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[Protobuf] Make it possible to override field numbers using x-protobuf-index

Merged Administrator requested to merge github/fork/k0ral/protobuf-index into master

Created by: k0ral

As described in issue https://github.com/OpenAPITools/openapi-generator/issues/6756 . Cc: @dobl1 .

The protobuf generator currently does not guarantee that generated field numbers are preserved across multiple generations: it always generates them as a sequence from 0 to n (through the x-index vendor extension). If fields are re-ordered, added, or deleted, field numbers will almost always shift. This is a big deal, as field numbers are part of the binary interface of protobuf serialization format.

This change is making it possible to specify protobuf field numbers using the vendor extension x-protobuf-index. This is an opt-in feature at field level: if x-protobuf-index is not specified, then the current behavior based on x-index still applies. In other words: this is backward compatible.

For example, the following data definition:

Pet:
  type: object
  properties:
    id:
      type: integer
      x-protobuf-index: 3
    name:
      type: string
      x-protobuf-index: 1

... will generate the below protobuf message:

message Pet {
  int64 id = 3;
  string name = 1;
}

I do not expect this pull-request to be merged as-is, I rather see it as a conversation-starter: do you agree with the general approach ? How would you suggest I improve the implementation ? I'd rather get feedback before investing time in writing automated tests, for example. For now, I've just manually validated the behavior on a toy example.

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