bpo-36786: Run compileall in parallel during "make install" (GH-13078)

Lib/compileall.py

@@ -67,20 +67,20 @@ def compile_dir(dir, maxlevels=10, ddir=None, force=False, rx=None,
     invalidation_mode: how the up-to-dateness of the pyc will be checked
     """
     ProcessPoolExecutor = None
-    if workers is not None:
-        if workers < 0:
-            raise ValueError('workers must be greater or equal to 0')
-        elif workers != 1:
-            try:
-                # Only import when needed, as low resource platforms may
-                # fail to import it
-                from concurrent.futures import ProcessPoolExecutor
-            except ImportError:
-                workers = 1
+    if workers < 0:
+        raise ValueError('workers must be greater or equal to 0')
+    if workers != 1:
+        try:
+            # Only import when needed, as low resource platforms may
+            # fail to import it
+            from concurrent.futures import ProcessPoolExecutor
+        except ImportError:
+            workers = 1
     files = _walk_dir(dir, quiet=quiet, maxlevels=maxlevels,
                       ddir=ddir)
     success = True
-    if workers is not None and workers != 1 and ProcessPoolExecutor is not None:
+    if workers != 1 and ProcessPoolExecutor is not None:
+        # If workers == 0, let ProcessPoolExecutor choose
         workers = workers or None
         with ProcessPoolExecutor(max_workers=workers) as executor:
             results = executor.map(partial(compile_file,
@@ -290,9 +290,6 @@ def main():
                 print("Error reading file list {}".format(args.flist))
             return False
 
-    if args.workers is not None:
-        args.workers = args.workers or None
-
     if args.invalidation_mode:
         ivl_mode = args.invalidation_mode.replace('-', '_').upper()
         invalidation_mode = py_compile.PycInvalidationMode[ivl_mode]