gh-154855: Ask non-ncurses curses for one more character

Passing n to the library is ncurses' reading of n: it stores n characters
and adds a terminator.  NetBSD curses counts the terminator in n.

Ask a library that is neither ncurses nor PDCurses for n + 1, and read
again if it stored more than asked; truncating could split a multibyte
character.  This is not possible for input, so getstr() and get_wstr() are
left as they are.

instr() now takes the length from the value returned by winnstr(), as
X/Open specifies, instead of searching for a terminator which it does not.

Author: serhiy-storchaka

Modules/_cursesmodule.c

@@ -309,6 +309,14 @@ curses_window_set_null_error(PyCursesWindowObject *win,
     _curses_set_null_error(state, curses_funcname, python_funcname);
 }
 
+/* ncurses and PDCurses store n characters and add a terminator; NetBSD
+   curses counts the terminator in n.  Ask an unknown library for one more. */
+#if defined(NCURSES_VERSION) || defined(PDCURSES)
+#  define CURSES_STR_EXTRA  0
+#else
+#  define CURSES_STR_EXTRA  1
+#endif
+
 /* Utility Checking Procedures */
 
 /*
@@ -3796,25 +3804,33 @@ curses_window_instr_bytes(PyCursesWindowObject *self, PyObject *args,
         return NULL;
     }
 
-    n = Py_MIN(n, max_buf_size - 1);
+    n = Py_MIN(n, max_buf_size - 1 - CURSES_STR_EXTRA);
+    n += CURSES_STR_EXTRA;
     PyBytesWriter *writer = PyBytesWriter_Create(n + 1);
     if (writer == NULL) {
         return NULL;
     }
     char *buf = PyBytesWriter_GetData(writer);
 
-    if (use_xy) {
-        rtn = mvwinnstr(self->win, y, x, buf, n);
-    }
-    else {
-        rtn = winnstr(self->win, buf, n);
+    /* Read again if the library stored more than asked: truncating could
+       split a multibyte character. */
+    for (unsigned int want = n - CURSES_STR_EXTRA; ; n = want) {
+        if (use_xy) {
+            rtn = mvwinnstr(self->win, y, x, buf, n);
+        }
+        else {
+            rtn = winnstr(self->win, buf, n);
+        }
+        if (rtn == ERR || (unsigned int)rtn <= want) {
+            break;
+        }
     }
 
     if (rtn == ERR) {
         PyBytesWriter_Discard(writer);
         return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
     }
-    return PyBytesWriter_FinishWithSize(writer, strlen(buf));
+    return PyBytesWriter_FinishWithSize(writer, rtn);
 }
 
 static PyObject *
@@ -3939,17 +3955,25 @@ PyCursesWindow_in_wstr(PyObject *op, PyObject *args)
         return NULL;
     }
 
-    n = Py_MIN(n, max_buf_size - 1);
+    n = Py_MIN(n, max_buf_size - 1 - CURSES_STR_EXTRA);
+    n += CURSES_STR_EXTRA;
     wchar_t *buf = PyMem_New(wchar_t, n + 1);
     if (buf == NULL) {
         return PyErr_NoMemory();
     }
 
-    if (use_xy) {
-        rtn = mvwinnwstr(self->win, y, x, buf, n);
-    }
-    else {
-        rtn = winnwstr(self->win, buf, n);
+    /* Read again if the library stored more than asked: truncating could
+       separate a combining character from its base. */
+    for (unsigned int want = n - CURSES_STR_EXTRA; ; n = want) {
+        if (use_xy) {
+            rtn = mvwinnwstr(self->win, y, x, buf, n);
+        }
+        else {
+            rtn = winnwstr(self->win, buf, n);
+        }
+        if (rtn == ERR || (unsigned int)rtn <= want) {
+            break;
+        }
     }
 
     if (rtn == ERR) {
@@ -4003,7 +4027,8 @@ PyCursesWindow_in_wchstr(PyObject *op, PyObject *args)
         return NULL;
     }
 
-    n = Py_MIN(n, max_buf_size - 1);
+    n = Py_MIN(n, max_buf_size - 1 - CURSES_STR_EXTRA);
+    n += CURSES_STR_EXTRA;
     cursesmodule_state *state = get_cursesmodule_state_by_win(self);
     /* Zero the cells: reading a cell back through getcchar() relies on the
        cchar_t text array being NUL-terminated, which some curses libraries
@@ -4026,6 +4051,7 @@ PyCursesWindow_in_wchstr(PyObject *op, PyObject *args)
         return PyCursesComplexStr_New(state, NULL, 0);
     }
 
+    n -= CURSES_STR_EXTRA;
     /* win_wchnstr() stores at most n cells and zero-terminates the array at
        the actual count; every real cell holds at least a space, so the first
        empty cell marks the end of the run. */
@@ -4058,6 +4084,7 @@ PyCursesWindow_in_wchstr(PyObject *op, PyObject *args)
         return PyCursesComplexStr_New(state, NULL, 0);
     }
 
+    n -= CURSES_STR_EXTRA;
     Py_ssize_t count = 0;
     while (count < (Py_ssize_t)n && buf[count] != 0) {
         count++;