Ensure global defers run on exit

[hadley]

Opening this primarily for discussion — this change means that when withr::defer() is called interactively (i.e. from the global environment), it will be run when you end the session.

I'm not 100% sure it's a good idea (or if it's been discussed and rejected before) but this is how I expected it to wrk. I started looking into because I noticed that my tests weren't always running cleanup code, and that lead to a simple experiment confirming that withr::defer(print("Hi")) is not executed when you quit R.

cc @jennybc, @jimhester, @lionel-

[jennybc]

We have discussed this before, on the original PR, starting around here.

[jennybc]

In terms of the workflow aspect, I have something in my .Rprofile that makes dr a shortcut for withr::deferred_run(), which I use heavily when I'm developing, e.g. tests, that use defer().

[jennybc]

See also #136 no, actually not related

[hadley]

I now feel more strongly that the non-finalizer approach is suboptimal because it relies on me to remember that I have to run deferred_run(), which is very hard.

[jennybc]

I don't have a strong opinion about this. I guess I prefer the status quo and I am certainly very used to it, but I can also make peace with the proposed change.

[lionel-]

The session finalizer comes in addition to the existing features, so I guess it makes sense. I wouldn't rely on it being run for important things though.

In general I don't have any strong feeling about global defer() except that we need to fix #136 so that the message isn't displayed in non-top-level global evaluations which are actually cleaned up automatically (I'm still happy to send a PR).

[hadley]

@jimhester if you want a test, I think I'd have to add a dependency on callr. I can't see any other way except to start another R process.

[jimhester]

Could you add the PR number here?

[jimhester]

I think it is ok to skip the test here.

OTOH callr is a testthat dependency anyway, so adding it here for the tests is basically free.

[hadley]

Will need an update after #176