php - Add Missing Image Dimensions when Updating/Saving Post

I would like to automatically add missing image dimensions (width/height) to images (internal/external) when post is updated/saved. I found a way to do it when page is loaded but obviously it slows down the loading of the page.

This is original code. When added to the functions it does the job by adding image dimensions.

function add_img_size($content){
  $pattern = '/<img [^>]*?src="(https?:\/\/[^"]+?)"[^>]*?>/iu';
  preg_match_all($pattern, $content, $imgs);
  foreach ( $imgs[0] as $i => $img ) {
    if ( false !== strpos( $img, 'width=' ) && false !== strpos( $img, 'height=' ) ) {
      continue;
    }
    $img_url = $imgs[1][$i];
    $img_size = @getimagesize( $img_url );
      
    if ( false === $img_size ) {
      continue;
    }
    $replaced_img = str_replace( '<img ', '<img ' . $img_size[3] . ' ', $imgs[0][$i] );
    $content = str_replace( $img, $replaced_img, $content );
  }
  return $content;
}
add_filter('the_content','add_img_size');

I tried editing it by using add_action save_post/pre_post_update but it does nothing.

add_action('save_post','add_img_size'); function add_img_size($post_id){

What am i missing? Can someone help me edit it so it adds image dimensions when post is updated/saved? Seemed simpler than it is..

Solution:

function set_image_dimensions( $id, $post ) {
if ( ! wp_is_post_revision( $post ) ){
remove_action('save_post', 'set_image_dimensions', 10, 2 );
$content = $post->post_content;
$pattern = '/<img [^>]*?src="(https?:\/\/[^"]+?)"[^>]*?>/iu';
preg_match_all($pattern, $content, $imgs);
foreach ( $imgs[0] as $i => $img ) {
    if ( false !== strpos( $img, 'width=' ) && false !== strpos( $img, 'height=' ) ) {
        continue;}
$img_url = $imgs[1][$i];
$img_size = @getimagesize( $img_url );
    if ( false === $img_size ) {
        continue;}
$replaced_img = str_replace( '<img ', '<img ' . $img_size[3] . ' ', $imgs[0][$i] );
$content = str_replace( $img, $replaced_img, $content );
}
$post->post_content = $content;
wp_update_post( $post );
add_action( 'save_post', 'set_image_dimensions', 10, 2 );
}}
add_action( 'save_post', 'set_image_dimensions', 10, 2 );

This works as i wanted. Adds missing image dimension on update.

In the end when i started with post_content and ended with wp_update_post had no errors to deal with and it didn't work. The culprit was using $content = $post->post_content; instead the other way around the second time. Got around infinite loop with help of link Shoelaced posted.