[3.9] bpo-46502: Remove "How do I tell incomplete input" from FAQ (GH-30925) (GH-30934)

Since,

- Py_CompileString no longer allows to distinguish "incomplete input"
  from "invalid input"
- there is no alternative solution available from the Python C API
  due to how the new parser works (rewritten in 3.9)
- the only supported way is to manually import the codeop module from C
  and use its API as IDLE does, and accept its own complications

it is desirable to remove this Q&A from the official FAQ..
(cherry picked from commit f0a648152f)

Co-authored-by: Mateusz Łoskot <mateusz@loskot.net>

Co-authored-by: Mateusz Łoskot <mateusz@loskot.net>

Doc/faq/extending.rst

@@ -254,7 +254,6 @@ For Red Hat, install the python-devel RPM to get the necessary files.
 
 For Debian, run ``apt-get install python-dev``.
 
-
 How do I tell "incomplete input" from "invalid input"?
 ------------------------------------------------------
 
@@ -273,161 +272,6 @@ you. You can also set the :c:func:`PyOS_ReadlineFunctionPointer` to point at you
 custom input function. See ``Modules/readline.c`` and ``Parser/myreadline.c``
 for more hints.
 
-However sometimes you have to run the embedded Python interpreter in the same
-thread as your rest application and you can't allow the
-:c:func:`PyRun_InteractiveLoop` to stop while waiting for user input.  The one
-solution then is to call :c:func:`PyParser_ParseString` and test for ``e.error``
-equal to ``E_EOF``, which means the input is incomplete.  Here's a sample code
-fragment, untested, inspired by code from Alex Farber::
-
-   #define PY_SSIZE_T_CLEAN
-   #include <Python.h>
-   #include <node.h>
-   #include <errcode.h>
-   #include <grammar.h>
-   #include <parsetok.h>
-   #include <compile.h>
-
-   int testcomplete(char *code)
-     /* code should end in \n */
-     /* return -1 for error, 0 for incomplete, 1 for complete */
-   {
-     node *n;
-     perrdetail e;
-
-     n = PyParser_ParseString(code, &_PyParser_Grammar,
-                              Py_file_input, &e);
-     if (n == NULL) {
-       if (e.error == E_EOF)
-         return 0;
-       return -1;
-     }
-
-     PyNode_Free(n);
-     return 1;
-   }
-
-Another solution is trying to compile the received string with
-:c:func:`Py_CompileString`. If it compiles without errors, try to execute the
-returned code object by calling :c:func:`PyEval_EvalCode`. Otherwise save the
-input for later. If the compilation fails, find out if it's an error or just
-more input is required - by extracting the message string from the exception
-tuple and comparing it to the string "unexpected EOF while parsing".  Here is a
-complete example using the GNU readline library (you may want to ignore
-**SIGINT** while calling readline())::
-
-   #include <stdio.h>
-   #include <readline.h>
-
-   #define PY_SSIZE_T_CLEAN
-   #include <Python.h>
-   #include <object.h>
-   #include <compile.h>
-   #include <eval.h>
-
-   int main (int argc, char* argv[])
-   {
-     int i, j, done = 0;                          /* lengths of line, code */
-     char ps1[] = ">>> ";
-     char ps2[] = "... ";
-     char *prompt = ps1;
-     char *msg, *line, *code = NULL;
-     PyObject *src, *glb, *loc;
-     PyObject *exc, *val, *trb, *obj, *dum;
-
-     Py_Initialize ();
-     loc = PyDict_New ();
-     glb = PyDict_New ();
-     PyDict_SetItemString (glb, "__builtins__", PyEval_GetBuiltins ());
-
-     while (!done)
-     {
-       line = readline (prompt);
-
-       if (NULL == line)                          /* Ctrl-D pressed */
-       {
-         done = 1;
-       }
-       else
-       {
-         i = strlen (line);
-
-         if (i > 0)
-           add_history (line);                    /* save non-empty lines */
-
-         if (NULL == code)                        /* nothing in code yet */
-           j = 0;
-         else
-           j = strlen (code);
-
-         code = realloc (code, i + j + 2);
-         if (NULL == code)                        /* out of memory */
-           exit (1);
-
-         if (0 == j)                              /* code was empty, so */
-           code[0] = '\0';                        /* keep strncat happy */
-
-         strncat (code, line, i);                 /* append line to code */
-         code[i + j] = '\n';                      /* append '\n' to code */
-         code[i + j + 1] = '\0';
-
-         src = Py_CompileString (code, "<stdin>", Py_single_input);
-
-         if (NULL != src)                         /* compiled just fine - */
-         {
-           if (ps1  == prompt ||                  /* ">>> " or */
-               '\n' == code[i + j - 1])           /* "... " and double '\n' */
-           {                                               /* so execute it */
-             dum = PyEval_EvalCode (src, glb, loc);
-             Py_XDECREF (dum);
-             Py_XDECREF (src);
-             free (code);
-             code = NULL;
-             if (PyErr_Occurred ())
-               PyErr_Print ();
-             prompt = ps1;
-           }
-         }                                        /* syntax error or E_EOF? */
-         else if (PyErr_ExceptionMatches (PyExc_SyntaxError))
-         {
-           PyErr_Fetch (&exc, &val, &trb);        /* clears exception! */
-
-           if (PyArg_ParseTuple (val, "sO", &msg, &obj) &&
-               !strcmp (msg, "unexpected EOF while parsing")) /* E_EOF */
-           {
-             Py_XDECREF (exc);
-             Py_XDECREF (val);
-             Py_XDECREF (trb);
-             prompt = ps2;
-           }
-           else                                   /* some other syntax error */
-           {
-             PyErr_Restore (exc, val, trb);
-             PyErr_Print ();
-             free (code);
-             code = NULL;
-             prompt = ps1;
-           }
-         }
-         else                                     /* some non-syntax error */
-         {
-           PyErr_Print ();
-           free (code);
-           code = NULL;
-           prompt = ps1;
-         }
-
-         free (line);
-       }
-     }
-
-     Py_XDECREF(glb);
-     Py_XDECREF(loc);
-     Py_Finalize();
-     exit(0);
-   }
-
-
 How do I find undefined g++ symbols __builtin_new or __pure_virtual?
 --------------------------------------------------------------------