the write() function of a heif_writer may now return a NULL message when there is no error (#1493)

Author: farindk

libheif/api/libheif/heif.cc

@@ -2663,7 +2663,14 @@ struct heif_error heif_context_write(struct heif_context* ctx,
   const auto& data = swriter.get_data();
   heif_error writer_error = writer->write(ctx, data.data(), data.size(), userdata);
   if (!writer_error.message) {
-    return heif_error{heif_error_Usage_error, heif_suberror_Null_pointer_argument, "heif_writer callback returned a null error text"};
+    // It is now allowed to return a NULL error message on success. It will be replaced by "Success". An error message is still required when there is an error.
+    if (writer_error.code == heif_error_Ok) {
+      writer_error.message = Error::kSuccess;
+      return writer_error;
+    }
+    else {
+      return heif_error{heif_error_Usage_error, heif_suberror_Null_pointer_argument, "heif_writer callback returned a null error text"};
+    }
   }
   else {
     return writer_error;

libheif/api/libheif/heif.h

@@ -2059,6 +2059,8 @@ struct heif_writer
   int writer_api_version;
 
   // --- version 1 functions ---
+
+  // On success, the returned heif_error may have a NULL message. It will automatically be replaced with a "Success" string.
   struct heif_error (* write)(struct heif_context* ctx, // TODO: why do we need this parameter?
                               const void* data,
                               size_t size,