Correct writing buffer to file in PyEncoder encode_to_file()

Tests/test_imagefile.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import tempfile
 from io import BytesIO
 from typing import Any
 
@@ -275,7 +276,7 @@ def __init__(self, mode: str, *args: Any) -> None:
         super().__init__(mode, *args)
 
     def encode(self, bufsize: int) -> tuple[int, int, bytes]:
-        return 1, 1, b""
+        return 1, 1, b"a"
 
     def cleanup(self) -> None:
         self.cleanup_called = True
@@ -437,6 +438,13 @@ def test_encode(self) -> None:
         with pytest.raises(NotImplementedError):
             encoder.encode_to_file(0, 0)
 
+        mock_encoder = MockPyEncoder("")
+        with tempfile.TemporaryFile() as fp:
+            mock_encoder.encode_to_file(fp.fileno(), 1)
+
+            fp.seek(0)
+            assert fp.read() == b"a"
+
     def test_zero_height(self) -> None:
         with pytest.raises(UnidentifiedImageError):
             Image.open("Tests/images/zero_height.j2k")

src/PIL/ImageFile.py

@@ -930,7 +930,7 @@ def encode_to_file(self, fh: int, bufsize: int) -> int:
         """
         errcode = 0
         while errcode == 0:
-            status, errcode, buf = self.encode(bufsize)
-            if status > 0:
-                os.write(fh, buf[status:])
+            bytes_consumed, errcode, buf = self.encode(bufsize)
+            if bytes_consumed > 0:
+                os.write(fh, buf[:bytes_consumed])
         return errcode

src/encode.c

@@ -121,7 +121,7 @@ static PyObject *
 _encode(ImagingEncoderObject *encoder, PyObject *args) {
     PyObject *buf;
     PyObject *result;
-    int status;
+    int bytes_consumed;
 
     /* Encode to a Python string (allocated by this method) */
 
@@ -136,16 +136,16 @@ _encode(ImagingEncoderObject *encoder, PyObject *args) {
         return NULL;
     }
 
-    status = encoder->encode(
+    bytes_consumed = encoder->encode(
         encoder->im, &encoder->state, (UINT8 *)PyBytes_AsString(buf), bufsize
     );
 
     /* adjust string length to avoid slicing in encoder */
-    if (_PyBytes_Resize(&buf, (status > 0) ? status : 0) < 0) {
+    if (_PyBytes_Resize(&buf, (bytes_consumed > 0) ? bytes_consumed : 0) < 0) {
         return NULL;
     }
 
-    result = Py_BuildValue("iiO", status, encoder->state.errcode, buf);
+    result = Py_BuildValue("iiO", bytes_consumed, encoder->state.errcode, buf);
 
     Py_DECREF(buf); /* must release buffer!!! */
 
@@ -155,25 +155,25 @@ _encode(ImagingEncoderObject *encoder, PyObject *args) {
 static PyObject *
 _encode_to_pyfd(ImagingEncoderObject *encoder, PyObject *args) {
     PyObject *result;
-    int status;
+    int bytes_consumed;
 
     if (!encoder->pushes_fd) {
         // UNDONE, appropriate errcode???
         result = Py_BuildValue("ii", 0, IMAGING_CODEC_CONFIG);
         return result;
     }
 
-    status = encoder->encode(encoder->im, &encoder->state, (UINT8 *)NULL, 0);
+    bytes_consumed = encoder->encode(encoder->im, &encoder->state, (UINT8 *)NULL, 0);
 
-    result = Py_BuildValue("ii", status, encoder->state.errcode);
+    result = Py_BuildValue("ii", bytes_consumed, encoder->state.errcode);
 
     return result;
 }
 
 static PyObject *
 _encode_to_file(ImagingEncoderObject *encoder, PyObject *args) {
     UINT8 *buf;
-    int status;
+    int bytes_consumed;
     ImagingSectionCookie cookie;
 
     /* Encode to a file handle */
@@ -198,10 +198,10 @@ _encode_to_file(ImagingEncoderObject *encoder, PyObject *args) {
         /* This replaces the inner loop in the ImageFile _save
            function. */
 
-        status = encoder->encode(encoder->im, &encoder->state, buf, bufsize);
+        bytes_consumed = encoder->encode(encoder->im, &encoder->state, buf, bufsize);
 
-        if (status > 0) {
-            if (write(fh, buf, status) < 0) {
+        if (bytes_consumed > 0) {
+            if (write(fh, buf, bytes_consumed) < 0) {
                 ImagingSectionLeave(&cookie);
                 free(buf);
                 return PyErr_SetFromErrno(PyExc_OSError);