Add server interceptor acting as a middleware

[ThHareau]

Fix for #591

Example of usage:

class SentryInstrumenterInterceptor extends $grpc.ServerInterceptor {
  @override
  Stream<R> interceptStreaming<Q, R>(
      $grpc.ServiceCall call,
      $grpc.ServiceMethod<Q, R> method,
      Stream<Q> requests,
      $grpc.ServerStreamingInvoker<Q, R> invoker) async* {
    final name = method.name;
    final transaction = Sentry.startTransaction(method.name, 'grpc');
    print('Before $name');

    yield* super.interceptStreaming(call, method, requests, invoker);

    print('After $name');
    transaction.finish();
  }
}

---

I did not check if this PR solves #611, anyway if it does not it would probably be nice to open a second PR, this one is complex enough

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ThHareau / name: Thomas Hareau (1bacbe6, 66feb5d, e645884, 045384c, db9e02c, fde74fe)

[ThHareau]

The goal here is to be close to the Client implementation. As the handler is a Stream, I did not add unary. It is probably doable tho.

[github-actions]

PR Health

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ⚠️

File	Coverage
lib/grpc.dart	💔 Not covered
lib/src/server/handler.dart	💚 96 %
lib/src/server/interceptor.dart	💚 100 %
lib/src/server/server.dart	💚 82 % ⬆️ 0 %
lib/src/server/service.dart	💚 97 % ⬆️ 1 %

This check for test coverage is informational (issues shown here will not fail the PR).

This check can be disabled by tagging the PR with skip-coverage-check.

API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols
grpc	Any $1.Duration

This check can be disabled by tagging the PR with skip-leaking-check.

[ThHareau]

Should I do anything about the changelog health check?

[mosuem]

Should I do anything about the changelog health check?

No, the breaking change should not be regarded as breaking and is from a different PR (note to self: the check should show that the breakage was introduced elsewhere..). The version is probably fine.

I'll take a closer look at this once I get to it, quite busy ATM unfortunately. But thanks a lot already for the contribution!

[ThHareau]

Should I do anything about the changelog health check?

No, the breaking change should not be regarded as breaking and is from a different PR (note to self: the check should show that the breakage was introduced elsewhere..). The version is probably fine.

I'll take a closer look at this once I get to it, quite busy ATM unfortunately. But thanks a lot already for the contribution!

No worries, it can wait a bit 🙂 Just fyi, I'll be away from the 12th to end of march, so I won't be able to answer during this period. But I'll reply afterward.

[ThHareau]

CI should pass now :)

[ThHareau]

@mosuem Is the changelog failure something I can act on?

[mosuem]

Sorry - forgot to press the submit review button...

[mosuem]

error: pubspec version (4.0.4) and changelog (4.1.0) don't agree

[ThHareau]

It should be fixed @mosuem

[ThHareau]

@mosuem any idea on how to make the CI pass? I see that both checks failed with error Error: Process completed with exit code 143., which corresponds to a SIGTERM if I understood correctly. Could you try relaunching it?

[mosuem]

Locally, the tests hang as well, which can be helped by commenting out serverInterceptors: serverInterceptors..insert(0, serverInterceptor), - which seems to be the culprit.

[ThHareau]

My bad @mosuem , the issue was in the inlinement of invoker and delegate. invoker is apparently reassigned in the same scope as the callback execution, creating an infinite loop.

I've rollbacked my changes and added a comment explaining the reason.

Small repro of the issue if you wanna try in a dartpad

invoker() {
  print('Invoked');
}

final wrappers = <Function(Function())>[
  (cb) {
    print('before function');
    cb();
    print('after function');
  },
];

void main() {
  var flow = invoker;

  for (final interceptor in wrappers.reversed) {
    flow = () => interceptor(flow);
  }

  flow();
}