gh-143008: fix TextIOWrapper.truncate via re-entrant flush

Lib/test/test_io/test_textio.py

@@ -1560,6 +1560,54 @@ def closed(self):
         wrapper = self.TextIOWrapper(raw)
         wrapper.close()  # should not crash
 
+    def test_reentrant_detach_during_flush(self):
+        # gh-143008: Reentrant detach() during flush should raise RuntimeError
+        # instead of crashing.
+        wrapper = None
+
+        class BadRaw(self.RawIOBase):
+            def write(self, b): return len(b)
+            def read(self, n=-1): return b''
+            def readable(self): return True
+            def writable(self): return True
+            def seekable(self): return True
+            def seek(self, pos, whence=0): return 0
+            def tell(self): return 0
+
+        class EvilBuffer(self.BufferedRandom):
+            detach_on_write = False
+
+            def flush(self):
+                if wrapper is not None and not self.detach_on_write:
+                    wrapper.detach()
+                return super().flush()
+
+            def write(self, b):
+                if wrapper is not None and self.detach_on_write:
+                    wrapper.detach()
+                return len(b)
+
+        tests = [
+            ('truncate', lambda: wrapper.truncate(0)),
+            ('close', lambda: wrapper.close()),
+            ('detach', lambda: wrapper.detach()),
+            ('seek', lambda: wrapper.seek(0)),
+            ('tell', lambda: wrapper.tell()),
+            ('reconfigure', lambda: wrapper.reconfigure(line_buffering=True)),
+        ]
+        for name, method in tests:
+            with self.subTest(name):
+                wrapper = self.TextIOWrapper(EvilBuffer(BadRaw()), encoding='utf-8')
+                self.assertRaisesRegex(RuntimeError, "reentrant", method)
+                wrapper = None
+
+        with self.subTest('read via writeflush'):
+            EvilBuffer.detach_on_write = True
+            wrapper = self.TextIOWrapper(EvilBuffer(BadRaw()), encoding='utf-8')
+            wrapper.write('x')
+            self.assertRaisesRegex(RuntimeError, "reentrant", wrapper.read)
+            wrapper = None
+
 
 class PyTextIOWrapperTest(TextIOWrapperTest, PyTestCase):
     shutdown_error = "LookupError: unknown encoding: ascii"

Misc/NEWS.d/next/Library/2025-12-21-17-56-37.gh-issue-143008.aakErJ.rst

@@ -0,0 +1 @@
+Fix crash in :class:`io.TextIOWrapper` when reentrant :meth:`io.TextIOBase.detach` called.

Modules/_io/textio.c

@@ -667,6 +667,7 @@ struct textio
     PyObject_HEAD
     int ok; /* initialized? */
     int detached;
+    int flushing; /* prevent reentrant detach during flush */
     Py_ssize_t chunk_size;
     PyObject *buffer;
     PyObject *encoding;
@@ -725,6 +726,16 @@ struct textio
 
 #define textio_CAST(op) ((textio *)(op))
 
+/* gh-143007 need to check for reentrant flush */
+static inline int
+_textiowrapper_flush(textio *self)
+{
+    self->flushing = 1;
+    int result = _PyFile_Flush((PyObject *)self);
+    self->flushing = 0;
+    return result;
+}
+
 static void
 textiowrapper_set_decoded_chars(textio *self, PyObject *chars);
 
@@ -1108,6 +1119,7 @@ _io_TextIOWrapper___init___impl(textio *self, PyObject *buffer,
 
     self->ok = 0;
     self->detached = 0;
+    self->flushing = 0;
 
     if (encoding == NULL) {
         PyInterpreterState *interp = _PyInterpreterState_GET();
@@ -1422,7 +1434,7 @@ _io_TextIOWrapper_reconfigure_impl(textio *self, PyObject *encoding,
         return NULL;
     }
 
-    if (_PyFile_Flush((PyObject *)self) < 0) {
+    if (_textiowrapper_flush(self) < 0) {
         return NULL;
     }
     self->b2cratio = 0;
@@ -1565,7 +1577,12 @@ _io_TextIOWrapper_detach_impl(textio *self)
 {
     PyObject *buffer;
     CHECK_ATTACHED(self);
-    if (_PyFile_Flush((PyObject *)self) < 0) {
+    if (self->flushing) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "reentrant call to detach() is not allowed");
+        return NULL;
+    }
+    if (_textiowrapper_flush(self) < 0) {
         return NULL;
     }
     buffer = self->buffer;
@@ -1636,9 +1653,11 @@ _textiowrapper_writeflush(textio *self)
     Py_DECREF(pending);
 
     PyObject *ret;
+    self->flushing = 1;
     do {
         ret = PyObject_CallMethodOneArg(self->buffer, &_Py_ID(write), b);
     } while (ret == NULL && _PyIO_trap_eintr());
+    self->flushing = 0;
     Py_DECREF(b);
     // NOTE: We cleared buffer but we don't know how many bytes are actually written
     // when an error occurred.
@@ -2583,7 +2602,7 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
             goto fail;
         }
 
-        if (_PyFile_Flush((PyObject *)self) < 0) {
+        if (_textiowrapper_flush(self) < 0) {
             goto fail;
         }
 
@@ -2630,7 +2649,7 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
         goto fail;
     }
 
-    if (_PyFile_Flush((PyObject *)self) < 0) {
+    if (_textiowrapper_flush(self) < 0) {
         goto fail;
     }
 
@@ -2757,7 +2776,7 @@ _io_TextIOWrapper_tell_impl(textio *self)
 
     if (_textiowrapper_writeflush(self) < 0)
         return NULL;
-    if (_PyFile_Flush((PyObject *)self) < 0) {
+    if (_textiowrapper_flush(self) < 0) {
         goto fail;
     }
 
@@ -2967,7 +2986,7 @@ _io_TextIOWrapper_truncate_impl(textio *self, PyObject *pos)
 {
     CHECK_ATTACHED(self)
 
-    if (_PyFile_Flush((PyObject *)self) < 0) {
+    if (_textiowrapper_flush(self) < 0) {
         return NULL;
     }
 
@@ -3165,7 +3184,7 @@ _io_TextIOWrapper_close_impl(textio *self)
                 PyErr_Clear();
             }
         }
-        if (_PyFile_Flush((PyObject *)self) < 0) {
+        if (_textiowrapper_flush(self) < 0) {
             exc = PyErr_GetRaisedException();
         }