Uploading lots of small files can be made significantly faster by parallelizing the `s3.put_object` calls. In testing, an UPLOAD_CONCURRENCY of 10 made a large restore 10x faster. An UPLOAD_CONCURRENCY of 20 made the same restore 18x faster.
This commit is careful to parallelize as little as possible, to reduce the chance of concurrency issues. In the worker threads, no database transactions are performed. All modification of shared objects is controlled with a mutex.
Unfortunately we do not have any existing tests for the `ToS3Migration` class. This change has been tested with a large site backup (120k uploads totalling 45GB)
`GlobalSetting.relative_url_root` comes from the destination site. We
can't be sure whether it was the same on the original site. It's safer
to use a wildcard here, so we can backup/restore sites with different
relative_url_root values.
* strip out the href and xlink:href attributes from use element that
are _not_ anchors in svgs which can be used for XSS
* adding the content-disposition: attachment ensures that
uploaded SVGs cannot be opened and executed using the XSS exploit.
svgs embedded using an img tag do not suffer from the same exploit
The `uplaods:migrate_to_s3` rake task should always use the environment variables, because you usually don't want to break your site's uploads during the migration. But restoring a backup should work with site settings as well as environment variables, otherwise you can't restore uploads to S3 from the web interface.
The rake task aborted the migration with "Already migrated" when all upload URLs linked to the correct S3 bucket even though the files didn't exist on S3. By removing the first check we force the rake task to check for the existance of uploads on S3.
