[3.15] gh-154855: Ask non-ncurses curses for one more character (GH-154870) (GH-156283)

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() is left as it is.

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.

(cherry picked from commit 43a1869)

Author: serhiy-storchaka

Misc/NEWS.d/next/Library/2026-07-29-14-20-40.gh-issue-154855.vfF453.rst

@@ -0,0 +1,3 @@
+Fix :meth:`curses.window.instr` returning one character too few when the
+:mod:`curses` module is built against a curses library that counts the
+terminator in the requested length, such as the NetBSD one.

Modules/_cursesmodule.c

@@ -310,6 +310,14 @@ 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 */
 
 /*
@@ -2217,25 +2225,33 @@ PyCursesWindow_instr(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;
     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);
 }
 
 /*[clinic input]