Issue #21396: Fix TextIOWrapper(..., write_through=True) to not force a flush() on the underlying binary stream.

Patch by akira.

Lib/test/test_io.py

@@ -2615,6 +2615,38 @@ class TextIOWrapperTest(unittest.TestCase):
         txt.write('5')
         self.assertEqual(b''.join(raw._write_stack), b'123\n45')
 
+    def test_bufio_write_through(self):
+        # Issue #21396: write_through=True doesn't force a flush()
+        # on the underlying binary buffered object.
+        flush_called, write_called = [], []
+        class BufferedWriter(self.BufferedWriter):
+            def flush(self, *args, **kwargs):
+                flush_called.append(True)
+                return super().flush(*args, **kwargs)
+            def write(self, *args, **kwargs):
+                write_called.append(True)
+                return super().write(*args, **kwargs)
+
+        rawio = self.BytesIO()
+        data = b"a"
+        bufio = BufferedWriter(rawio, len(data)*2)
+        textio = self.TextIOWrapper(bufio, encoding='ascii',
+                                    write_through=True)
+        # write to the buffered io but don't overflow the buffer
+        text = data.decode('ascii')
+        textio.write(text)
+
+        # buffer.flush is not called with write_through=True
+        self.assertFalse(flush_called)
+        # buffer.write *is* called with write_through=True
+        self.assertTrue(write_called)
+        self.assertEqual(rawio.getvalue(), b"") # no flush
+
+        write_called = [] # reset
+        textio.write(text * 10) # total content is larger than bufio buffer
+        self.assertTrue(write_called)
+        self.assertEqual(rawio.getvalue(), data * 11) # all flushed
+
     def test_read_nonbytes(self):
         # Issue #17106
         # Crash when underlying read() returns non-bytes