[matshch]

Some environments are too restricted to set setgid, so check that we can do that before expecting the rest of the code to do that.

[folbricht]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

The probe mirrors the sequence the code under test actually uses — prepareDirWrite creates the directory with n.Mode.Perm() (setgid stripped) and chmods the bit back in — so os.Mkdir followed by os.Chmod(…|os.ModeSetgid) is a faithful capability check, and the stat re-read covers the case where chmod reports success but the bit is not preserved.

desync/localfs_other.go

Lines 106 to 114 in f1a54c4

	}
	// Newly created directories are chmod'ed as well, both to undo a umask
	// that stripped the bits we need and to set the setuid/setgid/sticky bits
	// that mkdir doesn't apply. A setgid directory has to carry that bit while
	// its contents are created for them to inherit the group.
	if created || existing&0300 != 0300 {
	_ = r.Chmod(n.Name, n.Mode|0700)
	}
	}

The skip is additive: the assertions are untouched, so coverage is unchanged wherever setgid works. It also matches the existing convention for environment-dependent skips in this package.

desync/localfs_dirmeta_test.go

Lines 110 to 124 in 7092cc7

	// the contents of a directory are written, so they inherit its group.
	func TestLocalFSDirSetgidWhilePopulated(t *testing.T) {
	// Some build sandboxes prevent us from setting setgid bit
	probe := filepath.Join(t.TempDir(), "setgid")
	require.NoError(t, os.Mkdir(probe, 0755))
	if err := os.Chmod(probe, 0775|os.ModeSetgid); err != nil {
	t.Skipf("setgid on a directory is not permitted here: %v", err)
	}
	// Some environments do not preserve setgid, e.g. macOS with TMPDIR=/tmp
	info, err := os.Stat(probe)
	require.NoError(t, err)
	if info.Mode()&os.ModeSetgid == 0 {
	t.Skip("setgid on a directory is not preserved here")
	}

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