feat: sort longturtle blank nodes

[edmondchuc]

Summary of changes

Fixes #1890 - Sorting Turtle output?

This change improves git diffing Turtle data serialized with the longturtle serializer. Previously, blank nodes were not sorted, and round-trips using RDFLib's turtle parser and longturtle serializer would have blank node objects flip flop around, making it difficult to read real changes using git diff.

This PR fixes the above by implementing a sort on values where triples in the object position are blank nodes. The blank nodes are sorted by grabbing their concise-bounded description graph and sorting it as a string in their longturtle serialization.

This adds an additional cost to the longturtle serializer, but I think the cost is worth it if we are after a deterministic output with blank nodes.

Note that this depends on RDF data parsed using the turtle parser and its behaviour of how it assigns blank nodes. Exact serialization with data added via the graph object cannot be guaranteed. This would require implementing RDF Canonicalization to guarantee the same blank node identifiers in the graph.

Once RDF Canonicalization is implemented, sorting by the blank node identifier directly will be enough to guarantee deterministic serialization, and the expensive CBD sorting can be removed.

Update: looks like top-level blank nodes with no inbound relationships don't get sorted. I think this makes sense since we're only applying the sort to blank nodes in the object position. If we cared about sorting blank nodes in the subject position, we can apply the same kind of sorting based on the text serialization of the CBD onto the subject blank nodes.

rdflib/rdflib/plugins/serializers/longturtle.py

Lines 106 to 112 in 08dd4b7

	for subject in subjects_list:
	if self.isDone(subject):
	continue
	if firstTime:
	firstTime = False
	if self.statement(subject) and not firstTime:
	self.write("\n")

Fixes #2767 - Bug in longturtle serialization

This PR also fixes the missing trailing whitespace in the special case described by @mschiedon.

The longturtle serializer fails to emit a whitespace separator between a predicate and a list of objects if one of these objects is a blank node (and the blank node cannot be 'inlined', i.e. is used more than once)

This fix was previously fixed in PR #2700 but was inadvertently reverted in PR #2731 when I was trying to fix the ruff linting rule in the test case. To get around the ruff linting rule, I've now moved the target result of the test case into a text file.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement.
  • Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[edmondchuc]

@ajnelson-nist would be good to see if you can test this with some of the data you work with and confirm that this PR fixes the blank node sorting issues in your workflows.

[coveralls]

coverage: 90.276% (+0.03%) from 90.247%
when pulling 4ef7733 on edmond/longturtle-sort
into 08dd4b7 on main.

[nicholascar]

Awesome stuff @edmondchuc!