1.php

check_admin_referer('update-post_' . $post_id);

$post_id = edit_post();

// Session cookie flag that the post was saved
if ( isset( $_COOKIE['wp-saving-post'] ) && $_COOKIE['wp-saving-post'] === $post_id . '-check' ) {
	setcookie( 'wp-saving-post', $post_id . '-saved', time() + DAY_IN_SECONDS, ADMIN_COOKIE_PATH, COOKIE_DOMAIN, is_ssl() );
}

redirect_post($post_id); // Send user on their way while we keep working

exit();

2.php

function hcf_save( $post_id ) {
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}
	if ( $parent_id = wp_is_post_revision( $post_id ) ) {
		$post_id = $parent_id;
	}
	$field_list = [
		'hcf_author',
		'hcf_published_date',
		'hcf_price',
	];
	foreach ( $field_list as $fieldName ) {
		if ( array_key_exists( $fieldName, $_POST ) ) {
			update_post_meta(
				$post_id,
				$fieldName,
				sanitize_text_field( $_POST[ $fieldName ] )
			);
		}
	}
}
add_action( 'save_post', 'hcf_save' );

3.php

function get_default_post_to_edit( $post_type = 'post', $create_in_db = false ) {
	...
	if ( $create_in_db ) {
		$post_id = wp_insert_post( array(
          'post_title' => __( 'Auto Draft' ),
          'post_type' => $post_type,
          'post_status' => 'auto-draft' ));
	...

4.php

function wp_insert_post( $postarr, $wp_error = false ) {
	...
	do_action( 'save_post', $post_ID, $post, $update );
	...
	return $post_ID;
}

5.php

function hcf_save( $post_id ) {
	...
	if (get_post_status($post_id) === 'auto-draft') {
		return;
	}
	...
}

6.php

function wp_autosave( $post_data ) {
	// Back-compat
	if ( ! defined( 'DOING_AUTOSAVE' ) )
		define( 'DOING_AUTOSAVE', true );
	...
	if ( ! wp_check_post_lock( $post->ID )
         && get_current_user_id() == $post->post_author
         && ( 'auto-draft' == $post->post_status
              || 'draft' == $post->post_status ) ) {
// Drafts and auto-drafts are just overwritten by autosave for the same user if the post is not locked
		return edit_post( wp_slash( $post_data ) );
	} else {
// Non drafts or other users drafts are not overwritten. The autosave is stored in a special post revsion.
		return wp_create_post_autosave( wp_slash( $post_data ) );
	}
}

7.php

function edit_post( $post_data = null ) {
	...
	update_post_meta( $post_ID, '_edit_last', get_current_user_id() );

	$success = wp_update_post( $post_data );
	...
	return $post_ID;
}

8.php

function update_post_meta( $post_id, $meta_key, $meta_value, $prev_value = '' ) {
	// Make sure meta is added to the post, not a revision.
	if ( $the_post = wp_is_post_revision($post_id) )
		$post_id = $the_post;

	$updated = update_metadata( 'post', $post_id, $meta_key, $meta_value, $prev_value );
	if ( $updated ) {
		wp_cache_set( 'last_changed', microtime(), 'posts' );
	}
	return $updated;
}

9.php

function hcf_save( $post_id ) {
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}
	if (get_post_status($post_id) === 'auto-draft') {
		return;
	}
	$field_list = [
		'hcf_author',
		'hcf_published_date',
		'hcf_price',
	];
	foreach ( $field_list as $fieldName ) {
		if ( array_key_exists( $fieldName, $_POST ) ) {
			update_post_meta(
				$post_id,
				$fieldName,
				sanitize_text_field( $_POST[ $fieldName ] )
			);
		}
	}
}
add_action( 'save_post', 'hcf_save' );