grpc-dotnet + Blazor sets Safari and Firefox User-Agent to "grpc-dotnet/$assemblyVersion"

Describe the bug

When using grpc-dotnet and Blazor together to call a GRPC API, the User-Agent is set to “grpc-dotnet/$assemblyVersion” in Safari and Firefox. In Chrome and Edge it is the actual browser User-Agent.

https://docs.microsoft.com/en-us/aspnet/core/grpc/services?view=aspnetcore-5.0 for example describes how to read the User-Agent:

public override Task<ExampleResponse> UnaryCall(ExampleRequest request, ServerCallContext context)
{
    var userAgent = context.RequestHeaders.GetValue("user-agent");
    // ...

    return Task.FromResult(new ExampleResponse());
}

(Migrated from https://github.com/dotnet/AspNetCore.Docs/issues/21178)

To Reproduce

Use GRPC with Blazor as described at https://docs.microsoft.com/en-us/aspnet/core/grpc/browser?view=aspnetcore-5.0 and look at the traffic in Safari. The User-Agent will be for example “grpc-dotnet/2.34.0.0”.

You can also check out https://our.gatekeeper.page in Safari or Firefox and look at the request to the “WhoAmI” endpoint to see the header being wrong. The code for this can be found at https://github.com/getgatekeeper/server.

The Blazor example from https://github.com/grpc/grpc-dotnet/tree/master/examples/Blazor should also have the same issue.

The issue seems potentially related to the following piece of code in Grpc.Net.Client/Internal/GrpcCall.cs. I wonder if Chrome/Edge just ignore the header being overwritten whilst Safari and Firefox obeys it:

// User agent is optional but recommended.
headers.TryAddWithoutValidation(GrpcProtocolConstants.UserAgentHeader, GrpcProtocolConstants.UserAgentHeaderValue);

Test plan

Browser	Sends correct User-Agent	Screenshot
Safari	❌
Firefox	❌
Chrome	✅
Edge	✅

[BrennanConroy]

SignalR sets the “X-SignalR-User-Agent” header.

https://github.com/dotnet/aspnetcore/blob/020145ded2fd7ea4657f220e000d64d41232bec3/src/SignalR/clients/ts/signalr/src/Utils.ts#L201

I believe the reason we don’t set “User-Agent” is because it used to be a forbidden header but no longer is. Also, Chrome just drops the header? https://bugs.chromium.org/p/chromium/issues/detail?id=571722

[JamesNK]

gRPC fixed. Re-opening for SignalR.