[FEATURE]: ProtoBuf - make license.id an (external) enum

[jkowalleck]

currently(CDX 1.6), we have the following situation:

• for JSON, the known SPDX licence IDs are in an own schema store: https://github.com/CycloneDX/specification/blob/master/schema/spdx.schema.json
• for XML, the known SPDX licence IDs are in an own schema store: https://github.com/CycloneDX/specification/blob/master/schema/spdx.xsd
• for ProtoBuf, the known SPDX licence IDs are not an enum at all, they are just a free text(string):

  specification/schema/bom-1.6.proto

  Lines 397 to 400 in b50ff0d

  	message License {
  	oneof license {
  	// A valid SPDX license identifier. If specified, this value must be one of the enumeration of valid SPDX license identifiers defined in the spdx.schema.json (or spdx.xml) subschema which is synchronized with the official SPDX license list.
  	string id = 1;

problem

Using arbitrary strings for license.id means, that there is just no baked-in safety.
To improve this, I propose to use a dedicated ProtoBuf package that can be maintained and released outside the regular CDX release cycle, just like the enum for JSON and XML.

possible solution

• have a file spdx.proto
  • use an own package cyclonedx.spdx
  • declare the like

    enum LicenseId {
      LICENSEID_UNSPECIFIED = 0
      // 0BSD
      LICENSEID_0BSD = 1
      // ...
      // Apache-1.0
      LICENSEID_Apache_1_0
      // ...
    }

  • have this file checked against breaking changes
    • see https://github.com/CycloneDX/specification/blob/master/tools/src/test/proto/buf_breaking-remote.yaml
    • see https://github.com/CycloneDX/specification/blob/master/tools/src/test/proto/buf_breaking-version.yaml
    • have the license file updated with the other spdx.* schema files
• in the bom-1.x.proto file, use that enum (pseudocode)

  import "cyclonedx.spdx";
  message License {
    oneof license {
      // A known SPDX license identifier.
      cyclonedx.spdx.LicenseId = 1;
      // ...
    }
    // ...
  }

consideration & research

• ❗ this might be a breaking change - need to investigate
• ❕ need to investigate how/where to publish the schema file, so that has the intended effect

[jkowalleck]

this is currently in RFC phase.
If you have any opinion on that, please let us know.