IMDSv2 support

[condour]

Please ensure the following before submitting a PR:

• if suggesting code changes or improvements, open an issue first
• for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
• add code or new test files to /tests for any new functionality or bug fix
• make sure R CMD check runs without error before submitting the PR

[laurenkahn]

Test 1: EC2 using IMDSv1, Sys.setenv(USE_IMDS_TOKEN="TRUE")
Expected result: Should be able to access S3.
Actual result: Success, able to access S3.

Test 2: EC2 using IMDSv1, Sys.setenv(USE_IMDS_TOKEN="FALSE") or not set at all.
Expected result: Should be able to access S3, as in the original code.
Actual result: Failure - see comment. Header cannot be NULL.

Test 3: EC2 using IMDSv2, Sys.setenv(USE_IMDS_TOKEN="TRUE")
Expected result: Should be able to access S3 (unlike before).
Actual result: Success, able to access S3.

Test 4: EC2 using IMDSv2, Sys.setenv(USE_IMDS_TOKEN="FALSE") or not set at all.
Expected result: Should not be able to access S3, as in the original code.
Actual result: Not able to access S3, but for wrong reason (header = NULL). Addressing test #2's failure will fix this.

[laurenkahn]

This fails if token = NULL. For example if I use an EC2 using IMDSv1, and I don't set USE_IMDS_TOKEN to TRUE, and I run aws.ec2metadata::metadata$iam_role_names() I get the following error:

Error in curl::handle_setheaders(handle, X-aws-ec2-metadata-token = token): All headers must be strings.

Using a blank string instead seems to work when a token isn't required (IMDSv1).

[condour]

I've added a check for is.null prior to setting the header. I'm unable to test at the moment, so I would appreciate if you can verify that it solves the issue. Thanks!

[laurenkahn]

This works! The tests described above all have the expected results now.

[condour]

Thanks! Now I guess we just need someone to merge?

[daltschu22]

Is there a reason this hasnt been merged?

[tyner]

Would be great to see it merged

[rokhmanov]

Please!