Fix spack install chgrp on symlinks (#30743)

Fixes missing chgrp on symlinks in package installations, and errors on
symlinks referencing non-existent or non-writable locations.

Note: `os.chown(.., follow_symlinks=False)` is python3 only, but
`os.lchown` exists in both versions.

lib/spack/llnl/util/filesystem.py

@@ -302,13 +302,16 @@ def group_ids(uid=None):
     return [g.gr_gid for g in grp.getgrall() if user in g.gr_mem]
 
 
-def chgrp(path, group):
+def chgrp(path, group, follow_symlinks=True):
     """Implement the bash chgrp function on a single path"""
     if isinstance(group, six.string_types):
         gid = grp.getgrnam(group).gr_gid
     else:
         gid = group
-    os.chown(path, -1, gid)
+    if follow_symlinks:
+        os.chown(path, -1, gid)
+    else:
+        os.lchown(path, -1, gid)
 
 
 def chmod_x(entry, perms):

lib/spack/spack/test/installer.py

@@ -834,7 +834,7 @@ def test_setup_install_dir_grp(install_mockery, monkeypatch, capfd):
     def _get_group(spec):
         return mock_group
 
-    def _chgrp(path, group):
+    def _chgrp(path, group, follow_symlinks=True):
         tty.msg(mock_chgrp_msg.format(path, group))
 
     monkeypatch.setattr(prefs, 'get_package_group', _get_group)

lib/spack/spack/util/file_permissions.py

@@ -44,7 +44,7 @@ def set_permissions(path, perms, group=None):
     fs.chmod_x(path, perms)
 
     if group:
-        fs.chgrp(path, group)
+        fs.chgrp(path, group, follow_symlinks=False)
 
 
 class InvalidPermissionsError(SpackError):