Index: subprocess.py
===================================================================
--- subprocess.py	(revision 39605)
+++ subprocess.py	(working copy)
@@ -411,13 +411,19 @@
     True = 1
 
 _active = []
 
 def _cleanup():
     for inst in _active[:]:
-        inst.poll()
+        if inst.poll(_deadstate=sys.maxint) >= 0:        
+            try:
+                _active.remove(inst)
+            except ValueError:
+                # This can happen if two threads create a new Popen instance.
+                # It's harmless that it was already removed, so ignore.
+                pass
 
 PIPE = -1
 STDOUT = -2
 
 
 def call(*popenargs, **kwargs):
@@ -589,19 +595,26 @@
         if errread:
             if universal_newlines:
                 self.stderr = os.fdopen(errread, 'rU', bufsize)
             else:
                 self.stderr = os.fdopen(errread, 'rb', bufsize)
 
-        _active.append(self)
-
 
     def _translate_newlines(self, data):
         data = data.replace("\r\n", "\n")
         data = data.replace("\r", "\n")
         return data
+
+
+    def __del__(self):
+        # In case the child hasn't been waited on, check if it's done.
+        self.poll(_deadstate=sys.maxint)
+        if self.returncode is None:
+            # Child is still running, keep us alive until we can wait on it.
+            _active.append(self)
+
 
     def communicate(self, input=None):
         """Interact with process: Send data to stdin.  Read data from
         stdout and stderr, until end-of-file is reached.  Wait for
         process to terminate.  The optional input argument should be a
         string to be sent to the child process, or None, if no data
@@ -792,29 +805,27 @@
             if c2pwrite is not None:
                 c2pwrite.Close()
             if errwrite is not None:
                 errwrite.Close()
 
 
-        def poll(self):
+        def poll(self, _deadstate=None):
             """Check if child process has terminated.  Returns returncode
             attribute."""
             if self.returncode is None:
                 if WaitForSingleObject(self._handle, 0) == WAIT_OBJECT_0:
                     self.returncode = GetExitCodeProcess(self._handle)
-                    _active.remove(self)
             return self.returncode
 
 
         def wait(self):
             """Wait for child process to terminate.  Returns returncode
             attribute."""
             if self.returncode is None:
                 obj = WaitForSingleObject(self._handle, INFINITE)
                 self.returncode = GetExitCodeProcess(self._handle)
-                _active.remove(self)
             return self.returncode
 
 
         def _readerthread(self, fh, buffer):
             buffer.append(fh.read())
 
@@ -1039,25 +1050,24 @@
             elif os.WIFEXITED(sts):
                 self.returncode = os.WEXITSTATUS(sts)
             else:
                 # Should never happen
                 raise RuntimeError("Unknown child exit status!")
 
-            _active.remove(self)
-
 
         def poll(self):
             """Check if child process has terminated.  Returns returncode
             attribute."""
             if self.returncode is None:
                 try:
                     pid, sts = os.waitpid(self.pid, os.WNOHANG)
                     if pid == self.pid:
                         self._handle_exitstatus(sts)
                 except os.error:
-                    pass
+                    if _deadstate is not None:
+                        self.returncode = _deadstate
             return self.returncode
 
 
         def wait(self):
             """Wait for child process to terminate.  Returns returncode
             attribute."""