bpc-29877: compileall: import ProcessPoolExecutor only when needed

Importing ProcessPoolExecutor may hang or cause an error when the import
accesses urandom on a low resource platform

Author: virtuald

Lib/compileall.py

@@ -16,10 +16,9 @@
 import py_compile
 import struct
 
-try:
-    from concurrent.futures import ProcessPoolExecutor
-except ImportError:
-    ProcessPoolExecutor = None
+# Only import when needed, as low resource platforms may fail to import it
+ProcessPoolExecutor = None
+
 from functools import partial
 
 __all__ = ["compile_dir","compile_file","compile_path"]
@@ -70,13 +69,21 @@ def compile_dir(dir, maxlevels=10, ddir=None, force=False, rx=None,
     workers:   maximum number of parallel workers
     invalidation_mode: how the up-to-dateness of the pyc will be checked
     """
-    if workers is not None and workers < 0:
-        raise ValueError('workers must be greater or equal to 0')
-
+    global ProcessPoolExecutor
+    if workers is not None:
+        if workers < 0:
+            raise ValueError('workers must be greater or equal to 0')
+        elif workers > 1 and ProcessPoolExecutor is None:
+            try:
+                from concurrent.futures import ProcessPoolExecutor as PPE
+            except ImportError:
+                ProcessPoolExecutor = False
+            else:
+                ProcessPoolExecutor = PPE
     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 is not None and workers != 1 and ProcessPoolExecutor:
         workers = workers or None
         with ProcessPoolExecutor(max_workers=workers) as executor:
             results = executor.map(partial(compile_file,

Misc/NEWS.d/next/Library/2017-12-16-11-40-52.bpo-29877.SfWhmz.rst

@@ -0,0 +1,2 @@
+compileall: import ProcessPoolExecutor only when needed, preventing hangs on
+low resource platforms