bug: Gorums does not forward context

[meling]

Gorums fails to forward context to the server-side, and thus is not able to cancel or pass values from the client to the server via context.

We discussed this issue a while back when @aleksander-vedvik was working on the multiparty implementation, and there seems to be some customized support for cancelation in that code. But looking at the code it does not appear to work via the context.CancelFunc. Moreover, it does not pass the context.Value entries.

We need to add support for passing context. We can use the metadata package from the gRPC library. Here are some resources:

• Metadata guide
• Examples
• Metadata docs
• The metadata package

Tasks:

• Set up test cases to showcase the problem.
• Review the docs above.
• Investigate what gRPC does in their runtime.
• Implement solution.

Looking at the examples, we might actually have to use the metadata package explicitly. I'm not sure, but I have always assumed that we can use the context.Context we pass to the client call to cancel the request, or pass a value via its context from the client to the server-side. Need to check this with gRPC. Looking at the Examples listed above it looks less clear that that is the approach.

But since we already use the ordering.Metadata message to pass our own Gorums metadata, we should be able to pass the content of metadata.MD as part of our own Metadata message, and repopulate the context.Context values before we pass it to the server-side implementation via ServerCtx.

[meling]

More context (pun intended) about this: We currently have a few tests that allow passing metadata. However, the metadata is passed via two manager options:

• gorums.WithPerNodeMetadata(perNodeMD)
• gorums.WithMetadata(md)

These can only be used to pass metadata during connection establishment. Hence, there is no support for passing metadata per call, which is thus a missing feature.

// WithMetadata returns a ManagerOption that sets the metadata that is sent to each node
// when the connection is initially established. This metadata can be retrieved from the
// server-side method handlers.

// WithPerNodeMetadata returns a ManagerOption that allows you to set metadata for each
// node individually.

[meling]

Here is some more background on the cancellation issue. If I understand correctly, gRPC creates a new stream for each unary RPC and uses HTTP/2 stream reset (RST_STREAM) to cancel a unary RPC. However, we can't do that with Gorums since we want to retain the NodeStream connection, even if a single unary RPC should be canceled. Hence, we would have to implement our own cancellation mechanism within Gorums. Right now, perhaps it is not worth it since we don't depend on cancellations from client to server.

However, passing metadata is easy and will be fixed soon.