gh-119506: fix `_io.TextIOWrapper.write()` write during flush (#119507)

Co-authored-by: Inada Naoki <songofacandy@gmail.com>

Lib/test/test_io.py

@@ -4016,6 +4016,28 @@ class CTextIOWrapperTest(TextIOWrapperTest):
         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):
+            written = False
+            def write(self, data):
+                if not self.written:
+                    self.written = True
+                    t.write("middle")
+                return super().write(data)
+
+        buf = MockIO()
+        t = self.TextIOWrapper(buf)
+        t.write("abc")
+        t.write("def")
+        # writing data which size >= chunk_size cause flushing buffer before write.
+        t.write("g" * chunk_size)
+        t.flush()
+
+        self.assertEqual([b"abcdef", b"middle", b"g"*chunk_size],
+                         buf._write_stack)
+
 
 class PyTextIOWrapperTest(TextIOWrapperTest):
     io = pyio

Misc/NEWS.d/next/Library/2024-05-24-14-32-24.gh-issue-119506.-nMNqq.rst

@@ -0,0 +1 @@
+Fix :meth:`!io.TextIOWrapper.write` method breaks internal buffer when the method is called again during flushing internal buffer.

Modules/_io/textio.c

@@ -1719,16 +1719,26 @@ _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) {
-        // Prevent to concatenate more than chunk_size data.
-        if (_textiowrapper_writeflush(self) < 0) {
-            Py_DECREF(b);
-            return NULL;
+    // We should avoid concatinating huge data.
+    // Flush the buffer before adding b to the buffer if b is not small.
+    // https://github.com/python/cpython/issues/87426
+    if (bytes_len >= self->chunk_size) {
+        // _textiowrapper_writeflush() calls buffer.write().
+        // self->pending_bytes can be appended during buffer->write()
+        // or other thread.
+        // We need to loop until buffer becomes empty.
+        // https://github.com/python/cpython/issues/118138
+        // https://github.com/python/cpython/issues/119506
+        while (self->pending_bytes != NULL) {
+            if (_textiowrapper_writeflush(self) < 0) {
+                Py_DECREF(b);
+                return NULL;
+            }
         }
+    }
+
+    if (self->pending_bytes == NULL) {
+        assert(self->pending_bytes_count == 0);
         self->pending_bytes = b;
     }
     else if (!PyList_CheckExact(self->pending_bytes)) {
@@ -1737,6 +1747,9 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
             Py_DECREF(b);
             return NULL;
         }
+        // Since Python 3.12, allocating GC object won't trigger GC and release
+        // GIL. See https://github.com/python/cpython/issues/97922
+        assert(!PyList_CheckExact(self->pending_bytes));
         PyList_SET_ITEM(list, 0, self->pending_bytes);
         PyList_SET_ITEM(list, 1, b);
         self->pending_bytes = list;