Improve WebP encoder error handling

Tests/test_file_webp.py

@@ -2,7 +2,6 @@
 
 import io
 import re
-import sys
 import warnings
 from typing import Any
 
@@ -158,21 +157,13 @@ def test_write_unsupported_mode_P(self, tmp_path: Path) -> None:
 
         self._roundtrip(tmp_path, "P", 50.0)
 
-    @pytest.mark.skipif(sys.maxsize <= 2**32, reason="Requires 64-bit system")
-    def test_write_encoding_error_message(self, tmp_path: Path) -> None:
-        im = Image.new("RGB", (15000, 15000))
-        with pytest.raises(ValueError, match="encoding error 6"):
-            im.save(tmp_path / "temp.webp", method=0)
-
-    @pytest.mark.skipif(sys.maxsize <= 2**32, reason="Requires 64-bit system")
     def test_write_encoding_error_bad_dimension(self, tmp_path: Path) -> None:
         im = Image.new("L", (16384, 1))
-        with pytest.raises(ValueError) as e:
+        with pytest.raises(
+            ValueError,
+            match="encoding error 5: image size exceeds WebP limit of 16383 pixels",
+        ):
             im.save(tmp_path / "temp.webp")
-        assert (
-            str(e.value)
-            == "encoding error 5: Image size exceeds WebP limit of 16383 pixels"
-        )
 
     def test_WebPEncode_with_invalid_args(self) -> None:
         """

docs/releasenotes/13.0.0.rst

@@ -228,3 +228,9 @@ position or contents.
 
 :py:meth:`~PIL.Image.Image.tobytes` also now explicitly cleans up its encoder on
 both success and failure, rather than relying on object destruction.
+
+WebP encoding errors now include a description
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When libwebp fails to encode an image, the raised :py:exc:`ValueError` now
+consistently describes the error rather than only reporting a numeric code.

src/_webp.c

@@ -73,6 +73,30 @@ HandleMuxError(WebPMuxError err, char *chunk) {
     return NULL;
 }
 
+/* -------------------------------------------------------------------- */
+/* WebP Encoder Error Mapping                                           */
+/* -------------------------------------------------------------------- */
+
+#define WEBP_STR_HELPER(x) #x
+#define WEBP_STR(x) WEBP_STR_HELPER(x)
+
+static const char *const kEncoderErrorMessages[VP8_ENC_ERROR_LAST] = {
+    "ok",
+    "out of memory allocating objects",
+    "out of memory re-allocating byte buffer",
+    "NULL parameter passed to function",
+    "configuration is invalid",
+    "image size exceeds WebP limit of " WEBP_STR(WEBP_MAX_DIMENSION) " pixels",
+    "partition #0 is bigger than 512K",
+    "partition is bigger than 16M",
+    "picture writer returned an I/O error",
+    "file would be bigger than 4G",
+    "encoding aborted by user"
+};
+
+#undef WEBP_STR
+#undef WEBP_STR_HELPER
+
 /* -------------------------------------------------------------------- */
 /* Frame import                                                         */
 /* -------------------------------------------------------------------- */
@@ -647,17 +671,12 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
     ret_size = writer.size;
 
     if (!ok) {
-        int error_code = (&pic)->error_code;
-        char message[50] = "";
-        if (error_code == VP8_ENC_ERROR_BAD_DIMENSION) {
-            sprintf(
-                message,
-                ": Image size exceeds WebP limit of %d pixels",
-                WEBP_MAX_DIMENSION
-            );
-        }
-        PyErr_Format(PyExc_ValueError, "encoding error %d%s", error_code, message);
         free(output);
+        WebPEncodingError error_code = pic.error_code;
+        const char *message = error_code > VP8_ENC_OK && error_code < VP8_ENC_ERROR_LAST
+                                  ? kEncoderErrorMessages[error_code]
+                                  : "unknown error";
+        PyErr_Format(PyExc_ValueError, "encoding error %d: %s", error_code, message);
         return NULL;
     }