Handle race condition between File.length() and FileChannel.size()

agrona/src/main/java/org/agrona/MarkFile.java

@@ -19,8 +19,10 @@
 import org.agrona.concurrent.UnsafeBuffer;
 
 import java.io.File;
+import java.io.IOException;
 import java.nio.MappedByteBuffer;
 import java.nio.channels.FileChannel;
+import java.nio.file.StandardOpenOption;
 import java.util.function.Consumer;
 import java.util.function.IntConsumer;
 
@@ -375,6 +377,8 @@ public static MappedByteBuffer mapExistingMarkFile(
             sleep(16);
         }
 
+        waitForFileChannelContents(markFile, timeoutMs, epochClock, startTimeMs);
+
         final MappedByteBuffer byteBuffer = mapExistingFile(markFile, logger);
         final UnsafeBuffer buffer = new UnsafeBuffer(byteBuffer);
 
@@ -547,6 +551,30 @@ private static void validateOffsets(final int versionFieldOffset, final int time
         }
     }
 
+    private static void waitForFileChannelContents(
+        final File markFile,
+        final long timeoutMs,
+        final EpochClock epochClock,
+        final long startTimeMs)
+    {
+        try (FileChannel markFileChannel = FileChannel.open(markFile.toPath(), StandardOpenOption.READ))
+        {
+            while (markFileChannel.size() < 4)
+            {
+                if (epochClock.time() > (startTimeMs + timeoutMs))
+                {
+                    throw new IllegalStateException("Mark file is created but not populated.");
+                }
+
+                sleep(16);
+            }
+        }
+        catch (final IOException e)
+        {
+            throw new IllegalStateException("Cannot open mark file for reading.");
+        }
+    }
+
     static void sleep(final long durationMs)
     {
         try

agrona/src/test/java/org/agrona/MarkFileTest.java

@@ -0,0 +1,36 @@
+package org.agrona;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardOpenOption;
+
+import org.agrona.concurrent.SystemEpochClock;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+public class MarkFileTest
+{
+    @Rule
+    public TemporaryFolder tmpDir = new TemporaryFolder();
+
+    @Test(expected = IllegalStateException.class)
+    public void shouldWaitForMarkFileToContainEnoughDataForVersionCheck() throws IOException
+    {
+        final File directory = tmpDir.newFolder();
+        final String filename = "markfile.dat";
+        final Path markFilePath = directory.toPath().resolve(filename);
+        Files.createFile(markFilePath);
+
+        try (FileChannel channel = FileChannel.open(markFilePath, StandardOpenOption.WRITE))
+        {
+            channel.write(ByteBuffer.allocate(1));
+        }
+        new MarkFile(directory, filename, 0,
+          16, 10, new SystemEpochClock(), v -> {}, msg -> {});
+    }
+}