Failed upgrade to 3.0.0
Error: file_put_contents(/var/www/html/craftcms/composer.lock): failed to open stream: Permission denied


Whatever user:group PHP is running as on your server doesn't have the permissions to write to /car/www/html/craftcms


  It also looks like Craft didn't take a backup, as my most recent backup is from RC4, back in January.


Craft will only do a backup during updating if mysqldump or pg_dump is available to PHP (by default), or if your custom backupCommand config setting executes successfully.

If Craft can't make a backup during updating, it will halt the update process and prompt you asking what you'd like to do with one of the options being to continue without making a backup.


  I'm not sure how to recover from this,


To restore the Craft files to the previous version, you can open your project's composer.json file, find the craftcms/cms line, set the version to the pre-update attempt version, then run composer update from the command line.

If there were no database migrations that ran during the update attempt, that should be all you need to do.

If there were database migrations that ran, you'll probably get an message when you try to access the Control Panel saying that you can't backtrack to that version of Craft because now the database is out of sync with the files.

The only practical way around that is to restore from a pre-update attempt database backup.