save_post function is not called - wordpress

Question

I am trying to call the save_post or publish_post hook with my function it the function cannot be triggered.
My function works well on single.php but it only assigns thumbnails after browsing the post. So I decided to put it inside functions.php but I could not get it triggered with save_post or publish_post hooks.
Here is the code
function catch_that_image($post_id) {
if(is_single()){
global $post, $posts;
$first_img = '';
ob_start();
ob_end_clean();
$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
$first_img = $matches [1] [0];
if(empty($first_img)){ //Defines a default image
$first_img = "https://images.arabicpdfs.com/المكتبة-المفتوحة.jpg";
}
//return $first_img;
if ( has_post_thumbnail() ) {
return;
}
else{
require_once(ABSPATH . 'wp-admin/includes/media.php');
require_once(ABSPATH . 'wp-admin/includes/file.php');
require_once(ABSPATH . 'wp-admin/includes/image.php');
$url =$first_img;
$desc = "image description";
$post_id =$post->ID;
$image = media_sideload_image( $url, $post_id, $desc,'id' );
set_post_thumbnail( $post_id, $image );
//echo $post_id;
}
}}
add_action( 'publish_post', 'catch_that_image', 10, 1);
I could not figure out what I am missing

Answer 1

The function is_single does not work in this hook (even with post id as parameter). According to documentation the hook publish_post already specifies that it is a post see here and therefore the if-statement is not necessary. The suffix _post defines the post type for which the hook is valid.
I would also pass the post as a parameter.
function catch_that_image($post_id, $post)
{
...
}
add_action('publish_post', 'catch_that_image', 10, 2);
As an alternative I would recommend the transition_post_status hook see here

Answer 2

I would recommend you the hook transition_post_status
function custom_transition_post_status($new_status, $old_status, $post)
{
// here you can do your action something like this
if(is_single() && $post->post_type == 'post') {
...
}
}
add_action('transition_post_status', 'custom_transition_post_status', 10, 3);
The insert or update should be done with the function wp_insert_post. Add the field ID if you want do update your post.
$postData = [
'post_title' => 'your title',
'post_status' => 'publish',
'post_type' => 'post'
];
$postId = wp_insert_post($postData);