prevent double linking of datasets and dataverses #7259

[pdurbin]

What this PR does / why we need it:

Users have been able to link a dataset to the same dataverse twice (or more) via API. This prevents it.

Update: in testing, it was found that it's also possible to link a dataverse to the same dataverse more than once. A fix has been added for this too.

Which issue(s) this PR closes:

Closes #7259

Special notes for your reviewer:

I tacked a test on an existing test method but I'm happy to create a dedicated test method if that's preferred. Update: a new LinkIT test class was added.

Suggestions on how to test this:

• Try to link the same dataset to the same dataverse via API multiple times.
• Try to link the same dataverse to the same dataverse via API multiple times.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

No.

Is there a release notes update needed for this change?:

Probably not worth it. Just a bug fix.

Additional documentation:

None.

[scolapasta]

Added a few comments / questions. Also, do we know if the same issue exists for linking dataverses? if so, should we add similar logic to LinkDataverseCommand?

[coveralls]

Coverage decreased (-0.001%) to 19.479% when pulling 403a966 on 7259-linking into 40cda0e on develop.

[pdurbin]

Also, do we know if the same issue exists for linking dataverses? if so, should we add similar logic to LinkDataverseCommand?

Yes, the same issue exists. Fixed in 5d4a3dc.

[pdurbin]

I'm going to go ahead and put this into code review even though I might add another test for #7430 (as discussed in Slack). Here are the changes since the last code review:

• Tests have been moved to a dedicated class called LinkIT.
• In fact the same problem happens with linking dataverses. Fixed and I updated the description of this pull request.
• The potential performance problem noticed in the last review was fixed by switching to an "alreadyLinked" method.

[pdurbin]

I moved it back to in progress to reflect that I'm still hacking on tests.

[pdurbin]

In 403a966 I wrote some tests that demonstrate that #7430 is a real issue. Hopefully they will be a good starting point when we pick that issue up. I'm moving this back to code review.