diff -r ccce01988603 Lib/subprocess.py
--- a/Lib/subprocess.py	Thu Jul 28 09:55:13 2011 -0700
+++ b/Lib/subprocess.py	Thu Jul 28 22:51:39 2011 +0200
@@ -424,12 +424,16 @@
 except:
     MAXFD = 256
 
+# This lists holds Popen instances for which the underlying process had not
+# exited at the time its __del__ method got called: those processes are wait()ed
+# for synchronously from _cleanup() when a new Popen object is created, to avoid
+# zombie processes.
 _active = []
 
 def _cleanup():
     for inst in _active[:]:
         res = inst._internal_poll(_deadstate=sys.maxsize)
-        if res is not None and res >= 0:
+        if res is not None:
             try:
                 _active.remove(inst)
             except ValueError:
@@ -1272,6 +1276,7 @@
                             errread, errwrite,
                             errpipe_read, errpipe_write,
                             restore_signals, start_new_session, preexec_fn)
+                    self._child_created = True
                 finally:
                     # be sure the FD is closed no matter what
                     os.close(errpipe_write)
diff -r ccce01988603 Lib/test/test_subprocess.py
--- a/Lib/test/test_subprocess.py	Thu Jul 28 09:55:13 2011 -0700
+++ b/Lib/test/test_subprocess.py	Thu Jul 28 22:51:39 2011 +0200
@@ -1622,6 +1622,63 @@
             if dir is not None:
                 os.rmdir(dir)
 
+    def test_zombie_fast_process_del(self):
+        # Issue #12650: on Unix, if Popen.__del__() was called before the
+        # process exited, it wouldn't be added to subprocess._active, and would
+        # remain a zombie.
+        # spawn a Popen, and delete its reference before it exits
+        p = subprocess.Popen([sys.executable, "-c",
+                              'import sys, time;'
+                              'time.sleep(0.2)'],
+                             stdout=subprocess.PIPE,
+                             stderr=subprocess.PIPE)
+        ident = id(p)
+        pid = p.pid
+        del p
+        # check that p is in the active processes list
+        self.assertIn(ident, [id(o) for o in subprocess._active])
+
+        # sleep a little to let the process exit, and create a new Popen: this
+        # should trigger the wait() of p
+        time.sleep(1)
+        with self.assertRaises(EnvironmentError) as c:
+            with subprocess.Popen(['nonexisting_i_hope'],
+                                  stdout=subprocess.PIPE,
+                                  stderr=subprocess.PIPE) as proc:
+                pass
+        # p should have been wait()ed on, and removed from the _active list
+        self.assertRaises(OSError, os.waitpid, pid, 0)
+        self.assertNotIn(ident, [id(o) for o in subprocess._active])
+
+    def test_leak_fast_process_del_killed(self):
+        # Issue #12650: on Unix, if Popen.__del__() was called before the
+        # process exited, and the process got killed by a signal, it would never
+        # be removed from subprocess._active, which triggered a FD and memory
+        # leak.
+        # spawn a Popen, delete its reference and kill it
+        p = subprocess.Popen([sys.executable, "-c",
+                              'import time;'
+                              'time.sleep(3)'],
+                             stdout=subprocess.PIPE,
+                             stderr=subprocess.PIPE)
+        ident = id(p)
+        pid = p.pid
+        del p
+        os.kill(pid, signal.SIGKILL)
+        # check that p is in the active processes list
+        self.assertIn(ident, [id(o) for o in subprocess._active])
+
+        # let some time for the process to exit, and create a new Popen: this
+        # should trigger the wait() of p
+        time.sleep(0.2)
+        with self.assertRaises(EnvironmentError) as c:
+            with subprocess.Popen(['nonexisting_i_hope'],
+                                  stdout=subprocess.PIPE,
+                                  stderr=subprocess.PIPE) as proc:
+                pass
+        # p should have been wait()ed on, and removed from the _active list
+        self.assertRaises(OSError, os.waitpid, pid, 0)
+        self.assertNotIn(ident, [id(o) for o in subprocess._active])
 
 @unittest.skipUnless(getattr(subprocess, '_has_poll', False),
                      "poll system call not supported")