gh-85393: Preserve compatibility with writers returning None

Author: methane

Lib/_pyio.py

@@ -2311,7 +2311,8 @@ def write(self, s):
         while True:
             n = self.buffer.write(b)
             if n is None:
-                raise BlockingIOError("write returned None")
+                # Some legacy writers return None after writing all the data.
+                break
             if n < 0 or n > len(b) or (n == 0 and b):
                 raise OSError("write() returned incorrect number of bytes")
             b = b[n:]

Lib/test/test_io/test_textio.py

@@ -1032,10 +1032,16 @@ def write(self, b):
             self.assertEqual(txt.write('a\u20acb'), 3)
             self.assertEqual(b''.join(raw._write_stack), b'a\xe2\x82\xacb')
 
+    def test_rawio_write_none(self):
+        raw = self.MockRawIO()
+        raw.write = raw._write_stack.append
+        with self.TextIOWrapper(raw, encoding='ascii', write_through=True) as txt:
+            self.assertEqual(txt.write('abc'), 3)
+            self.assertEqual(raw._write_stack, [b'abc'])
+
     def test_rawio_write_invalid_result(self):
         for result, error in ((0, OSError), (-1, OSError), (4, OSError),
-                              (None, BlockingIOError), ('1', TypeError),
-                              (1.5, TypeError)):
+                              ('1', TypeError), (1.5, TypeError)):
             with self.subTest(result=result):
                 raw = self.MockRawIO()
                 raw.write = lambda b: result

Modules/_io/textio.c

@@ -1735,9 +1735,9 @@ _textiowrapper_writeflush(textio *self)
             goto error;
         }
         if (ret == Py_None) {
+            /* Some legacy writers return None after writing all the data. */
             Py_DECREF(ret);
-            PyErr_SetString(PyExc_BlockingIOError, "write returned None");
-            goto error;
+            break;
         }
         Py_ssize_t n = PyNumber_AsSsize_t(ret, PyExc_OverflowError);
         Py_DECREF(ret);