test: cover NativeObject lifecycle via a test subclass (#31)

NativeObject is abstract, so a minimal concrete TestObject exercises every
branch: ptr() while open, close() runs tryClose exactly once with the owned
pointer, repeated close is idempotent, ptr() after close throws, a throwing
tryClose is swallowed (destructors must not throw) yet still marks the object
closed, constructing with NULL never calls tryClose, and 32 concurrent close()
calls release exactly once.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

zstd/src/test/java/io/github/dfa1/zstd/NativeObjectTest.java

@@ -0,0 +1,171 @@
+package io.github.dfa1.zstd;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.lang.foreign.MemorySegment;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.assertj.core.api.ThrowableAssert.ThrowingCallable;
+import org.junit.jupiter.api.Test;
+
+class NativeObjectTest {
+
+    // A non-NULL stand-in pointer; never dereferenced, only compared by identity.
+    private static final MemorySegment POINTER = MemorySegment.ofAddress(0x1234);
+
+    @Test
+    void ptrReturnsTheOwnedPointerWhileOpen() {
+        // Given an open native object
+        TestObject sut = new TestObject(POINTER);
+
+        // When its pointer is read
+        MemorySegment p = sut.pointer();
+
+        // Then it is the pointer it was constructed with
+        assertThat(p).isEqualTo(POINTER);
+    }
+
+    @Test
+    void closeCallsTryCloseExactlyOnceWithThePointer() {
+        // Given an open native object
+        TestObject sut = new TestObject(POINTER);
+
+        // When closed
+        sut.close();
+
+        // Then tryClose ran once, with the owned pointer
+        assertThat(sut.closeCount.get()).isEqualTo(1);
+        assertThat(sut.closedWith).isEqualTo(POINTER);
+    }
+
+    @Test
+    void closeIsIdempotent() {
+        // Given an open native object
+        TestObject sut = new TestObject(POINTER);
+
+        // When closed repeatedly
+        sut.close();
+        sut.close();
+        sut.close();
+
+        // Then the resource is released only once
+        assertThat(sut.closeCount.get()).isEqualTo(1);
+    }
+
+    @Test
+    void ptrFailsAfterClose() {
+        // Given a closed native object
+        TestObject sut = new TestObject(POINTER);
+        sut.close();
+
+        // When its pointer is read
+        ThrowingCallable result = sut::pointer;
+
+        // Then it fails fast
+        assertThatThrownBy(result)
+                .isInstanceOf(IllegalStateException.class)
+                .hasMessageContaining("closed");
+    }
+
+    @Test
+    void closeSwallowsTryCloseFailures() {
+        // Given an object whose native free throws
+        TestObject sut = new TestObject(POINTER);
+        sut.failOnClose = true;
+
+        // When closed
+        ThrowingCallable result = sut::close;
+
+        // Then the exception does not escape (destructors must not throw)
+        assertThatCode(result).doesNotThrowAnyException();
+        // And it is still considered closed
+        assertThat(sut.closeCount.get()).isEqualTo(1);
+        assertThatThrownBy(sut::pointer).isInstanceOf(IllegalStateException.class);
+    }
+
+    @Test
+    void constructingWithNullNeverCallsTryClose() {
+        // Given an object that owns no pointer
+        TestObject sut = new TestObject(MemorySegment.NULL);
+
+        // When closed
+        sut.close();
+
+        // Then there is nothing to release, and the pointer is unavailable
+        assertThat(sut.closeCount.get()).isZero();
+        assertThatThrownBy(sut::pointer).isInstanceOf(IllegalStateException.class);
+    }
+
+    @Test
+    void concurrentCloseReleasesExactlyOnce() throws Exception {
+        // Given an open object and many threads poised to close it at once
+        TestObject sut = new TestObject(POINTER);
+        int threads = 32;
+        ExecutorService pool = Executors.newFixedThreadPool(threads);
+        CountDownLatch ready = new CountDownLatch(threads);
+        CountDownLatch go = new CountDownLatch(1);
+        try {
+            List<Future<?>> futures = new ArrayList<>();
+            for (int i = 0; i < threads; i++) {
+                futures.add(pool.submit(() -> {
+                    ready.countDown();
+                    awaitUninterruptibly(go);
+                    sut.close();
+                }));
+            }
+            ready.await();
+
+            // When they all fire
+            go.countDown();
+            for (Future<?> f : futures) {
+                f.get();
+            }
+
+            // Then the resource is released exactly once
+            assertThat(sut.closeCount.get()).isEqualTo(1);
+        } finally {
+            pool.shutdownNow();
+        }
+    }
+
+    private static void awaitUninterruptibly(CountDownLatch latch) {
+        try {
+            latch.await();
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new IllegalStateException(e);
+        }
+    }
+
+    /// Minimal concrete [NativeObject] that records how its native resource was released.
+    private static final class TestObject extends NativeObject {
+
+        private final AtomicInteger closeCount = new AtomicInteger();
+        private volatile MemorySegment closedWith;
+        private volatile boolean failOnClose;
+
+        private TestObject(MemorySegment owningPointer) {
+            super(owningPointer);
+        }
+
+        private MemorySegment pointer() {
+            return ptr();
+        }
+
+        @Override
+        protected void tryClose(MemorySegment ptr) {
+            closeCount.incrementAndGet();
+            closedWith = ptr;
+            if (failOnClose) {
+                throw new RuntimeException("native free failed");
+            }
+        }
+    }
+}