Source

File: wp-includes/revision.php

function _wp_upgrade_revisions_of_post( $post, $revisions ) {
	global $wpdb;

	// Add post option exclusively.
	$lock   = "revision-upgrade-{$post->ID}";
	$now    = time();
	$result = $wpdb->query( $wpdb->prepare( "INSERT IGNORE INTO `$wpdb->options` (`option_name`, `option_value`, `autoload`) VALUES (%s, %s, 'no') /* LOCK */", $lock, $now ) );
	if ( ! $result ) {
		// If we couldn't get a lock, see how old the previous lock is.
		$locked = get_option( $lock );
		if ( ! $locked ) {
			// Can't write to the lock, and can't read the lock.
			// Something broken has happened.
			return false;
		}

		if ( $locked > $now - 3600 ) {
			// Lock is not too old: some other process may be upgrading this post. Bail.
			return false;
		}

		// Lock is too old - update it (below) and continue.
	}

	// If we could get a lock, re-"add" the option to fire all the correct filters.
	update_option( $lock, $now );

	reset( $revisions );
	$add_last = true;

	do {
		$this_revision = current( $revisions );
		$prev_revision = next( $revisions );

		$this_revision_version = _wp_get_post_revision_version( $this_revision );

		// Something terrible happened.
		if ( false === $this_revision_version ) {
			continue;
		}

		// 1 is the latest revision version, so we're already up to date.
		// No need to add a copy of the post as latest revision.
		if ( 0 < $this_revision_version ) {
			$add_last = false;
			continue;
		}

		// Always update the revision version.
		$update = array(
			'post_name' => preg_replace( '/^(\d+-(?:autosave|revision))[\d-]*$/', '$1-v1', $this_revision->post_name ),
		);

		/*
		 * If this revision is the oldest revision of the post, i.e. no $prev_revision,
		 * the correct post_author is probably $post->post_author, but that's only a good guess.
		 * Update the revision version only and Leave the author as-is.
		 */
		if ( $prev_revision ) {
			$prev_revision_version = _wp_get_post_revision_version( $prev_revision );

			// If the previous revision is already up to date, it no longer has the information we need :(
			if ( $prev_revision_version < 1 ) {
				$update['post_author'] = $prev_revision->post_author;
			}
		}

		// Upgrade this revision.
		$result = $wpdb->update( $wpdb->posts, $update, array( 'ID' => $this_revision->ID ) );

		if ( $result ) {
			wp_cache_delete( $this_revision->ID, 'posts' );
		}
	} while ( $prev_revision );

	delete_option( $lock );

	// Add a copy of the post as latest revision.
	if ( $add_last ) {
		wp_save_post_revision( $post->ID );
	}

	return true;
}