Add Metaschema support for valid value validation

[david-waltermire]

User Story:

As an OSCAL model developer, I need to be able to define a set of valid values that are then checked for in the JSON and XML schema generated from my Metaschema-based model.

Goals:

The Metaschema allows for a set of valid values to be defined for a flag or field. For example:

<define-flag datatype="NCName" name="order">
  <formal-name>Order</formal-name>
  <description>A designation of how a selection of controls in a profile is to be ordered.</description>
  <valid-values>
    <value name="keep">Keep existing order</value>
    <value name="ascending">Sort values ascending</value>
    <value name="descending">Sort values descending</value>
  </valid-values>
</define-flag>

For a given list of valid values, the specified values can be inclusive, allowing other values to be provided that are not in the list, or exclusive (the default), restricting allowed values to only those on the list. The inclusive behavior is used if the <valid-values @allow-other="yes"/> is provided.

These values are displayed in the schema documentation, but when the exclusive case is selected, the values are not constrained by the generated XML or JSON schema. This support needs to be implemented.

A valid values list can be included in the JSON schema using the enum keyword as follows:

"properties": {
  "order": {
    "description": "An enumerated property",
    "type": "string",
    "enum": [
      "keep",
      "ascending",
      "descending"
    ]
    
  }
},

Dependencies:

This issue will also partially address issue #390, by providing for validation of valid values.

Acceptance Criteria

• All generated XML and JSON schema include validation for valid values. These valid values are checked by the CI/CD workflow.
• The Metaschema definitions are updated in a way that result in no changes to associated content.
• Documentation on the use of Metaschema is updated to fully document this feature.
• A single PR is approved that contains the above changes.

[wendellpiez]

Addressed in https://github.com/wendellpiez/OSCAL/tree/sprint22-metaschema

[iMichaela]

7/11/2019

This issue and the related ones (noted in #435 ) for the metaschema need to be reviewed together and unitested. However, more unit tests need to be created for the other features.

[david-waltermire]

@wendellpiez I don't think this has been fully implemented in JSON schema validation. We also don't have a unit test for this.

[wendellpiez]

Also to do on this Issue: wire datatype support into Metaschema Schematron so it can validate a set of valid values against a nominal (declared) datatype to ensure they are all acceptable as tokens of that type.

Also, @anweiss suggests Metaschema require flag/@as-type and perhaps field/@as-type, and not assuming strings. Since there are also boolean and numeric types I guess this is fair. @david-waltermire-nist what do you think?

Short list:

• Unit tests
• Working example both XSD and Schematron
• Metaschema checking for clashes

[david-waltermire]

Development on this is complete in PR #492. This is ready for review.