Passing enum values through Mojo

[Veranika Liaukevich]

Hi chromium-dev,

What is the recommended way to typemap enums that are passed through mojo and validate their values?

We have an enum, which is already defined somewhere in the code. We would like to pass it through mojo. I see several ways to do so, but I don't like any of them:

1. Use int32 to pass the value around
   
• Cons: need to validate the values in each place where it is used
  
2. Duplicate the enum definition in .mojom file and use typemapping between two types and to validate the passed values.
• Cons: duplicating enums is ugly by itself, need to ensure two definitions stay in sync
  
3. Use "[Native] enum EnumType" to define typemapping
• Seems to be to be the best possible option. But it requires using IPC_ENUM_TRAITS macros, which appears to be deprecated.
  
4. Create a custom mojo structure (struct EnumType { int32 value;} ) and type-map it to the already defined enum type.

Any suggestions?

[Robert Sesek]

Hi Veranika,

Can you move the declaration of the enum? Security Team recently updated the best practices around this: the recommendation is to, if possible, put the definition of the enum in the .mojom, and have code reference the values directly from the generated bindings. Otherwise, option (2) using mojo::EnumTraits<T,U> is the common practice, even though it does result in duplication.

- Robert

rsesek / @chromium.org

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[Veranika Liaukevich]

The original enum is also used for data reporting and is defined in a .proto file. Moving the declaration to .mojom sadly wouldn't work.

Is it possible to write a unit test of sorts to ensure that duplicated enums in proto and in mojo stay in sync? Protobuf provides EnumType_MIN, EnumType_MAX and EnumType_IsValid(), which effectively allows to iterate through all defined values. Is there a similar thing in mojo?

[Daniel Cheng]

On Wed, Apr 11, 2018 at 12:43 PM Veranika Liaukevich <vera...@chromium.org> wrote:

The original enum is also used for data reporting and is defined in a .proto file. Moving the declaration to .mojom sadly wouldn't work.

Is it possible to write a unit test of sorts to ensure that duplicated enums in proto and in mojo stay in sync? Protobuf provides EnumType_MIN, EnumType_MAX and EnumType_IsValid(), which effectively allows to iterate through all defined values. Is there a similar thing in mojo?

​Mojo now emits EnumType::kMaxValue​; it doesn't emit EnumType::kMinValue currently, though doing so would be trivial. It also has a helper for determining if a given enum value is valid: https://cs.chromium.org/chromium/src/out/Debug/gen/content/common/frame.mojom-shared.h?rcl=da86d15f7497bd91008863b1c9b56d3787a6a224&l=113 for an example.

So it should be possible to make this work, though Mojo enums are scoped so 'iterating' through them involves a lot of casting.

Daniel

 

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[Veranika Liaukevich]

Thanks!

Is there any specific reason why approach #4 is not advisable?