[ffigen] Allow blocks and protocols to keep their isolates alive

[liamappelbe]

Add a keepIsolateAlive parameter to the block constructors. When true, this creates a RawReceivePort and stores it adjacent to the closure in the block closure registry. When the block is GC'd, the port is closed.

Similarly, add a keepIsolateAlive parameter to the protocol constructor. This is also implemented using a RawReceivePort, but in this case the port holds a reference to itself, and closes itself when it receives any message. That message is sent by DOBJCDartProxy's dispose method.

Fixes #447

[github-actions]

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	6.0.0	7.0.0-wip	7.0.0	✔️

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License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 87.804% (+0.2%) from 87.64%
when pulling 54364d0 on blockalive
into 63b2a88 on main.

[HosseinYousefi]

To keep the APIs similar:

In JNIgen I use one port per implemented interface, I don't store the port anywhere (should I?) but it does reference itself and when it receives a message from Java it closes itself:

(_$impls is a map from native ports to the implementation object that contains the Dart functions)

    late final jni$_.RawReceivePort $p;
    $p = jni$_.RawReceivePort(($m) {
      if ($m == null) {
        _$impls.remove($p.sendPort.nativePort);
        $p.close();
        return;
      }
      final $i = jni$_.MethodInvocation.fromMessage($m);
      final $r = _$invokeMethod($p.sendPort.nativePort, $i);
      jni$_.ProtectedJniExtensions.returnResult($i.result, $r);
    });

If I want to add keepIsolateAlive, do I have to simply set it for $p like $p.keepIsolateAlive = keepIsolateAlive?

We also need to add the mechanism we talked about with the finalizers so the objc/java side knows that the callback is no longer valid.

[HosseinYousefi]

Isn't it safer to have this default to true?

[liamappelbe]

That would be a breaking change, so I was avoiding it. But thinking about it again, you're right, it should default to true.

[liamappelbe]

In JNIgen I use one port per implemented interface, I don't store the port anywhere (should I?) but it does reference itself and when it receives a message from Java it closes itself:

Yeah, that's how I implemented it for protocols too.

If I want to add keepIsolateAlive, do I have to simply set it for $p like $p.keepIsolateAlive = keepIsolateAlive?

Yep, I think that's probably all you need.

We also need to add the mechanism we talked about with the finalizers so the objc/java side knows that the callback is no longer valid.

Yeah, it's on my list.