Skip session recording reservation files (filessesion)

[gabrielcorado]

Closes #13640 by skipping reservation files when completing a session recording upload. To do so, now we create the reservation files with a different extension so we can differentiate them from regular upload parts.

Context

Session recording modes introduce "disk reservation", where Teleport creates placeholder files to guarantee that the node has enough disk space to record sessions. This reservation happens every time a new part file is required. Those files are replaced with recording contents on the UploadPart flow.

In scenarios where the upload couldn't run (for example, if the node is killed during a session), the reservation file was left on the node. When the UploadComplete was executed, it created broken recording files (.tar), causing the uploads to fail (as we can see in the logs presented on the related issue).

[espadolini]

Side note: don't we lose the disk space reservation the moment we open the file for writing again in openUploadPart?

[gabrielcorado]

Side note: don't we lose the disk space reservation the moment we open the file for writing again in openUploadPart?

@espadolini Do you mean it because we truncate the file when opening?

[espadolini]

Do you mean it because we truncate the file when opening?

Yes, as opposed to writing whatever content we need and then truncating before closing.

[gabrielcorado]

@zmb3 I've updated the PR with @espadolini suggestion: create the reservation files with a different extension, so we don't need to check their content.

Yes, as opposed to writing whatever content we need and then truncating before closing.

I have updated the PR with this also. Thanks for pointing it out.

[espadolini]

Never mucked around the session recording code so take this approval with a grain of salt