Prompt users during make() in interactive mode

[wlandau]

Summary

drake gets initial dependencies from the user's environment rather than script files. However, environments are brittle, so targets can falsely invalidate if you run make() in a stale R session. This PR implements safeguards and reminders for make() in interactive mode. If the R session is interactive and some targets are out of date, make() prompts you with a menu to keep you from accidentally mangling your project. It also reminds you to read #761 regardless of how your targets are doing.

> interactive()
[1] TRUE
> library(drake)
> plan <- drake_plan(x = 1, y = 2)
> make(plan)
2 outdated targets:
  x
  y

Really run make() in interactive mode?
Considerations: https://github.com/ropensci/drake/issues/761 

1: yes
2: no

Selection: 1
target x
target y
make() in interactive mode requires extra care:
https://github.com/ropensci/drake/issues/761
> make(plan)
All targets are already up to date.
make() in interactive mode requires extra care:
https://github.com/ropensci/drake/issues/761

You can disable the menu with make(force = TRUE) or options(drake_force_interactive = TRUE).

cc @MilesMcBain, @ha0ye

Related GitHub issues and pull requests

• Ref: Environments, reproducibility, and consistent target validation #761 (not necessarily fixed)

Checklist

• I have read drake's code of conduct, and I agree to follow its rules.
• I have listed any substantial changes in the development news.
• I have added testthat unit tests to tests/testthat to confirm that any new features or functionality work correctly.
• I have tested this pull request locally with devtools::check()
• This pull request is ready for review.
• I think this pull request is ready to merge.

[codecov-io]

Codecov Report

Merging #762 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #762   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          72     72           
  Lines        6106   6122   +16     
=====================================
+ Hits         6106   6122   +16

Impacted Files	Coverage Δ
R/exec-backend.R	100% <ø> (ø)	⬆️
R/vis-color.R	100% <ø> (ø)	⬆️
R/api-make.R	100% <100%> (ø)	⬆️
R/exec-session.R	100% <100%> (ø)	⬆️
R/vis-console.R	100% <100%> (ø)	⬆️
R/preprocess-config.R	100% <100%> (ø)	⬆️

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