[jni] Add a call to jvm.DetachCurrentThread when the thread detaches

[HosseinYousefi]

Closes #557.

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// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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pkgs/ffi/example/main.dart
pkgs/ffigen/example/libclang-example/generated_bindings.dart
pkgs/ffigen/example/shared_bindings/generate.dart
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pkgs/ffigen/example/simple/generated_bindings.dart
pkgs/ffigen/lib/src/config_provider/config_spec.dart
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pkgs/jni/lib/src/lang/jcharacter.dart
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/jni/lib/src/third_party/global_env_extensions.dart
pkgs/jni/lib/src/third_party/jni_bindings_generated.dart
pkgs/jnigen/android_test_runner/lib/main.dart
pkgs/jnigen/example/in_app_java/lib/android_utils.dart
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Package	Version	Status
package:ffi	2.1.2	already published at pub.dev
package:ffigen	13.0.0-wip	WIP (no publish necessary)
package:jni	0.9.3-wip	WIP (no publish necessary)
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package:objective_c	1.0.1	already published at pub.dev
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[coveralls]

coverage: 90.986% (-4.0%) from 94.98%
when pulling 4f04ef0 on detach-current-thread
into 652badc on main.

[mkustermann]

With comments addressed: lgtm

[mkustermann]

sidenote: This DllMain is assuming the C code in package:jni gets compiled into it's own DLL (?). That would prevent statically linking it into a bigger library (which may have already a different DllMain). It's a little bit of a smell I think.

If we can use C++ here (if not: why?) then a global C++ object can do things in it's constructor and destructor (e.g. initializer locks and free them, create TLS keys, free them, ...) that get run on shared library load & unload times.

(Arguably this doesn't solve the issue of detaching the thread from java at exit)

[HosseinYousefi]

We could use C++. We would have to wrap the functions we want to call from Dart in extern "C" but otherwise it should not be a problem. We also generate another C file from jni.h which wraps the API and does things like conversion to global refs and exception checking.

In general, I did not really refactor the C code we had since 2 years ago. I think it's worth it to open an issue and do this in the near future. #1169