Do not initialize previous store state in "use_store" (#45268)

The "use_store" context manager is used to swap the value
of a global variable (spack.store.STORE), while keeping
another global variable consistent (spack.config.CONFIG).

When doing that it tries to evaluate the previous value
of the store, if that was not done already. This is wrong,
since the configuration might be in an "intermediate" state
that was never meant to trigger side effects.

Remove that operation, and add a unit test to
prevent regressions.

lib/spack/spack/store.py

@@ -371,7 +371,6 @@ def use_store(
         data.update(extra_data)
 
     # Swap the store with the one just constructed and return it
-    ensure_singleton_created()
     spack.config.CONFIG.push_scope(
         spack.config.InternalConfigScope(name=scope_name, data={"config": {"install_tree": data}})
     )

lib/spack/spack/test/bootstrap.py

@@ -228,3 +228,25 @@ def test_source_is_disabled(mutable_config):
     spack.config.add("bootstrap:trusted:{0}:{1}".format(conf["name"], False))
     with pytest.raises(ValueError):
         spack.bootstrap.core.source_is_enabled_or_raise(conf)
+
+
+@pytest.mark.regression("45247")
+def test_use_store_does_not_try_writing_outside_root(tmp_path, monkeypatch, mutable_config):
+    """Tests that when we use the 'use_store' context manager, there is no attempt at creating
+    a Store outside the given root.
+    """
+    initial_store = mutable_config.get("config:install_tree:root")
+    user_store = tmp_path / "store"
+
+    fn = spack.store.Store.__init__
+
+    def _checked_init(self, root, *args, **kwargs):
+        fn(self, root, *args, **kwargs)
+        assert self.root == str(user_store)
+
+    monkeypatch.setattr(spack.store.Store, "__init__", _checked_init)
+
+    spack.store.reinitialize()
+    with spack.store.use_store(user_store):
+        assert spack.config.CONFIG.get("config:install_tree:root") == str(user_store)
+    assert spack.config.CONFIG.get("config:install_tree:root") == initial_store