func to hook into WP's attachment_url_to_postid filter to try to resolve attachment URLs for intermediate/backup sizes

shc_attachment_url_to_postid_filter.php

<?php
if ( version_compare( get_bloginfo( 'version' ), '4.2', '>=') ) :
	/*
	 * WP 4.0 introduced attachment_url_to_postid().  However, it fails when the
	 * URL is for an intermediate sized image (e.g., `.../filename-150x150.jpg`).
	 *
	 * WP 4.2.0 introduced the attachment_url_to_postid filter. The filter func
	 * defined below hooks into that filter and attempts remedy that limitation.
	 */
	add_filter( 'attachment_url_to_postid' , 'shc_attachment_url_to_postid_filter', 10, 2 );

	/**
	 * Tries to convert an attachment URL (for intermediate/edited sized image) into a post ID.
	 *
	 * @param string $url The URL to resolve.
	 * @return int The found post ID, or 0 on failure.
	 *
	 * @see attachment_url_to_postid()
	 *
	 * @todo produces incorrect results after the following sequence:
	 * 		 1) set thumbnail site to 150x150;
	 * 		 2) upload foo-150x150.jpg;
	 * 		 3) upload foo.jpg;
	 * 		 4) call attachment_url_to_post_id( 'http://host/wp-content/uploads/foo-150x150.jpg' )
	 *
	 * 		 this is due to a bug in WP Core,
	 * 		 see @link https://core.trac.wordpress.org/ticket/44095
	 *
	 * 		 Actually, it is attachment_url_to_postid() that produces the incorrect
	 * 		 results, and we don't try to work around that bug
	 *
	 * @todo produces incorrect results after the following sequence:
	 * 		 1) set thumbnail site to 150x150;
	 * 		 2) upload a 300x300 image foo.jpg;
	 * 		 3) edit foo.jpg and scale to 200x200;
	 * 		 4) regenerate intermediate sized images (e.g., with https://wordpress.org/plugins/regenerate-thumbnails/)
	 * 		 5) call attachment_url_to_post_id( 'http://host/wp-content/uploads/foo-150x150.jpg' )
	 *
	 * 		 this is due to what may/may not be considered a bug in WP core, but the fix/enhancement suggested in
	 * 		 @link https://core.trac.wordpress.org/ticket/44127 would allow a slight mod to this filter
	 * 		 to correctly resolve that case
	 */
	function shc_attachment_url_to_postid_filter( $post_id, $url ) {
		global $wpdb;

 		if ( $post_id ) {
 			// attachment_url_to_postid() found the attachment, nothing for us to do
 			// however, see the 1st @todo above for one know case where attachment_url_to_postid()
 			// returns a false positive.  We could try to work around that bug, but I'd
 			// rather the bug be fixed rather than try to work around it
 			return $post_id;
 		}

 		// start by setting up a few vars the same way attachment_url_to_postid() does
		$dir = wp_get_upload_dir();
		$path = $url;

		$site_url = parse_url( $dir['url'] );
		$image_path = parse_url( $path );

		//force the protocols to match if needed
		if ( isset( $image_path['scheme'] ) && ( $image_path['scheme'] !== $site_url['scheme'] ) ) {
			$path = str_replace( $image_path['scheme'], $site_url['scheme'], $path );
		}

		if ( 0 === strpos( $path, $dir['baseurl'] . '/' ) ) {
			$path = substr( $path, strlen( $dir['baseurl'] . '/' ) );
		}

		$basename = wp_basename( $path );
		$dirname = dirname( $path );

		// note: the "LIKE" we search for is the serialized form of $basename to reduce the
		//		 number of false positives we have to deal with
		$sql = $wpdb->prepare(
			"SELECT post_id, meta_key, meta_value
			 FROM $wpdb->postmeta
			 WHERE meta_key IN ( '_wp_attachment_metadata', '_wp_attachment_backup_sizes' ) AND meta_value LIKE %s",
			'%' . serialize( $basename ) . '%' );

		$results = $wpdb->get_results( $sql );
		foreach ( $results as $row ) {
			if ( '_wp_attachment_metadata' === $row->meta_key ) {
				$meta = maybe_unserialize( $row->meta_value );

				if ( dirname( $meta['file'] ) === $dirname && in_array( $basename, wp_list_pluck( $meta['sizes'], 'file' ) ) ) {
					// URL is for a registered intermediate size
					$post_id = $row->post_id;

					break;
				}
			}
			else {
				// see if URL is for a "backup" of an edited image
				$backup_sizes = maybe_unserialize( $row->meta_value );

				if ( in_array( $basename, wp_list_pluck( $backup_sizes, 'file' ) ) ) {
					// URL is possibly for a "backup" of an edited image
					// get the meta for the "original" attachment and perform the equivalent
					// test we did above for '_wp_attachment_metadata' === $row->meta_key
					$sql = $wpdb->prepare(
						"SELECT meta_value
						 FROM $wpdb->postmeta
						 WHERE
							post_id = %d AND meta_key = '_wp_attachment_metadata'",
						$row->post_id
					);
					$meta = maybe_unserialize( $wpdb->get_var( $sql ) );
					if ( isset( $meta['file'] ) && dirname( $meta['file'] ) === $dirname ) {
						// URL is for a "backup" of an edited image
						$post_id = $row->post_id;

						break;
					}
				}
			}
		}

		return $post_id;
	}
endif; // function_exists( 'attachment_url_to_postid' )