Fix a bug with automatic tag detection (#45696)

Extracted from #45638

When adding the "detectable" tag to a package class that has the
"tag" attribute inherited from a base class, we need to copy it to
avoid modifying the base class.

lib/spack/spack/package_base.py

@@ -197,13 +197,12 @@ def __init__(cls, name, bases, attr_dict):
         # that "foo" was a possible executable.
 
         # If a package has the executables or libraries  attribute then it's
-        # assumed to be detectable
+        # assumed to be detectable. Add a tag, so finding them is faster
         if hasattr(cls, "executables") or hasattr(cls, "libraries"):
-            # Append a tag to each detectable package, so that finding them is faster
-            if not hasattr(cls, "tags"):
-                setattr(cls, "tags", [DetectablePackageMeta.TAG])
-            elif DetectablePackageMeta.TAG not in cls.tags:
-                cls.tags.append(DetectablePackageMeta.TAG)
+            # To add the tag, we need to copy the tags attribute, and attach it to
+            # the current class. We don't use append, since it might modify base classes,
+            # if "tags" is retrieved following the MRO.
+            cls.tags = getattr(cls, "tags", []) + [DetectablePackageMeta.TAG]
 
             @classmethod
             def platform_executables(cls):

lib/spack/spack/test/cmd/external.py

@@ -114,11 +114,31 @@ def test_find_external_cmd_not_buildable(mutable_config, working_env, mock_execu
 @pytest.mark.parametrize(
     "names,tags,exclude,expected",
     [
-        # find --all
-        (None, ["detectable"], [], ["builtin.mock.find-externals1"]),
+        # find -all
+        (
+            None,
+            ["detectable"],
+            [],
+            [
+                "builtin.mock.find-externals1",
+                "builtin.mock.gcc",
+                "builtin.mock.llvm",
+                "builtin.mock.intel-oneapi-compilers",
+            ],
+        ),
         # find --all --exclude find-externals1
-        (None, ["detectable"], ["builtin.mock.find-externals1"], []),
-        (None, ["detectable"], ["find-externals1"], []),
+        (
+            None,
+            ["detectable"],
+            ["builtin.mock.find-externals1"],
+            ["builtin.mock.gcc", "builtin.mock.llvm", "builtin.mock.intel-oneapi-compilers"],
+        ),
+        (
+            None,
+            ["detectable"],
+            ["find-externals1"],
+            ["builtin.mock.gcc", "builtin.mock.llvm", "builtin.mock.intel-oneapi-compilers"],
+        ),
         # find cmake (and cmake is not detectable)
         (["cmake"], ["detectable"], [], []),
     ],

lib/spack/spack/test/tag.py

@@ -153,7 +153,7 @@ def test_tag_no_tags(mock_packages):
 
 
 def test_tag_update_package(mock_packages):
-    mock_index = spack.repo.PATH.tag_index
+    mock_index = mock_packages.tag_index
     index = spack.tag.TagIndex(repository=mock_packages)
     for name in spack.repo.all_package_names():
         index.update_package(name)