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.

2 files changed, 3 insertions, 13 deletions

diff --git a/Lib/multiprocessing/forkserver.py b/Lib/multiprocessing/forkserver.py
index 215ac39..22a911a 100644
--- a/Lib/multiprocessing/forkserver.py
+++ b/Lib/multiprocessing/forkserver.py
@@ -237,14 +237,8 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None):
                             break
                         child_w = pid_to_fd.pop(pid, None)
                         if child_w is not None:
-                            if os.WIFSIGNALED(sts):
-                                returncode = -os.WTERMSIG(sts)
-                            else:
-                                if not os.WIFEXITED(sts):
-                                    raise AssertionError(
-                                        "Child {0:n} status is {1:n}".format(
-                                            pid,sts))
-                                returncode = os.WEXITSTATUS(sts)
+                            returncode = os.waitstatus_to_exitcode(sts)
+
                             # Send exit code to client process
                             try:
                                 write_signed(child_w, returncode)
diff --git a/Lib/multiprocessing/popen_fork.py b/Lib/multiprocessing/popen_fork.py
index a65b06f..625981c 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):