Avoid downloading data when rehashing URL

[noamross]

Summary

It seems that current behavior for rehashing URL still downloads the whole contents of that URL to memory. This change avoids doing so at all by telling curl to only get response headers, no body.

I'm not sure where or how in the test framework to set up a test for "Does not download the whole URL content body when testing for changes at the URL."

Related GitHub issues and pull requests

• Ref: #

Checklist

• I understand and agree to drake's code of conduct.
• I have listed any substantial changes in the development news.
• I have added testthat unit tests to tests/testthat for any new functionality.
  • I have made an in-line runtime test condition.
• This pull request is not a draft.

[wlandau]

Thanks for catching this, @noamross. Would you mention it in NEWS.md?

For testing, I think we could mock assert_useful_headers() in rehash_url() or assert_useful_headers() to make sure we download just the headers.

[wlandau]

Hmm... instead of mockr, what about stopifnot(length(req$content) < 1L) just after req <- curl::curl_fetch_memory(url, handle = curl::new_handle(nobody = TRUE)) in rehash_url()?

[wlandau]

A couple more comments:

1. Don't worry about the Travis failure. I am almost positive it is caused by 'UseMethod' used in an inappropriate fashion r-lib/covr#392. Feel free to switch back to CRAN covr in .travis.yml.
2. If you would like, please feel free to add yourself as a contributor in the DESCRIPTION. It's a small PR, but it uncovered an important performance issue.

[wlandau]

In fact, for (1), I will rerun Travis since the issue with covr should now be fixed.

[codecov-io]

Codecov Report

Merging #982 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #982   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          46     46           
  Lines        6336   6337    +1     
=====================================
+ Hits         6336   6337    +1

Impacted Files	Coverage Δ
R/drake_meta_.R	100% <100%> (ø)	⬆️

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[wlandau]

Looks great, Noam. Thanks again!