Issue #18762: Print debug info on failure to create new forkserver process.

Also modify test code to hopefully avoid deadlock on failure.

Lib/multiprocessing/forkserver.py

@@ -66,6 +66,21 @@ def connect_to_new_process(fds):
         try:
             reduction.sendfds(client, allfds)
             return parent_r, parent_w
+        except OSError:
+            # XXX This is debugging info for Issue #18762
+            import fcntl
+            L = []
+            for fd in allfds:
+                try:
+                    flags = fcntl.fcntl(fd, fcntl.F_GETFL)
+                except OSError as e:
+                    L.append((fd, e))
+                else:
+                    L.append((fd, flags))
+            print('*** connect_to_new_process: %r' % L, file=sys.stderr)
+            os.close(parent_r)
+            os.close(parent_w)
+            raise
         except:
             os.close(parent_r)
             os.close(parent_w)

Lib/test/_test_multiprocessing.py

@@ -3386,7 +3386,8 @@ class TestForkAwareThreadLock(unittest.TestCase):
         if n > 1:
             p = multiprocessing.Process(target=cls.child, args=(n-1, conn))
             p.start()
-            p.join()
+            conn.close()
+            p.join(timeout=5)
         else:
             conn.send(len(util._afterfork_registry))
         conn.close()
@@ -3397,8 +3398,9 @@ class TestForkAwareThreadLock(unittest.TestCase):
         old_size = len(util._afterfork_registry)
         p = multiprocessing.Process(target=self.child, args=(5, w))
         p.start()
+        w.close()
         new_size = r.recv()
-        p.join()
+        p.join(timeout=5)
         self.assertLessEqual(new_size, old_size)
 
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