JobSystemThreadPool: work around deadlocks on Windows

src/main/java/com/github/stephengold/joltjni/JobSystemThreadPool.java

@@ -60,8 +60,26 @@ public JobSystemThreadPool(int maxJobs, int maxBarriers, int numThreads) {
         setVirtualAddress(systemVa, true);
     }
     // *************************************************************************
+    // JobSystem methods
+
+    /**
+     * Unassign the assigned native object, assuming there is one. Free the
+     * native object if the current system owns it.
+     */
+    @Override
+    public void close() {
+        if (ownsNativeObject()) {
+            long systemVa = va();
+            free(systemVa);
+        }
+
+        unassignNativeObject();
+    }
+    // *************************************************************************
     // native private methods
 
     native private static long createJobSystem(
             int maxJobs, int maxBarriers, int numThreads);
+
+    native private static void free(long systemVa);
 }

src/main/native/glue/com_github_stephengold_joltjni_JobSystemThreadPool.cpp

@@ -40,3 +40,17 @@ JNIEXPORT jlong JNICALL Java_com_github_stephengold_joltjni_JobSystemThreadPool_
             = new JobSystemThreadPool(maxJobs, maxBarriers, numThreads);
     return reinterpret_cast<jlong> (pPool);
 }
+
+/*
+ * Class:     com_github_stephengold_joltjni_JobSystemThreadPool
+ * Method:    free
+ * Signature: (J)V
+ */
+JNIEXPORT void JNICALL Java_com_github_stephengold_joltjni_JobSystemThreadPool_free
+  (JNIEnv *, jclass, jlong systemVa) {
+#ifndef WIN32
+    // Attempting to delete a JobSystemThreadPool on Windows causes deadlock!
+    const JobSystem * const pSystem = reinterpret_cast<JobSystem *> (systemVa);
+    delete pSystem;
+#endif
+}