gh-119506: fix _io.TextIOWrapper.write() write during flush

* check if call to `_textiowrapper_writeflush()` has left any data in `self->pending_bytes`. If so, store them in `self->pending_bytes` after `b` * add test
python · methane · Jun 3, 2024 · May 24, 2024 · May 24, 2024 · May 24, 2024
commit 5b924dc728ea6c8a49014d35326e07a88e7205f8
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
@@ -4016,6 +4016,22 @@ def write(self, data):
         t.write("x"*chunk_size)
         self.assertEqual([b"abcdef", b"ghi", b"x"*chunk_size], buf._write_stack)
 
+    def test_issue119506(self):
+        chunk_size = 8192
+
+        class MockIO(self.MockRawIO):
+            def write(self, data):
+                t.write("efg")
+                return super().write(data)
+
+        buf = MockIO()
+        t = self.TextIOWrapper(buf)
+        t.write("a" * (chunk_size - 1))
+        t.write("bcd")
+        t.flush()
+
+        self.assertEqual([b"a" * (chunk_size - 1), b"bcdefg"], buf._write_stack)
+
 
 class PyTextIOWrapperTest(TextIOWrapperTest):
     io = pyio

diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
@@ -1719,16 +1719,23 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
         bytes_len = PyBytes_GET_SIZE(b);
     }
 
-    if (self->pending_bytes == NULL) {
-        self->pending_bytes_count = 0;
-        self->pending_bytes = b;
-    }
-    else if (self->pending_bytes_count + bytes_len > self->chunk_size) {
+    if (self->pending_bytes_count + bytes_len > self->chunk_size) {
         // Prevent to concatenate more than chunk_size data.
         if (_textiowrapper_writeflush(self) < 0) {
             Py_DECREF(b);
             return NULL;
         }
+        if (self->pending_bytes) {
+            // gh-119506: call to _textiowrapper_writeflush()
+            // can write new data to pending_bytes
+            PyObject *tmp = self->pending_bytes;
+            self->pending_bytes = b;
+            b = tmp;
+        }
+    }
+
+    if (self->pending_bytes == NULL) {
+        self->pending_bytes_count = 0;
         self->pending_bytes = b;
     }
     else if (!PyList_CheckExact(self->pending_bytes)) {