8259027: NullPointerException in makeMappedSegment due to NULL Unmapp…

…er when length of segment is 0

Reviewed-by: chegar, uschindler

src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/MappedMemorySegmentImpl.java

@@ -116,13 +116,17 @@ public static MemorySegment makeMappedSegment(Path path, long bytesOffset, long
         if (bytesOffset < 0) throw new IllegalArgumentException("Requested bytes offset must be >= 0.");
         try (FileChannelImpl channelImpl = (FileChannelImpl)FileChannel.open(path, openOptions(mapMode))) {
             UnmapperProxy unmapperProxy = channelImpl.mapInternal(mapMode, bytesOffset, bytesSize);
-            MemoryScope scope = MemoryScope.createConfined(null, unmapperProxy::unmap, null);
             int modes = defaultAccessModes(bytesSize);
             if (mapMode == FileChannel.MapMode.READ_ONLY) {
                 modes &= ~WRITE;
             }
-            return new MappedMemorySegmentImpl(unmapperProxy.address(), unmapperProxy, bytesSize,
-                    modes, scope);
+            if (unmapperProxy != null) {
+                MemoryScope scope = MemoryScope.createConfined(null, unmapperProxy::unmap, null);
+                return new MappedMemorySegmentImpl(unmapperProxy.address(), unmapperProxy, bytesSize,
+                        modes, scope);
+            } else {
+                return new EmptyMappedMemorySegmentImpl(modes);
+            }
         }
     }
 
@@ -135,4 +139,32 @@ private static OpenOption[] openOptions(FileChannel.MapMode mapMode) {
             throw new UnsupportedOperationException("Unsupported map mode: " + mapMode);
         }
     }
+
+    static class EmptyMappedMemorySegmentImpl extends MappedMemorySegmentImpl {
+
+        public EmptyMappedMemorySegmentImpl(int modes) {
+            super(0, null, 0, modes,
+                    MemoryScope.createConfined(null, MemoryScope.DUMMY_CLEANUP_ACTION, null));
+        }
+
+        @Override
+        public void load() {
+            // do nothing
+        }
+
+        @Override
+        public void unload() {
+            // do nothing
+        }
+
+        @Override
+        public boolean isLoaded() {
+            return true;
+        }
+
+        @Override
+        public void force() {
+            // do nothing
+        }
+    };
 }

test/jdk/java/foreign/TestByteBuffer.java

@@ -464,12 +464,30 @@ public void testBadMapNegativeOffset() throws IOException {
         MemorySegment.mapFile(f.toPath(), -1, 1, FileChannel.MapMode.READ_WRITE);
     }
 
+    @Test
     public void testMapZeroSize() throws IOException {
         File f = new File("testPos1.out");
         f.createNewFile();
         f.deleteOnExit();
+        //RW
         try (MemorySegment segment = MemorySegment.mapFile(f.toPath(), 0L, 0L, FileChannel.MapMode.READ_WRITE)) {
             assertEquals(segment.byteSize(), 0);
+            assertEquals(segment.isMapped(), true);
+            assertTrue((segment.accessModes() & (READ | WRITE)) == (READ | WRITE));
+            MappedMemorySegments.force(segment);
+            MappedMemorySegments.load(segment);
+            MappedMemorySegments.isLoaded(segment);
+            MappedMemorySegments.unload(segment);
+        }
+        //RO
+        try (MemorySegment segment = MemorySegment.mapFile(f.toPath(), 0L, 0L, FileChannel.MapMode.READ_ONLY)) {
+            assertEquals(segment.byteSize(), 0);
+            assertEquals(segment.isMapped(), true);
+            assertTrue((segment.accessModes() & (READ | WRITE)) == READ);
+            MappedMemorySegments.force(segment);
+            MappedMemorySegments.load(segment);
+            MappedMemorySegments.isLoaded(segment);
+            MappedMemorySegments.unload(segment);
         }
     }