fix: fix headers not completing when call is terminated

[c-lucera-pvotal]

This PR attempt to solve the issue with headers not being correctly completed after a call is closed as pointed out in #727

To fix the issue I have added in the _terminate() method a check to complete with errors any completer still going (I found _headers and _trailers)

[linux-foundation-easycla]

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• ✅ login: c-lucera-pvotal (5203500, ab73121, f954ba0, d47e246, 8f08c49)

[r-durao-pvotal]

@kevmoo who would be the right person to take a look at this PR? Thanks in advance!

[mraleph]

Please add a regression test and then we can land it.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
grpc	None	4.0.0	4.0.1	4.0.0	✔️

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Package	Changed Files

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File	Coverage
lib/src/client/call.dart	💚 85 %

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Package publish validation ✔️

Package	Version	Status
package:grpc	4.0.1	ready to publish

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[c-lucera-pvotal]

@mraleph
Added the requested test and fixed CI issues like Changelog notes.

However I had to change the previous logic from throwing an error

    if (!_headers.isCompleted) {
      _headers.completeError(GrpcError.unimplemented('Request terminated before headers'));
    }

To completing with an empty value

    if (!_headers.isCompleted) {
      _headers.complete({});
    }

The reason about the change was that the error throwing was really distruptive for the current implementations, instead of receving a error only on active listeners (I.e whoever was actively looking at that completer) all the cancelled calls received 3 throws, one for the cancellation, one for the headers not completed and one for the trailers not completed, this meant that all the tests in this project were failing due to the not catched exceptions, and at the same time, anyone cancelling the call had to deal with 3 different throws, I hope this change make sense to you.

[c-lucera-pvotal]

@mraleph saw some failing tests, but those seems unrelated to my changes, any hint about those?

[mosuem]

These keepalive tests can be flaky, but this doesn't look that way. Let me check.

[c-lucera-pvotal]

It looks like that the tearDown somehow is not awaiting the closing before the next setUp is running, maybe they are running in parallel?

in line 52 of keep_alive_test.dart, if you use a different port for each serve you will see the tests not failing anymore

//before tests setup
int port = 8081;

//Line 52
 await server.serve(address: 'localhost', port: port++);

Want me to add this?

[c-lucera-pvotal]

Another thing I discovered, if I run only keep alive tests it works out of the box, so probably tests are running in parallel with other tests using the port 8081, another fix is to use the port 8282 on line 52

/Line 52
 await server.serve(address: 'localhost', port: 8082);

[c-lucera-pvotal]

Basically those two tests
https://github.com/grpc/grpc-dart/blob/master/test/keepalive_test.dart
https://github.com/grpc/grpc-dart/blob/master/test/server_cancellation_test.dart

uses port 8081, this is not an issue when run one by one, but running the whole suite with flutter test causes he keep_alive failing because the server cancellation test runs first and occupies the port 8081, I can fix this in my pr by changing the port if you want

[c-lucera-pvotal]

Pushed the fix for keepalive_test.dart let me know if you prefer to revert it and solve in a different way