bpo-40094: Add os.waitstatus_to_exitcode() (GH-19201)

Add os.waitstatus_to_exitcode() function to convert a wait status to an
exitcode.

Suggest waitstatus_to_exitcode() usage in the documentation when
appropriate.

Use waitstatus_to_exitcode() in:

* multiprocessing, os, subprocess and _bootsubprocess modules;
* test.support.wait_process();
* setup.py: run_command();
* and many tests.

1 files changed, 1 insertions, 5 deletions

diff --git a/Lib/multiprocessing/popen_fork.py b/Lib/multiprocessing/popen_fork.py
index a65b06f1b2c..625981cf476 100644
--- a/Lib/multiprocessing/popen_fork.py
+++ b/Lib/multiprocessing/popen_fork.py
@@ -30,11 +30,7 @@ class Popen(object):
                 # e.errno == errno.ECHILD == 10
                 return None
             if pid == self.pid:
-                if os.WIFSIGNALED(sts):
-                    self.returncode = -os.WTERMSIG(sts)
-                else:
-                    assert os.WIFEXITED(sts), "Status is {:n}".format(sts)
-                    self.returncode = os.WEXITSTATUS(sts)
+                self.returncode = os.waitstatus_to_exitcode(sts)
         return self.returncode
 
     def wait(self, timeout=None):