bpo-40094: Add _bootsubprocess._waitstatus_to_exitcode (GH-19264)

* Add _waitstatus_to_exitcode() helper function to _bootsubprocess.
* Enhance check_output() error message if the command fails.

_bootsubprocess no longer handles WIFSTOPPED() case: it now raises a
ValueError.

Lib/_bootsubprocess.py

@@ -6,6 +6,15 @@ subprocess is unavailable. setup.py is not used on Windows.
 import os
 
 
+def _waitstatus_to_exitcode(status):
+    if os.WIFEXITED(status):
+        return os.WEXITSTATUS(status)
+    elif os.WIFSIGNALED(status):
+        return -os.WTERMSIG(status)
+    else:
+        raise ValueError(f"invalid wait status: {status!r}")
+
+
 # distutils.spawn used by distutils.command.build_ext
 # calls subprocess.Popen().wait()
 class Popen:
@@ -27,15 +36,8 @@ class Popen:
                 os._exit(1)
         else:
             # Parent process
-            pid, status = os.waitpid(pid, 0)
-            if os.WIFSIGNALED(status):
-                self.returncode = -os.WTERMSIG(status)
-            elif os.WIFEXITED(status):
-                self.returncode = os.WEXITSTATUS(status)
-            elif os.WIFSTOPPED(status):
-                self.returncode = -os.WSTOPSIG(status)
-            else:
-                raise Exception(f"unknown child process exit status: {status!r}")
+            _, status = os.waitpid(pid, 0)
+            self.returncode = _waitstatus_to_exitcode(status)
 
         return self.returncode
 
@@ -85,8 +87,10 @@ def check_output(cmd, **kwargs):
     try:
         # system() spawns a shell
         status = os.system(cmd)
-        if status:
-            raise ValueError(f"Command {cmd!r} failed with status {status!r}")
+        exitcode = _waitstatus_to_exitcode(status)
+        if exitcode:
+            raise ValueError(f"Command {cmd!r} returned non-zero "
+                             f"exit status {exitcode!r}")
 
         try:
             with open(tmp_filename, "rb") as fp: