Installations: don't set group permissions when they match what is desired (#38036)

* When installing a package Spack will attempt to set group permissions on
the install prefix even when the configuration does not specify a group.

Co-authored-by: David Gomez <dvdgomez@users.noreply.github.com>

lib/spack/llnl/util/filesystem.py

@@ -610,6 +610,8 @@ def chgrp(path, group, follow_symlinks=True):
         gid = grp.getgrnam(group).gr_gid
     else:
         gid = group
+    if os.stat(path).st_gid == gid:
+        return
     if follow_symlinks:
         os.chown(path, -1, gid)
     else:

lib/spack/spack/test/llnl/util/filesystem.py

@@ -502,6 +502,40 @@ def test_filter_files_with_different_encodings(regex, replacement, filename, tmp
         assert replacement in f.read()
 
 
+@pytest.mark.skipif(sys.platform == "win32", reason="chgrp isn't used on Windows")
+def test_chgrp_dont_set_group_if_already_set(tmpdir, monkeypatch):
+    with fs.working_dir(tmpdir):
+        os.mkdir("test-dir_chgrp_dont_set_group_if_already_set")
+
+    def _fail(*args, **kwargs):
+        raise Exception("chrgrp should not be called")
+
+    class FakeStat(object):
+        def __init__(self, gid):
+            self.st_gid = gid
+
+    original_stat = os.stat
+
+    def _stat(*args, **kwargs):
+        path = args[0]
+        if path == "test-dir_chgrp_dont_set_group_if_already_set":
+            return FakeStat(gid=1001)
+        else:
+            # Monkeypatching stat can interfere with post-test cleanup, so for
+            # paths that aren't part of the test, we want the original behavior
+            # of stat
+            return original_stat(*args, **kwargs)
+
+    monkeypatch.setattr(os, "chown", _fail)
+    monkeypatch.setattr(os, "lchown", _fail)
+    monkeypatch.setattr(os, "stat", _stat)
+
+    with fs.working_dir(tmpdir):
+        with pytest.raises(Exception):
+            fs.chgrp("test-dir_chgrp_dont_set_group_if_already_set", 1002)
+        fs.chgrp("test-dir_chgrp_dont_set_group_if_already_set", 1001)
+
+
 def test_filter_files_multiple(tmpdir):
     # All files given as input to this test must satisfy the pre-requisite
     # that the 'replacement' string is not present in the file initially and