util: simplify pipefilter and avoid subprocess race The subprocess module is not thread safe. Spawning a thread to read the output leads to exceptions like this when Mercurial exits: Exception exceptions.TypeError: TypeError("'NoneType' object is not callable",) in <bound method Popen.__del__ of <subprocess.Popen object at 0x9ed0dcc>> ignored The bug is already reported in the Python bug tracker: http://bugs.python.org/issue1731717