Detach from any threads we have attached to.

Fixes: swiftlang#172

Sources/JavaKit/JavaKitVM/JavaVirtualMachine.swift

@@ -24,6 +24,11 @@ public final class JavaVirtualMachine: @unchecked Sendable {
   /// The JNI version that we depend on.
   static let jniVersion = JNI_VERSION_1_6
 
+  /// Thread-local storage to detach from thread on exit
+  private static let destroyTLS = ThreadLocalStorage { _ in
+    try? JavaVirtualMachine.shared().detachCurrentThread()
+  }
+
   /// The Java virtual machine instance.
   private let jvm: JavaVMPointer
 
@@ -146,12 +151,14 @@ extension JavaVirtualMachine {
       throw attachError
     }
 
+    JavaVirtualMachine.destroyTLS.set(environment!)
+
     return environment!.assumingMemoryBound(to: JNIEnv?.self)
   }
 
   /// Detach the current thread from the Java Virtual Machine. All Java
   /// threads waiting for this thread to die are notified.
-  public func detachCurrentThread() throws {
+  func detachCurrentThread() throws {
     if let resultError = VMError(fromJNIError: jvm.pointee!.pointee.DetachCurrentThread(jvm)) {
       throw resultError
     }

Sources/JavaKit/JavaKitVM/ThreadLocalStorage.swift

@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Bionic)
+import Bionic
+#elseif canImport(Glibc)
+import Glibc
+#elseif canImport(Musl)
+import Musl
+#elseif canImport(WinSDK)
+import WinSDK
+#endif
+
+#if !(canImport(Darwin) || canImport(Bionic) || canImport(Glibc) || canImport(Musl) || canImport(WinSDK))
+private var _globalTlsValue: UnsafeMutableRawPointer?
+#endif
+
+package struct ThreadLocalStorage: ~Copyable {
+#if canImport(Darwin) || canImport(Bionic) || canImport(Glibc) || canImport(Musl)
+  private typealias PlatformKey = pthread_key_t
+#elseif canImport(WinSDK)
+  private typealias PlatformKey = DWORD
+#else
+  private typealias PlatformKey = Void
+#endif
+
+#if canImport(Darwin)
+  package typealias Value = UnsafeMutableRawPointer
+#else
+  package typealias Value = UnsafeMutableRawPointer?
+#endif
+
+  package typealias OnThreadExit = @convention(c) (_: Value) -> ()
+
+#if canImport(Darwin) || canImport(Bionic) || canImport(Glibc) || canImport(Musl)
+  private var _key: PlatformKey
+#elseif canImport(WinSDK)
+  private let _key: PlatformKey
+#endif
+
+  package init(onThreadExit: OnThreadExit) {
+#if canImport(Darwin) || canImport(Bionic) || canImport(Glibc) || canImport(Musl)
+    _key = 0
+    pthread_key_create(&_key, onThreadExit)
+#elseif canImport(WinSDK)
+    key = FlsAlloc(onThreadExit)
+#endif
+  }
+
+  package func get() -> UnsafeMutableRawPointer? {
+#if canImport(Darwin) || canImport(Bionic) || canImport(Glibc) || canImport(Musl)
+    pthread_getspecific(_key)
+#elseif canImport(WinSDK)
+    FlsGetValue(_key)
+#else
+    _globalTlsValue
+#endif
+  }
+
+  package func set(_ value: Value) {
+#if canImport(Darwin) || canImport(Bionic) || canImport(Glibc) || canImport(Musl)
+    pthread_setspecific(_key, value)
+#elseif canImport(WinSDK)
+    FlsSetValue(_key, value)
+#else
+    _globalTlsValue = value
+#endif
+  }
+
+  deinit {
+#if canImport(Darwin) || canImport(Bionic) || canImport(Glibc) || canImport(Musl)
+    pthread_key_delete(_key)
+#elseif canImport(WinSDK)
+    FlsFree(_key)
+#endif
+  }
+}