Temp file cleanup on process crash, or at least specify temp path

[cbecker]

I noticed two cases where temp files are created:

• A %Waffle.File is created with the binary field
• convert (or another tool) is called to transform the file

In production, this is dangerous: if the process handling the temp files crashes, temp files are not removed and will dangle, potentially occupying significant storage (even filling it if not cleaned up for a long time).

I see two options here, IMO they must be both implemented:

1. Allow to specify the temp dir for waffle, and use it in Waffle.File.generate_temporary_file
2. Briefly-like auto temp file deletion

(1) can be used to periodically clean up old files (e.g. > 1 hour)
(2) is the best option, but assumes a %Waffle.File does not cross process boundaries

@achempion I'd like to hear your opinion, maybe there is a better option.
I've seen this be a problem in production and eventually occupy all disk space with old temp files.

EDIT: I wonder if we could do without temp files at all. convert could output/input to/from stdout/stdin, but not sure where else we need explicit files.

[cbecker]

@achempion I'd like to get at least #145 merged if possible, as this is problematic during production. Would this fix work out for you?

[achempion]

Thank you for submitting the PR 👍 I've added a codereview and I can release a new version once merged

p.s. linux systems should be automatically cleaning up the system tmp directory on boot or every X hours, is the tmp directory growing too fast on your system?

[cbecker]

Thanks. My understanding is that linux systems clean up /tmp at boot, but not periodically.
My issues are when running inside a docker container.

I'd like to reimplement the code to only store on disk when needed, as it is state that is difficult to track- but for the time being it'll be fine to have a specific temp dir as in the PR

[achempion]

resolved with #145