GH-103929: handle long input lines with PyOS_InputHook

If:
 - stdin is in line buffer mode
 - the readline module is not loaded
 - not on windows
 - a GUI toolkit has installd PyOS_InputHook

Then, if the user enters lines longer than 98 characters into `input`:

 - user calls `input(...)`
 - user types/pastes a line longer than 98 characters + 1 newline
 - PyOS_InputHook returns
 - the first 99 characters are returned
 - the last character is not a new line so we go back to my_fgets
 - the PyOS_InputHook blocks because despite there being value in the buffer,
   stdin does not flag itself as ready to be read again
 - user hits enter again and to finish reading the input
 - the extra new line comes out the next time the user calls input

This fixes this bug by passing the currently read number of characters to
`my_fgets` to skip the input hook when reading out a long buffer.

closes #103929

Author: tacaswell

Doc/c-api/veryhigh.rst

@@ -167,6 +167,8 @@ the same library that the Python runtime is using.
    event loops, as done in the :file:`Modules/_tkinter.c` in the
    Python source code.
 
+   This function should block until stdin is readable.
+
    .. versionchanged:: 3.12
       This function is only called from the
       :ref:`main interpreter <sub-interpreter-support>`.

Parser/myreadline.c

@@ -36,7 +36,7 @@ int (*PyOS_InputHook)(void) = NULL;
    except if _PyOS_InterruptOccurred() returns true. */
 
 static int
-my_fgets(PyThreadState* tstate, char *buf, int len, FILE *fp)
+my_fgets(PyThreadState* tstate, char *buf, int len, FILE *fp, int n)
 {
 #ifdef MS_WINDOWS
     HANDLE handle;
@@ -51,7 +51,7 @@ my_fgets(PyThreadState* tstate, char *buf, int len, FILE *fp)
 #endif
 
     while (1) {
-        if (PyOS_InputHook != NULL &&
+        if (PyOS_InputHook != NULL && n == 0 &&
             // GH-104668: See PyOS_ReadlineFunctionPointer's comment below...
             _Py_IsMainInterpreter(tstate->interp))
         {
@@ -331,7 +331,7 @@ PyOS_StdioReadline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
             return NULL;
         }
         p = pr;
-        int err = my_fgets(tstate, p + n, (int)incr, sys_stdin);
+        int err = my_fgets(tstate, p + n, (int)incr, sys_stdin, n);
         if (err == 1) {
             // Interrupt
             PyMem_RawFree(p);