Fix deterministic ids

[chisholm]

Fixes #395, #396 .

Revamped deterministic ID generation code:

• "Primary" object initialization is now done first, and the resulting property values are what are used to create the deterministic ID instead of whatever kwargs had been passed into the constructor.

• Conversion to a JSON-serializable object (as preparation for canonicalization) should now handle all types of property values.

  The way this conversion should be done isn't clear from the spec. E.g. it says to use the "list of ID Contributing Properties" and create canonical JSON from that, but (a) not all property values are compatible with JSON, and (b) it doesn't describe what from each property to use (name+value or just value?). For (a), property values which are incompatible with JSON are first serialized according to the JSON serialization rules for the type. This produces a string, which is JSON compatible and so can be canonicalized. This notably applies to timestamps, for which the library uses instances of a datetime subclass for convenience, which will get their spec-prescribed textual format. For (b), the decision is that both names and values are important, so we create a canonical JSON object, rather than taking the word "list" as given in the spec, literally.

[codecov-commenter]

Codecov Report

Merging #402 into master will increase coverage by 0.01%.
The diff coverage is 98.27%.

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
+ Coverage   98.43%   98.44%   +0.01%     
==========================================
  Files         127      128       +1     
  Lines       14652    14694      +42     
==========================================
+ Hits        14423    14466      +43     
+ Misses        229      228       -1     

Impacted Files	Coverage Δ
stix2/base.py	98.14% <95.12%> (+0.25%)	⬆️
stix2/test/v21/test_deterministic_ids.py	100.00% <100.00%> (ø)
stix2/canonicalization/Canonicalize.py	60.64% <0.00%> (-0.41%)	⬇️
stix2/patterns.py	99.33% <0.00%> (-0.01%)	⬇️
stix2/v20/__init__.py	100.00% <0.00%> (ø)
stix2/v21/__init__.py	100.00% <0.00%> (ø)
stix2/test/v20/test_pattern_expressions.py	100.00% <0.00%> (ø)
stix2/pattern_visitor.py	90.67% <0.00%> (+0.06%)	⬆️

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[rpiazza]

Can you see any issues with using refs as contributing properties? There are a few objects that use refs. I know I have to be careful about the order in which I create the ids when creating the deterministic ids by hand, but maybe it would never be an issue using python-stix2.

[chisholm]

By hand, I guess you'd have to create the referent object IDs first, then you can populate the ref properties with those IDs and then canonicalize. It is similar with the stix2 library: you need some value to use for the ref property, so you have to create the referent object (or its ID) first.

Circular references are impossible if both SCOs require UUIDv5's. You'd have to switch one to a v4 to make that work.

[clenk]

LGTM, thanks @chisholm!