memory: Try to work around a rare crash on Windows

I've been seeing occasional crashes in selftest on Windows related to
Futexes on shared memory.

My understanding is that (at least on Windows) the memory needs to be
pinned/activated first, so let's fill it with zero bytes.

Add extra checks to Futex32, just to be sure.

Author: kohlschuetter

junixsocket-memory/src/main/java/org/newsclub/net/unix/memory/Futex32.java

@@ -22,6 +22,7 @@
 import java.lang.foreign.ValueLayout;
 import java.lang.foreign.ValueLayout.OfInt;
 import java.lang.invoke.VarHandle;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.newsclub.net.unix.MemoryImplUtilInternal;
 
@@ -34,50 +35,48 @@ final class Futex32 implements Futex {
 
   private final MemorySegment ms;
   private final boolean zeroOnClose;
-  private final long address;
-  private volatile boolean closed = false;
+  private final AtomicBoolean closed = new AtomicBoolean(false);
 
-  Futex32(MemorySegment addr, boolean zeroOnClose) throws IOException {
-    this.address = addr.address();
-    if ((address & 3) != 0) {
+  Futex32(MemorySegment ms, boolean zeroOnClose) throws IOException {
+    if (!ms.isAccessibleBy(Thread.currentThread())) {
+      throw new IllegalStateException("Cannot access this MemorySegment from the current thread");
+    }
+    if ((ms.address() & 3) != 0) {
       throw new IOException("Not aligned");
     }
-    if (addr.byteSize() != SharedMemory.FUTEX32_SEGMENT_SIZE) {
+    if (ms.byteSize() != SharedMemory.FUTEX32_SEGMENT_SIZE) {
       throw new IOException("MemorySegment must be exactly 4 bytes long");
     }
-    this.ms = addr;
+    this.ms = ms;
     this.zeroOnClose = zeroOnClose;
 
     // Make sure the 32-bit value is accessible (page-in memory)
-    SharedMemory.UTIL.madvise(address, SharedMemory.FUTEX32_SEGMENT_SIZE,
+    SharedMemory.UTIL.madvise(ms.address(), SharedMemory.FUTEX32_SEGMENT_SIZE,
         MemoryImplUtilInternal.MADV_WILLNEED, true);
   }
 
   @Override
   public void close() {
-    if (zeroOnClose && ms != null) {
-      ms.set(OfInt.JAVA_INT, 0, 0);
+    if (!closed.getAndSet(true)) {
+      if (zeroOnClose && ms != null) {
+        ms.set(OfInt.JAVA_INT, 0, 0);
+      }
     }
-    this.closed = true;
   }
 
   @Override
   public boolean tryWait(int ifValue, int timeoutMillis) throws IOException {
-    if (closed) {
-      return false;
-    }
-
-    return SharedMemory.UTIL.futexWait(address, ifValue, timeoutMillis);
+    return SharedMemory.UTIL.futexWait(ms.address(), ifValue, timeoutMillis);
   }
 
   @Override
   public boolean tryWake(boolean wakeAll) throws IOException {
-    return SharedMemory.UTIL.futexWake(address, wakeAll);
+    return SharedMemory.UTIL.futexWake(ms.address(), wakeAll);
   }
 
   @Override
   public boolean isClosed() {
-    return closed;
+    return closed.get();
   }
 
   MemorySegment getMemorySegment() {

junixsocket-memory/src/test/java/org/newsclub/net/unix/memory/SharedMemoryTest.java

@@ -551,7 +551,7 @@ public void testFutexCloseSharedMemoryFail() throws Exception {
   @Test
   public void testFutexAlignmentAndLength() throws Exception {
     try (SharedMemory mem = SharedMemory.createAnonymous(16)) {
-      MemorySegment ms = mem.asMappedMemorySegment(MapMode.READ_WRITE);
+      MemorySegment ms = mem.asMappedMemorySegment(MapMode.READ_WRITE).fill((byte) 0);
 
       mem.futex(ms.asSlice(0, SharedMemory.FUTEX32_SEGMENT_SIZE)); // OK
       assertThrows(IOException.class, () -> mem.futex(ms.asSlice(1,
@@ -573,7 +573,7 @@ public void testFutexAlignmentAndLength() throws Exception {
   @Test
   public void testFutexIs32Bit() throws Exception {
     try (SharedMemory mem = SharedMemory.createAnonymous(16)) {
-      MemorySegment ms = mem.asMappedMemorySegment(MapMode.READ_WRITE);
+      MemorySegment ms = mem.asMappedMemorySegment(MapMode.READ_WRITE).fill((byte) 0);
 
       ms.setAtIndex(OfInt.JAVA_INT, 1, 0xCAFEBABE);
       assertNotEquals(0, ms.get(OfLong.JAVA_LONG, 0));
@@ -606,18 +606,25 @@ public void testFutexReadOnlySegment() throws Exception {
 
   @Test
   @ForkedVMRequirement(forkSupported = true)
+  @SuppressWarnings("PMD.ExceptionAsFlowControl")
   public void testFutexSeparateVM() throws Exception {
     try (SharedMemory mem = SharedMemory.createAnonymous(FORKEDVM_FUTEX_SIZE)) {
       ForkedVM vm = new ForkedVM(FutexTestApp.class);
 
-      CompletableFuture<@Nullable IOException> cf = CompletableFuture.supplyAsync(() -> {
+      CompletableFuture<@Nullable Exception> cf = CompletableFuture.supplyAsync(() -> {
         try {
           MemorySegment ms = mem.asMappedMemorySegment(MapMode.READ_WRITE);
 
           int waitRemaining = 5000;
           long now = System.currentTimeMillis();
 
           try (Futex futex = mem.futex(ms.asSlice(0, 4))) {
+            if (!futex.isInterProcess()) {
+              throw new TestAbortedWithImportantMessageException(
+                  MessageType.TEST_ABORTED_SHORT_INFORMATIONAL,
+                  "On this system, SharedMemory Futexes cannot be shared between processes");
+            }
+
             do {
               if (!futex.tryWait(0, waitRemaining)) {
                 waitRemaining -= (int) (System.currentTimeMillis() - now);
@@ -629,7 +636,7 @@ public void testFutexSeparateVM() throws Exception {
               }
             } while (true); // NOPMD
           }
-        } catch (IOException e) {
+        } catch (Exception e) {
           return e;
         }
       });
@@ -641,7 +648,7 @@ public void testFutexSeparateVM() throws Exception {
 
       Process p = vm.fork();
 
-      IOException exc;
+      Exception exc;
       try {
         exc = cf.get(5, TimeUnit.SECONDS);
       } catch (ExecutionException e) {
@@ -664,7 +671,7 @@ private void testFutex(boolean closeMem, boolean wakeUp) throws Exception {
     try (SharedMemory mem = SharedMemory.createAnonymous(16)) {
       MemorySegment ms = mem.asMappedMemorySegment(MapMode.READ_WRITE);
 
-      try (Futex futex = mem.futex(ms.asSlice(4, 4), wakeUp)) {
+      try (Futex futex = mem.futex(ms.asSlice(4, 4).fill((byte) 0), wakeUp)) {
         assertTrue(futex.tryWait(0xDEADBEEF, 0)); // immediately returns; value doesn't match
 
         long time = System.currentTimeMillis();