Index: oak-segment/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/FileAccess.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-segment/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/FileAccess.java	(revision aef68138a849ca472dc8d9e979da3e4de0c43c92)
+++ oak-segment/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/FileAccess.java	(revision cf75cae5bc1036490541c7efa084af42f5b0c5b9)
@@ -97,7 +97,7 @@
         }
 
         @Override
-        public int length() throws IOException {
+        public synchronized int length() throws IOException {
             long length = file.length();
             checkState(length < Integer.MAX_VALUE);
             return (int) length;
\ No newline at end of file
Index: oak-segment/src/test/java/org/apache/jackrabbit/oak/plugins/segment/CompactionAndCleanupIT.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-segment/src/test/java/org/apache/jackrabbit/oak/plugins/segment/CompactionAndCleanupIT.java	(revision aef68138a849ca472dc8d9e979da3e4de0c43c92)
+++ oak-segment/src/test/java/org/apache/jackrabbit/oak/plugins/segment/CompactionAndCleanupIT.java	(revision cf75cae5bc1036490541c7efa084af42f5b0c5b9)
@@ -21,6 +21,7 @@
 
 import static com.google.common.collect.Lists.newArrayList;
 import static java.lang.Integer.getInteger;
+import static java.util.concurrent.Executors.newFixedThreadPool;
 import static java.util.concurrent.TimeUnit.MINUTES;
 import static org.apache.commons.io.FileUtils.byteCountToDisplaySize;
 import static org.apache.jackrabbit.oak.api.Type.STRING;
@@ -33,6 +34,7 @@
 import static org.apache.jackrabbit.oak.plugins.segment.compaction.CompactionStrategy.CleanupType.CLEAN_OLD;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 import static org.junit.Assume.assumeTrue;
@@ -47,9 +49,12 @@
 import java.util.UUID;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Future;
 import java.util.concurrent.FutureTask;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
 
 import javax.annotation.Nonnull;
@@ -59,6 +64,8 @@
 import org.apache.jackrabbit.oak.api.CommitFailedException;
 import org.apache.jackrabbit.oak.api.PropertyState;
 import org.apache.jackrabbit.oak.api.Type;
+import org.apache.jackrabbit.oak.commons.concurrent.ExecutorCloser;
+import org.apache.jackrabbit.oak.plugins.blob.ReferenceCollector;
 import org.apache.jackrabbit.oak.plugins.segment.compaction.CompactionStrategy;
 import org.apache.jackrabbit.oak.plugins.segment.file.FileStore;
 import org.apache.jackrabbit.oak.spi.commit.CommitInfo;
@@ -635,6 +642,77 @@
                 fail("Segment " + id + "should be gc'ed");
             } catch (SegmentNotFoundException ignore) {}
         } finally {
+            fileStore.close();
+        }
+    }
+
+    @Test
+    public void randomAccessFileConcurrentReadAndLength() throws Exception {
+        final FileStore fileStore = FileStore.builder(getFileStoreFolder())
+                .withMaxFileSize(1)
+                .withMemoryMapping(false)
+                .build();
+
+        final SegmentNodeStore nodeStore = SegmentNodeStore.builder(fileStore).build();
+        CompactionStrategy strategy = new CompactionStrategy(false, false, CLEAN_NONE, 0, (byte) 0) {
+            @Override
+            public boolean compacted(@Nonnull Callable<Boolean> setHead)
+            throws Exception {
+                return nodeStore.locked(setHead);
+            }
+        };
+
+        ExecutorService executorService = newFixedThreadPool(300);
+        final AtomicInteger counter = new AtomicInteger();
+        final ReferenceCollector dummyCollector = new ReferenceCollector() {
+
+            @Override
+            public void addReference(String reference, String nodeId) {
+                // do nothing
+            }
+        };
+
+        try {
+            Callable<Void> concurrentWriteTask = new Callable<Void>() {
+                @Override
+                public Void call() throws Exception {
+                    NodeBuilder builder = nodeStore.getRoot().builder();
+                    builder.setProperty("blob-" + counter.getAndIncrement(), createBlob(nodeStore, 25 * 25));
+                    nodeStore.merge(builder, EmptyHook.INSTANCE, CommitInfo.EMPTY);
+                    fileStore.flush();
+                    return null;
+                }
+            };
+
+            Callable<Void> concurrentCleanupTask = new Callable<Void>() {
+                @Override
+                public Void call() throws Exception {
+                    fileStore.cleanup();
+                    return null;
+                }
+            };
+
+            Callable<Void> concurrentReferenceCollector = new Callable<Void>() {
+                @Override
+                public Void call() throws Exception {
+                    fileStore.getTracker().collectBlobReferences(dummyCollector);
+                    return null;
+                }
+            };
+
+            List<Future<?>> results = newArrayList();
+            for (int i = 0; i < 100; i++) {
+                results.add(executorService.submit(concurrentWriteTask));
+                results.add(executorService.submit(concurrentCleanupTask));
+                results.add(executorService.submit(concurrentReferenceCollector));
+            }
+
+            for (Future<?> result : results) {
+                assertNull(result.get());
+            }
+
+        } finally {
+            new ExecutorCloser(executorService).close();
             fileStore.close();
         }
     }