Prevent reusing ImagingDecoder.setimage

Tests/test_image.py

@@ -1122,6 +1122,14 @@ def test_constants(self) -> None:
             for name in enum.__members__:
                 assert getattr(Image, name) == enum[name]
 
+    def test_decoder_setimage_once(self) -> None:
+        im = Image.new("L", (1, 1))
+        decoder = Image._getdecoder("L", "raw", "L")
+
+        decoder.setimage(im.im, None)
+        with pytest.raises(ValueError, match="decoder already has an image"):
+            decoder.setimage(im.im, None)
+
     @pytest.mark.parametrize(
         "path",
         [

src/decode.c

@@ -204,6 +204,10 @@ _setimage(ImagingDecoderObject *decoder, PyObject *args) {
         return NULL;
     }
 
+    if (decoder->im != NULL) {
+        PyErr_SetString(PyExc_ValueError, "decoder already has an image");
+        return NULL;
+    }
     decoder->im = im;
 
     state = &decoder->state;