Fix concurrent modification error in GrpcWebClientChannel.terminate

[isaldana]

Fixes #331

[linux-foundation-easycla]

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• ✅ Isaac Saldana (e8b961b)

[mit-mit]

cc @mraleph

[mit-mit]

@isaldana you mentioned in #306 (comment) that this might also fix #206 ?

[isaldana]

@mit-mit I think so. The cancel() call eventually calls terminate() on the stream as show here https://github.com/grpc/grpc-dart/blob/master/lib/src/client/call.dart#L328. So this addresses the issue with terminate() on web.

[mraleph]

Thanks for the contribution and sorry for the delay with reviews.

[mraleph]

I would suggest:

while (_requests.isNotEmpty) {
  final requests = List.of(_requests);
  _requests.clear();

    
  await Future.wait(requests.map((rq) => rq.terminate()));
}

this would terminate() all requests in parallel rather then sequentially.

[isaldana]

@mraleph AFAIK, there is no need to wait for the requests to terminate. It was just the nature of the loop since I didn't want to copy the requests list. I made the change to copy the list of requests and call terminate on them.

[isaldana]

@mraleph Please hold on before merging this. I've been running https://github.com/isaldana/grpc-dart for a while and I just realized that I forgot to include isaldana@0d23114 in this PR.

[mraleph]

@isaldana is this in a mergable? Could you rebase and add a test. We now have a gRPC-web testing harness you could use.

[mraleph]

I have added test and going to merge.