environment: be more defensive when deleting roots for old views (#29636)

Currently `old_root` is computed by reading the symlink at `self.root`.
We should be more defensive in removing it by checking that it is in the
same directory as the new root. Otherwise, in the worst case, when
someone runs `spack env create --with-view=./view -d .` and `view`
already exists and is a symlink to `/`, Spack effectively runs `rm -rf /`.

lib/spack/spack/environment/environment.py

@@ -541,8 +541,14 @@ def regenerate(self, concretized_specs):
             raise SpackEnvironmentViewError(msg)
         rename(tmp_symlink_name, self.root)
 
-        # remove old_root
+        # Remove the old root when it's in the same folder as the new root. This guards
-        if old_root and os.path.exists(old_root):
+        # against removal of an arbitrary path when the original symlink in self.root
+        # was not created by the environment, but by the user.
+        if (
+            old_root and
+            os.path.exists(old_root) and
+            os.path.samefile(os.path.dirname(new_root), os.path.dirname(old_root))
+        ):
             try:
                 shutil.rmtree(old_root)
             except (IOError, OSError) as e:

lib/spack/spack/test/cmd/env.py

@@ -2735,3 +2735,14 @@ def test_activate_temp(monkeypatch, tmpdir):
                           if ev.spack_env_var in line)
     assert str(tmpdir) in active_env_var
     assert ev.is_env_dir(str(tmpdir))
+
+
+def test_env_view_fail_if_symlink_points_elsewhere(tmpdir, install_mockery, mock_fetch):
+    view = str(tmpdir.join('view'))
+    # Put a symlink to an actual directory in view
+    non_view_dir = str(tmpdir.mkdir('dont-delete-me'))
+    os.symlink(non_view_dir, view)
+    with ev.create('env', with_view=view):
+        add('libelf')
+        install('--fake')
+    assert os.path.isdir(non_view_dir)