rsrsrs

So after perusing through at all the hooks (actions and filters) available i realized this is not possible using a hook because none is available for media_sideload_image or wp_handle_sideload or media_handle_sideload or download_url or wp_tempfnam. So i ended up writing a simple procedure in place of media_sideload_image() and download_url(). media_sideload_image_in_GAE() and download_url_in_GAE(). Here is the code for others benefit:

/**
 * Download an image from the specified URL and attach it to a post.
 *
 * Replaced media_sideload_image with media_sideload_image_in_GAE to take into account Google Appengine where wp_tempnam fails since there is no temp filesystem location
 *
 */
private function media_sideload_image_in_GAE($file, $post_id, $desc = null) {
        if ( ! empty($file) ) {
                // Download file to temp location
                $tmp = $this->store_tmpfile_in_GAE( $file );

                // Set variables for storage
                // fix file filename for query strings
                preg_match( '/[^\?]+\.(jpe?g|jpe|gif|png)\b/i', $file, $matches );
                $file_array['name'] = basename($matches[0]);
                $file_array['tmp_name'] = $tmp;

                // If error storing temporarily, unlink
                if ( is_wp_error( $tmp ) ) {
                        @unlink($file_array['tmp_name']);
                        $file_array['tmp_name'] = '';
                }

                // do the validation and storage stuff
                $id = media_handle_sideload( $file_array, $post_id, $desc );
                // If error storing permanently, unlink
                if ( is_wp_error($id) ) {
                        @unlink($file_array['tmp_name']);
                        return $id;
                }

                $src = wp_get_attachment_url( $id );
        }

        // Finally check to make sure the file has been saved, then return the html
        if ( ! empty($src) ) {
                $alt = isset($desc) ? esc_attr($desc) : '';
                $html = "<img src='$src' alt='$alt' />";
                return $html;
        }
}

private function store_tmpfile_in_GAE($filename) {
        $filesourceurl=$filename;
        $filename = basename($filename);
        $filename = preg_replace('|\..*$|', '.tmp', $filename);
        //$filename = $dir . wp_unique_filename($dir, $filename);

	$bucket_name = get_option('appengine_uploads_bucket', '');
	$filepath = "gs://" . $bucket_name . "/" . $filename;
        $filecontent = file_get_contents($filesourceurl);
        $fp = fopen($filepath, "w");
        //fwrite($fp, "");
        fwrite($fp, $filecontent);
        fclose($fp);
        return $filepath;
}