Periodically garbage collect

[Stebalien]

We should periodically garbage collect in case the user has garbage collection turned off. Otherwise, we'll never delete anything.

[raulk]

Why is gcDiscardRatio statically configured to 0.1? The struct field is unexported :-(

[Stebalien]

Not exporting it was probably a mistake, @magik6k?

[raulk]

Gotcha, I'll submit a PR.

Another thing, overriding the error here creates ambiguity: https://github.com/ipfs/go-ds-badger/blob/master/datastore.go#L286.

Badger returns nil if one file was collected, and callers may use this as a hint to continue collecting by calling GC again.

Overriding the "nothing was collected" error to nil makes it impossible to discriminate whether it's worth calling GC again or not :-(

If we want to decouple this from badger implementation-specific errors, we can return a second value to signal if something was collected, or not, or we don't know (some stores may not expose this event).

[Stebalien]

If we want to decouple this from badger implementation-specific errors, we can return a second value to signal if something was collected, or not, or we don't know (some stores may not expose this event).

That's the issue. However, for every datastore except badger, re-running GC till it removes nothing is just a waste of time so I'm not sure how useful that is.

See #56 that just does this automatically.

[magik6k]

Not exporting it was probably a mistake, @magik6k?

Yeah, I probably meant to export it, but also set some value that would mostly work to quickly test and never changed it

[aarshkshah1992]

@Stebalien Is this up for grabs ?

[Stebalien]

Yes! (it's also pretty easy to review so it should get merged pretty quickly).

[Stebalien]

Fixed in #72.