Allow interrupting safe import tasks

[Hawke]

Currently some long-running tasks cannot be interrupted by keyboard Ctrl+C. (e.g. 'chroma' plugin import scanning.

It would be nice to allow this to happen, assuming the task in question can be interrupted (e.g. chroma should be entirely read-only and should always be safe)

[sampsyo]

As I mentioned on IRC, this will require the pipeline module to optionally interrupt some threads. I can imagine this working one of two ways:

• Inform the thread that it's time to shut down and let it voluntarily return at a safe point.
• Have the application inform the pipeline which threads are safe to interrupt without warning.

[jmtar]

I would like to add that you also can't interrupt a simple "beet list" command.

I have 85k tracks, so this scrolls for a while if I mess up a query to something overly broad. I've taken to just Ctrl+Z and killing the process for now.

[sampsyo]

@rarn Really? Interrupting a beet list works fine for me.

[jmtar]

Must be a terminal emulator setting or something then. beets is the only program I've noticed the problem in though, but it must be on my end. Sorry!

[sampsyo]

No worries! Do let us know if you do track down the cause.

[FichteFoll]

I suppose rewriting beets asynchronously is not an option for the foreseeable future? At least auto-matching albums during import should be interruptable.