I just found the readline/GIL issue while working on #18203. I created #18205 but then I found this issue. I just closed #18205 as a duplicate. Here is a patch for Python 3.4.

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Copy of the initial message (msg191089):

The callback PyOS_ReadlineFunctionPointer (used to read a line from the standard input) must return a buffer allocated by PyMem_Malloc(), but PyOS_Readline() releases the GIL before calling PyOS_ReadlineFunctionPointer.

Simplified extract of PyOS_Readline():

    Py_BEGIN_ALLOW_THREADS
    if (!isatty (fileno (sys_stdin)) || !isatty (fileno (sys_stdout)))
        rv = PyOS_StdioReadline (sys_stdin, sys_stdout, prompt);
    else
        rv = (*PyOS_ReadlineFunctionPointer)(sys_stdin, sys_stdout,
                                             prompt);
    Py_END_ALLOW_THREADS

tok_nextc() calls PyOS_Readline() and calls PyMem_FREE() to release its result.

PyOS_ReadlineFunctionPointer should allocate memory using malloc(), not using PyMem_Malloc(). But PyOS_Readline() should copy the line into a buffer allocated by PyMem_Malloc() to keep backward compatibility.

See also issue #18203 and #3329.