pexpect read() leaves zombies on EOF

[jdemeyer]

If read_nonblocking() detects an EOF condition, it should call isalive() to reap the child process.

[dimpase]

any progress on this?

[jpflori]

Don't we also have an issue with sage-cleaner?

[embray]

From my experience on #18267 apparently now it does, as I'm getting tracebacks ending with:

      File "/home/embray/src/sagemath/sage/local/lib/python2.7/site-packages/pexpect/spawnbase.py", line 369, in expect_list
        return exp.expect_loop(timeout)
      File "/home/embray/src/sagemath/sage/local/lib/python2.7/site-packages/pexpect/expect.py", line 111, in expect_loop
        incoming = spawn.read_nonblocking(spawn.maxread, timeout)
      File "/home/embray/src/sagemath/sage/local/lib/python2.7/site-packages/pexpect/pty_spawn.py", line 469, in read_nonblocking
        self.isalive()
      File "/home/embray/src/sagemath/sage/local/lib/python2.7/site-packages/pexpect/pty_spawn.py", line 704, in isalive
        alive = ptyproc.isalive()
      File "/home/embray/src/sagemath/sage/local/lib/python2.7/contextlib.py", line 35, in __exit__
        self.gen.throw(type, value, traceback)
      File "/home/embray/src/sagemath/sage/local/lib/python2.7/site-packages/pexpect/pty_spawn.py", line 25, in _wrap_ptyprocess_err
        raise ExceptionPexpect(*e.args)
    ExceptionPexpect: isalive() encountered condition where "terminated" is 0, but there was no child process. Did someone else call waitpid() on our process?

Unfortunately, sometimes sage-cleaner reaps the zombie process during isalive() resulting in this exception, which we don't handle in some cases, but that's a separate issue IIUC.