Avoid Infinite Derivatives.

[rosiel]

GitHub Issue:

• [BUG] Endless repeating "Created a version in Fedora for [media name]" #903

Other Relevant Links (Google Groups discussion, related pull requests,
Release pull requests, etc.)

What does this Pull Request do?

Forbids "normal" derivatives ("normal" i.e. create another media and attach it to my parent node) where the source and target media are the same media.

Example: I have a MP4 that is both my Original File and Service File. Before, i'd be stuck in an infinite loop of replacing the file with worse and worse versions of that video. Now, I know to leave it alone.

What's new?

• AbstractGenerateDerivative checks and bails if the source media already has the destination term.

• EmitEvent knows how to catch this exception

• Trying to be good, I made a new "IslandoraDerivativeException".

• Does this change add any new dependencies? no

• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? no

• Could this change impact execution of existing code? yes, in a desirable way.

How should this be tested?

With contexts set up to make service file derivatives, create some content, and a media which is (e.g. image) both Original File and Service File.
Notice messages in the logs about infinite versions of the media.

With this PR:
That doesn't happen. You should get one message in the logs that the derivative was not created.

Documentation Status

• Does this change existing behaviour that's currently documented? not really
• Does this change require new pages or sections of documentation? no
• Who does this need to be documented for?
• Associated documentation pull request(s): ___ or documentation issue ___

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

Tag (@ mention) interested parties or, if unsure, @Islandora/committers

[rosiel]

Also directly addresses @dannylamb's reply to @nigelgbanks who mentioned this issue in Islandora-Devops/isle-buildkit#248

[nigelgbanks]

Reviewed and tested, works as intended. Prevents infinite derivatives when a file as both original file and service file, for example.