constraint readonlyAttr for gahr

testConstraint readonlyAttr 0
if {[testConstraint unix]} {
    set f [makeFile "whatever" probe]
    catch {
        if {[catch { file attributes $f -readonly 1; file attributes $f -readonly 0 }]} {
          switch -- $::tcl_platform(os) {
            "FreeBSD" - "Darwin" {
                # FreeBSD and macOS (BSD-based)
                # Fixes FreeBSD ZFS bug where unsetting masks failure
                exec chflags uchg $f
                exec chflags nouchg $f
            }
            "Linux" {
                # Linux (requires root/CAP_LINUX_IMMUTABLE to succeed)
                exec chattr +i $f
                exec chattr -i $f
            }
            "SunOS" {
                # Solaris / Illumos / OpenIndiana (ZFS native attributes)
                exec chmod +N readonly $f
                exec chmod -N readonly $f
            }
            default {
                # Fallback for NetBSD, OpenBSD, AIX, HP-UX, etc.
                file attributes $f -readonly
            }
          }
        }
        testConstraint readonlyAttr 1
    }
    removeFile probe
}