buildable"""
-
     allpkgs = spack.config.get("packages")
     all_buildable = allpkgs.get("all", {}).get("buildable", True)
+    so_far = all_buildable  # the default "so far"
 
     def _package(s):
         pkg_cls = spack.repo.path.get_pkg_class(s.name)
         return pkg_cls(s)
 
-    # Get the list of names for which all_buildable is overridden
-    reverse = [
-        name
+    # check whether any providers for this package override the default
+    if any(
+        _package(spec).provides(name) and entry.get("buildable", so_far) != so_far
         for name, entry in allpkgs.items()
-        if entry.get("buildable", all_buildable) != all_buildable
-    ]
-    # Does this spec override all_buildable
-    spec_reversed = spec.name in reverse or any(_package(spec).provides(name) for name in reverse)
-    return not all_buildable if spec_reversed else all_buildable
+    ):
+        so_far = not so_far
+
+    spec_buildable = allpkgs.get(spec.name, {}).get("buildable", so_far)
+    return spec_buildable
 
 
 def get_package_dir_permissions(spec):

lib/spack/spack/test/concretize_preferences.py

@@ -395,6 +395,23 @@ def test_buildable_false_all_true_virtual(self):
         spec = Spec("mpich")
         assert spack.package_prefs.is_spec_buildable(spec)
 
+    def test_buildable_false_virtual_true_pacakge(self):
+        conf = syaml.load_config(
+            """\
+mpi:
+  buildable: false
+mpich:
+  buildable: true
+"""
+        )
+        spack.config.set("packages", conf, scope="concretize")
+
+        spec = Spec("zmpi")
+        assert not spack.package_prefs.is_spec_buildable(spec)
+
+        spec = Spec("mpich")
+        assert spack.package_prefs.is_spec_buildable(spec)
+
     def test_config_permissions_from_all(self, configure_permissions):
         # Although these aren't strictly about concretization, they are
         # configured in the same file and therefore convenient to test here.