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

truncate() delegated to the buffer without an explicit position, so the stream
was resized to wherever the buffer had read ahead to rather than to the current
position of the text stream.  _pyio passes tell() through and already behaves
correctly, so this only changes the C implementation.

Author: tsayin4

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);
 }