How does MinVersion work on union type?

[Chung-sheng Wu]

Hi all,

On a stable union type, we can use MinVersion to add new fields. But what happen if a client receive a union from a newer server which have a newer value?

For enum type, the value will be mapped to the field with [Default] tag. However, this is not the case for union. Using "switch(MyUnion::Tag)" may cause the client crash because of unexpected value in MyUnion::Tag.

If the above understanding is correct, it seems like using MinVersion on union is not safe. What is the best practice of doing versioning on union types?

Thanks

Chungsheng

[Ken Rockot]

Unions tagged with a field unknown to the receiver are decoded as null values. For this to work well, the union-typed field must be marked as optional. For example:

  union Thing {

    int32 number;

    string name;

    [MinVersion=1] string? animal;

  };

  struct SomeOtherStruct {

    // This field will be null if someone sends an animal to a recipient

    // using Thing v0.

    Thing? thing;

  };

 

If, on the other hand, the union-typed field is not marked optional, then the receiver will behave like any other scenario where a non-optional field receives a null value: it will fail validation, reject the message, and disconnect the interface.

Thanks

Chungsheng

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[Chung-sheng Wu]

Thanks for the explanation!

Ken Rockot <roc...@google.com> 於 2021年10月12日 週二 下午11:57寫道：

[Chung-sheng Wu]

Hi Ken,

More questions.

1. What is the difference between |string| and |string?| in a union? Does it make string become optional<string>?

2. What if the users using |switch(MyUnion->which()) {...}| ? Does the switch get an unexpected enum value and crash?(assume that no |default:| in that switch statement.) What value will |which()| return from a null union?

Thanks

Chungsheng

Chung-sheng Wu <chung...@google.com> 於 2021年10月13日 週三 下午7:45寫道：

[Daniel Cheng]

1. With `string`, the union field type will be std::string. With `string?`, the union field type will be absl::optional<std::string>.

2. Using a versioned union requires that all uses of the union be marked nullable. Then, when Mojo sees an unknown union field, it will be deserialized as a null union instead—so there will be no which() to consult.

Daniel

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[Chung-sheng Wu]

Hi,

I did some experiment and faced some issues.

I try to send a union with the following type:

union U {
  int32 a;
  int64 b;
  [MinVersion=1] BarB? c;
};

And receive the union with the following type:

union U {
  int32 a;
  int64 b;

}; 

When the server sent the union with type BarB, I got "Invalid message: VALIDATION_ERROR_UNKNOWN_UNION_TAG (unknown tag in U)".

I also tried to wrap the union in the following struct:

struct UO {
  U? u;
};

But got the same result.

Does I misunderstand something? 

The experiment is in this cl.

Thanks

Chungsheng

Daniel Cheng <dch...@chromium.org> 於 2021年10月14日 週四 下午2:17寫道：

[Chung-sheng Wu]

Also, it seems like union doesn't support ordinal value?

Chung-sheng Wu <chung...@google.com> 於 2021年10月15日 週五 上午11:31寫道：

[Chung-sheng Wu]

Friendly  ping on this.

[Ken Rockot]

After taking a look at the union code generation, it seems like we never properly implemented union versioning after all, and we have only gotten away with this because nobody's tried using it until now. I don't see any logic in the generated code which would allow for extensibility of the tag enum.

[Chung-sheng Wu]

+@Meng-Huan Yu 

Hi, 

Is there any plan or schedule for the union? It is a critical blocker for us if the union cannot be versioned.

Thanks

Chungsheng

Ken Rockot <roc...@google.com> 於 2021年10月18日 週一 下午8:24寫道：

[Meng-Huan Yu]

Add more context:

We heavily use union in our interface. The interface is actively used between Chrome and Chrome OS and many (our team and our client teams) tests get broken frequently because they are out of sync between Chrome/ChromeOS. After some study, we think versioning is the best approach and we want to be versioning ASAP.

Please advise us how to deal with union versioning if this is not in your priority.

[Daniel Cheng]

We should almost certainly fix it. I've already filed a bug to track this: https://crbug.com/1261313.

As it's pretty late for most //mojo OWNERS though, we will discuss more about how to proceed tomorrow.

Daniel

[Chung-sheng Wu]

Sounds good! 

Thanks

Chungsheng

Daniel Cheng <dch...@chromium.org> 於 2021年10月19日 週二 下午1:04寫道：