gh-161275: Fix TextIOWrapper.truncate() with no argument

Lib/test/test_io/test_textio.py

@@ -577,6 +577,28 @@ def multi_line_test(self, f, enc):
             rlines.append((pos, line))
         self.assertEqual(rlines, wlines)
 
+    def test_truncate_after_read(self):
+        # gh-161275: truncate() without an argument resizes the stream to the
+        # current position.  The buffer may have read ahead of the text
+        # stream, so the buffer's own position must not be used.
+        with self.open(os_helper.TESTFN, "w", encoding="utf-8") as f:
+            f.write("abcde")
+        with self.open(os_helper.TESTFN, "r+", encoding="utf-8") as f:
+            self.assertEqual(f.read(2), "ab")
+            self.assertEqual(f.tell(), 2)
+            self.assertEqual(f.truncate(), 2)
+            self.assertEqual(f.tell(), 2)
+        with self.open(os_helper.TESTFN, "r", encoding="utf-8") as f:
+            self.assertEqual(f.read(), "ab")
+
+        # Also with a file larger than the read-ahead chunk.
+        with self.open(os_helper.TESTFN, "w", encoding="utf-8") as f:
+            f.write("0123456789" * 2000)
+        with self.open(os_helper.TESTFN, "r+", encoding="utf-8") as f:
+            self.assertEqual(f.read(10), "0123456789")
+            self.assertEqual(f.truncate(), 10)
+        self.assertEqual(os.path.getsize(os_helper.TESTFN), 10)
+
     def test_telling(self):
         f = self.open(os_helper.TESTFN, "w+", encoding="utf-8")
         p0 = f.tell()

Misc/NEWS.d/next/Library/2026-11-04-09-40-00.gh-issue-161275.Rt4Xw8.rst

@@ -0,0 +1,3 @@
+:meth:`~io.IOBase.truncate` on an :class:`io.TextIOWrapper` with no argument
+resized the stream to the position of the underlying buffer, which may have
+read ahead, instead of to the current position of the text stream.

Modules/_io/textio.c

@@ -3112,6 +3112,20 @@ _io_TextIOWrapper_truncate_impl(textio *self, PyObject *pos)
         return NULL;
     }
 
+    if (pos == Py_None) {
+        /* The buffer may have read ahead of this stream's position, so the
+           buffer's own position must not be used here.  Resize to the
+           position reported by tell(), as _pyio already does. */
+        PyObject *cookie = _io_TextIOWrapper_tell_impl(self);
+        if (cookie == NULL) {
+            return NULL;
+        }
+        PyObject *res = buffer_callmethod_onearg(self, &_Py_ID(truncate),
+                                                 cookie);
+        Py_DECREF(cookie);
+        return res;
+    }
+
     return buffer_callmethod_onearg(self, &_Py_ID(truncate), pos);
 }