Source

File: wp-includes/revision.php

function wp_save_post_revision( $post_id ) {
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}

	$post = get_post( $post_id );
	if ( ! $post ) {
		return;
	}

	if ( ! post_type_supports( $post->post_type, 'revisions' ) ) {
		return;
	}

	if ( 'auto-draft' === $post->post_status ) {
		return;
	}

	if ( ! wp_revisions_enabled( $post ) ) {
		return;
	}

	/*
	 * Compare the proposed update with the last stored revision verifying that
	 * they are different, unless a plugin tells us to always save regardless.
	 * If no previous revisions, save one.
	 */
	$revisions = wp_get_post_revisions( $post_id );
	if ( $revisions ) {
		// Grab the last revision, but not an autosave.
		foreach ( $revisions as $revision ) {
			if ( false !== strpos( $revision->post_name, "{$revision->post_parent}-revision" ) ) {
				$last_revision = $revision;
				break;
			}
		}

		/**
		 * Filters whether the post has changed since the last revision.
		 *
		 * By default a revision is saved only if one of the revisioned fields has changed.
		 * This filter can override that so a revision is saved even if nothing has changed.
		 *
		 * @since 3.6.0
		 *
		 * @param bool    $check_for_changes Whether to check for changes before saving a new revision.
		 *                                   Default true.
		 * @param WP_Post $last_revision     The last revision post object.
		 * @param WP_Post $post              The post object.
		 */
		if ( isset( $last_revision ) && apply_filters( 'wp_save_post_revision_check_for_changes', true, $last_revision, $post ) ) {
			$post_has_changed = false;

			foreach ( array_keys( _wp_post_revision_fields( $post ) ) as $field ) {
				if ( normalize_whitespace( $post->$field ) !== normalize_whitespace( $last_revision->$field ) ) {
					$post_has_changed = true;
					break;
				}
			}

			/**
			 * Filters whether a post has changed.
			 *
			 * By default a revision is saved only if one of the revisioned fields has changed.
			 * This filter allows for additional checks to determine if there were changes.
			 *
			 * @since 4.1.0
			 *
			 * @param bool    $post_has_changed Whether the post has changed.
			 * @param WP_Post $last_revision    The last revision post object.
			 * @param WP_Post $post             The post object.
			 */
			$post_has_changed = (bool) apply_filters( 'wp_save_post_revision_post_has_changed', $post_has_changed, $last_revision, $post );

			// Don't save revision if post unchanged.
			if ( ! $post_has_changed ) {
				return;
			}
		}
	}

	$return = _wp_put_post_revision( $post );

	// If a limit for the number of revisions to keep has been set,
	// delete the oldest ones.
	$revisions_to_keep = wp_revisions_to_keep( $post );

	if ( $revisions_to_keep < 0 ) {
		return $return;
	}

	$revisions = wp_get_post_revisions( $post_id, array( 'order' => 'ASC' ) );

	$delete = count( $revisions ) - $revisions_to_keep;

	if ( $delete < 1 ) {
		return $return;
	}

	$revisions = array_slice( $revisions, 0, $delete );

	for ( $i = 0; isset( $revisions[ $i ] ); $i++ ) {
		if ( false !== strpos( $revisions[ $i ]->post_name, 'autosave' ) ) {
			continue;
		}

		wp_delete_post_revision( $revisions[ $i ]->ID );
	}

	return $return;
}