grpc-web: Gracefully close a streaming connection

[ligustah]

Calling cancel() on the ResponseStream returned by a streaming RPC throws an exception.

version: 2.0.0+1

Repro steps

Don't have separate code for this right now, but could prepare something if required.

1. Call a server-side streaming RPC
2. Receive messages
3. Call cancel() on the ResponseStream

Expected result: The connection is closed is closed and I can handle any possible exceptions.

Actual result: The code will throw two exceptions, only one of which I can catch.

Details

I can see why adding an error to the stream when calling cancel() might be useful, but somewhere in the code it tries to add another error to one of the internal streams after that one was closed, which triggers the bad state exception seen below.

EXCEPTION: Bad state: Cannot add event after closing
STACKTRACE: 
dart:sdk_internal                                                                                addError
package:grpc/src/client/packages/grpc/src/client/call.dart 305:16                                [_onRequestError]
package:grpc/packages/grpc/src/client/transport/xhr_transport.dart 61:13                         <fn>

...

EXCEPTION: gRPC Error (1, Cancelled by client.)
STACKTRACE: 
package:grpc/src/client/packages/grpc/src/client/call.dart 324:18                                cancel
package:grpc/src/client/packages/grpc/src/client/common.dart 85:34                               cancel

Potentially related: #111

[softkot]

I am confirming this annoying bug in 2.3.0

[mraleph]

Duplicate of #67 wrong classification. This issue is about grpc-web

[aragonet]

The same is happening here on web with grpc version 2.8.0