[Bug] after apache/skywalking-java#457, cross-thread grpc streaming is broken.

[h0cheung]

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Apache SkyWalking Component

Java Agent (apache/skywalking-java)

What happened

If StreamObserver#onNext is called in another thread, we will see exception: IllegalArgumentException("ContextSnapshot can't be null.")

After apache/skywalking-java#457, we use a io.grpc.Context to pass the ContextSnapshot. But by default, io.grpc.Context uses ThreadLocal as storage. So it won't work across threads. Then the contextSnapshot will be null, which causes the exception above.

I think we should partially revert the PR. Using the constructor to pass contexts is good.

What you expected to happen

It should work normally

How to reproduce

the simplest way is write a handler like this:

    public StreamObserver<Request> connect(StreamObserver<Response> responseObserver) {
        new Thread(()-> {
            responseObserver.onNext(Response.newBuilder().build());
        }).start();
        return new StreamObserver<>() {
            @Override
            public void onNext(Request value) {
                
            }

            @Override
            public void onError(Throwable t) {

            }

            @Override
            public void onCompleted() {

            }
        }

Then call it with a client, such as grpcurl.

Anything else

No response

Are you willing to submit a pull request to fix on your own?

• Yes I am willing to submit a pull request on my own!

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