Upload multiple images to a Woocommerce product - wordpress

Question

I am trying to upload various images from an URL to a given woocommerce product. The issue I am facing with my code is that, although I see that the images are being uploaded to the server, when I go to see my post I only see the last image uploaded to the gallery.
Here is my code:
function generateSecondImage($image_url, $post_id)
{
$upload_dir = wp_upload_dir();
$image_data = file_get_contents($image_url);
$filename = basename($image_url);
if (wp_mkdir_p($upload_dir['path'])) {
$file = $upload_dir['path'] . '/' . $filename;
} else {
$file = $upload_dir['basedir'] . '/' . $filename;
}
file_put_contents($file, $image_data);
$wp_filetype = wp_check_filetype($filename, null);
$attachment = array(
'post_mime_type' => $wp_filetype['type'],
'post_title' => sanitize_file_name($filename),
'post_content' => '',
'post_status' => 'inherit',
);
$attach_id = wp_insert_attachment($attachment, $file, $post_id);
require_once ABSPATH . 'wp-admin/includes/image.php';
$attach_data = wp_generate_attachment_metadata($attach_id, $file);
$res1 = wp_update_attachment_metadata($attach_id, $attach_data);
$res3 = update_post_meta($post_id, '_product_image_gallery', $attach_id);
}
And here is how I call the function
for ($j=1; $j <$picCount ; $j++) {
generateSecondImage($pic[$j]->url, $post_id);
}
I am thinking that maybe, res3 is overwriting the gallery and showing only the last image posted, but if that is the case, how would I tell wordpress to include all of the images in the for loop?

Answer 1

Finally managed to solve it!
Here is my final code. My upload function goes like this:
function generateSecondImage($image_url, $post_id)
{
$upload_dir = wp_upload_dir();
$image_data = file_get_contents($image_url);
$filename = basename($image_url);
if (wp_mkdir_p($upload_dir['path'])) {
$file = $upload_dir['path'] . '/' . $filename;
} else {
$file = $upload_dir['basedir'] . '/' . $filename;
}
file_put_contents($file, $image_data);
$wp_filetype = wp_check_filetype($filename, null);
$attachment = array(
'post_mime_type' => $wp_filetype['type'],
'post_title' => sanitize_file_name($filename),
'post_content' => '',
'post_status' => 'inherit',
);
$attach_id = wp_insert_attachment($attachment, $file, $post_id);
require_once ABSPATH . 'wp-admin/includes/image.php';
$attach_data = wp_generate_attachment_metadata($attach_id, $file);
$res1 = wp_update_attachment_metadata($attach_id, $attach_data);
echo "Attach ID is".$attach_id;
return $attach_id;
}
And then I had to create a comma-separated list of the IDs and add it to the add_post_meta
So my loop changed to this:
for ($j=1; $j <$picCount ; $j++) {
$attarray[] = generateSecondImage($pic[$j]->url, $post_id);
}
$commaList = implode(', ', $attarray);
$res3 = add_post_meta($post_id, '_product_image_gallery', $commaList);
Hope this helps anyone else looking for a solution.