How can I check error types of gRPC calls?

[cpu...@gmail.com]

In my local code, I'm using things like

if errors.Is(err, api.ErrMustRetry) { ... }

How would I achieve the same on errors returned by the gRCP interface? I've noticed these are wrapped: 

rpc error: code = Unknown desc = must retry rpc error: code = Unknown desc = must retry

I assume the errors package won't work here as type information is not carried across gRPC: What is the best practice here: unwrap the root error from the RPC result (how) and perform string comparison?

Thanks,

Andreas

[Dan Kortschak]

There is the status package which provides tools for examining the gRPC
errors, https://pkg.go.dev/google.golang.org/grpc/status.

[Axel Wagner]

Generally, I would argue that the grpc package maintainers should, at some point, support native wrapping of errors.

In the meantime, they are providing this package: https://pkg.go.dev/google.golang.org/grpc/status

It predates (by a long time) the errors.Is APIs, which is why I assume they haven't yet implemented support

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[cpu...@gmail.com]

Thank you for the pointer. After looking at the API, I'm still confused how to use it. I can status.Convert(err) to get a status object but couldn't really figure how how to use that to extract the actual underlying base error?

[Brian Candler]

It's an alias for this:

https://pkg.go.dev/google.golang.org/grpc/internal/status#Status
So it should have Code() and Detail() methods.

[Axel Wagner]

You cant to use status.Code(err), which gives you the gRPC status code.

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[Axel Wagner]

*want, sorry, typo :)

[burak serdar]

Here is something I did to pass custom error information:

Based on the grpc code, if an error type has GRPCStatus()
*status.Status function, then grpc gets the status information using
that method. For the error types that can go through grpc, I have this
method below:

func (e MyError) GRPCStatus() *status.Status {
var code codes.Code
switch e.HTTPStatus {
case http.StatusBadRequest:
code = codes.InvalidArgument
case http.StatusUnauthorized:
code = codes.Unauthenticated
default:
code = codes.Unknown
}
x, _ := json.Marshal(e)
return status.New(code, string(x))
}


In this example, the error type includes an HTTP status code, and the
grpc status is decided based on that. The grpc error contains a JSON
marshaled error information.

On the receiving end, I have this function:

func FromGRPCError(err error) (MyError, bool) {
if x, ok := status.FromError(err); ok {
var e MyError
if json.Unmarshal([]byte(x.Message()), &e) == nil {
return e, true
}
}
return MyError{}, false
}

This decodes the error information from the incoming error.

This scheme can be extended to deal with multiple error types.

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