Implementing validation for prost -generated structs

[blaind]

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This is not a bug report or feature request, but rather a question about practical approaches for implementing validation for prost -generated structs (as a part of tonic grpc service).

Use case: I'd like to validate the user-submitted struct data on the server side (e.g. matches regexp, max length, etc.).

Possible ways to resolve this:

1. Create a new trait & implement it on structs

trait MyValidator {
  fn validate() -> bool;
}

impl MyValidator for MyProstStruct {
  fn validate(&self) -> bool {
    if !self.email.contains("@") {
      return false;
    }
    // ... etc
    true
  }
}

2. Use existing validation framework with derive-macros

For example, https://crates.io/crates/validator. The syntax for validator is:

#[derive(Debug, Validate, Deserialize)]
struct SignupData {
    #[validate(email)]
    mail: String
}

this brings a problem, how to actually have prost generate the required macro-markup per struct field. There is config.field_attribute(...) (https://docs.rs/prost-build/0.6.1/prost_build/struct.Config.html#method.field_attribute), but having the actual .proto files and the validation-derives separated in multiple files is hard to manage. Do .proto files have some way to supply metadata per field?

I looked for other alternative frameworks, but could not find one with moderate+ adoption rate.

3. Use existing validation framework without macros

Validator seems to support this, partly, but for example regex validation is missing.

Conclusion

Any ideas, how to proceed with this?

[danburkert]

I'm not aware of any widely used 'semantic' validation tools that are used across languages for protobuf. That being said it's been a pain point I've felt in any moderately complex system using protobufs, so I'm certainly open to ideas.

[Palmik]

One could use custom field options to define the validation for a field. For example, one could have a a very generic custom field option like prost_field_attribute that would translate to the equivalent of calling field_attribute on that field.

Example:

message Foo {
  string bar = 1 [(prost_field_attribute) = "#[validate(email)]"];
}

If custom field options get more support, one could perhaps build a more sophisticated companion library prost-validate.

[danburkert]

I think it'd have to work at the Message level, since quite often you also want to do cross field validations, i.e. 'the sum of fields a and b must be even'

[volfco]

There seems to be a standard developing around https://github.com/envoyproxy/protoc-gen-validate

[makorne]

There seems to be a standard developing around https://github.com/envoyproxy/protoc-gen-validate

But it does not have any rust code generator. Why not use this rust validator? https://github.com/KaiserKarel/prosit