os: implement waitstatus_to_exitcode

vm/src/stdlib/os.rs

@@ -1668,6 +1668,31 @@ pub(super) mod _os {
         Ok((loadavg[0], loadavg[1], loadavg[2]))
     }
 
+    #[cfg(any(unix, windows))]
+    #[pyfunction]
+    fn waitstatus_to_exitcode(status: i32, vm: &VirtualMachine) -> PyResult<i32> {
+        let status = u32::try_from(status)
+            .map_err(|_| vm.new_value_error(format!("invalid WEXITSTATUS: {}", status)))?;
+
+        cfg_if::cfg_if! {
+            if #[cfg(not(windows))] {
+                let status = status as libc::c_int;
+                if libc::WIFEXITED(status) {
+                    return Ok(libc::WEXITSTATUS(status));
+                }
+
+                if libc::WIFSIGNALED(status) {
+                    return Ok(-libc::WTERMSIG(status));
+                }
+
+                Err(vm.new_value_error(format!("Invalid wait status: {}", status)))
+            } else {
+                i32::try_from(status.rotate_right(8))
+                    .map_err(|_| vm.new_value_error(format!("invalid wait status: {}", status)))
+            }
+        }
+    }
+
     #[pyattr]
     #[pyclass(module = "os", name = "terminal_size")]
     #[derive(PyStructSequence)]