Take a hash of the previous state's directory structure and file contents, and then include that hash in the patch metadata (when creating a new patch). This allows the integrity of the layers to be validated as each one is applied.

src/test/java/org/qortal/test/arbitrary/ArbitraryDataDigestTests.java

@@ -0,0 +1,58 @@
+package org.qortal.test.arbitrary;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.qortal.arbitrary.ArbitraryDataDigest;
+import org.qortal.repository.DataException;
+import org.qortal.test.common.Common;
+
+import java.io.FileWriter;
+import java.io.IOException;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.UUID;
+
+import static org.junit.Assert.*;
+
+public class ArbitraryDataDigestTests extends Common {
+
+    @Before
+    public void beforeTest() throws DataException {
+        Common.useDefaultSettings();
+    }
+
+    @Test
+    public void testDirectoryDigest() throws IOException {
+        Path dataPath = Paths.get("src/test/resources/arbitrary/demo1");
+        String expectedHash58 = "59dw8CgVybcHAUL5GYgYUUfFffVVhiMKZLCnULPKT6oC";
+
+        // Ensure directory exists
+        assertTrue(dataPath.toFile().exists());
+        assertTrue(dataPath.toFile().isDirectory());
+
+        // Compute a hash
+        ArbitraryDataDigest digest = new ArbitraryDataDigest(dataPath);
+        digest.compute();
+        assertEquals(expectedHash58, digest.getHash58());
+
+        // Write a random file to .qortal/cache to ensure it isn't being included in the digest function
+        // We exclude all .qortal files from the digest since they can be different with each build, and
+        // we only care about the actual user files
+        FileWriter fileWriter = new FileWriter(Paths.get(dataPath.toString(), ".qortal", "cache").toString());
+        fileWriter.append(UUID.randomUUID().toString());
+        fileWriter.close();
+
+        // Recompute the hash
+        digest = new ArbitraryDataDigest(dataPath);
+        digest.compute();
+        assertEquals(expectedHash58, digest.getHash58());
+
+        // Now compute the hash 100 more times to ensure it's always the same
+        for (int i=0; i<100; i++) {
+            digest = new ArbitraryDataDigest(dataPath);
+            digest.compute();
+            assertEquals(expectedHash58, digest.getHash58());
+        }
+    }
+
+}

src/test/java/org/qortal/test/arbitrary/ArbitraryDataFileTests.java

@@ -1,4 +1,4 @@
-package org.qortal.test;
+package org.qortal.test.arbitrary;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -10,7 +10,7 @@ import java.util.Random;
 
 import static org.junit.Assert.*;
 
-public class DataTests extends Common {
+public class ArbitraryDataFileTests extends Common {
 
 	@Before
 	public void beforeTest() throws DataException {

src/test/resources/arbitrary/demo1/.qortal/cache

@@ -0,0 +1 @@
+2ea5a99a-3b85-4f1f-a259-436787f90bd1

src/test/resources/arbitrary/demo1/dir1/dir2/lorem5.txt

@@ -0,0 +1 @@
+Pellentesque laoreet laoreet dui ut volutpat.

src/test/resources/arbitrary/demo1/dir1/lorem4.txt

@@ -0,0 +1 @@
+Sed non lacus ante.

src/test/resources/arbitrary/demo1/lorem1.txt

@@ -0,0 +1 @@
+Lorem ipsum dolor sit amet, consectetur adipiscing elit.

src/test/resources/arbitrary/demo1/lorem2.txt

@@ -0,0 +1 @@
+Quisque viverra neque quis eros dapibus

src/test/resources/arbitrary/demo1/lorem3.txt

@@ -0,0 +1 @@
+Sed ac magna pretium, suscipit mauris sed, ultrices nunc.

src/test/resources/arbitrary/demo2/dir1/dir2/lorem5.txt

@@ -0,0 +1 @@
+Pellentesque laoreet laoreet dui ut volutpat.

src/test/resources/arbitrary/demo2/dir1/lorem4.txt

@@ -0,0 +1 @@
+Sed non lacus ante.

src/test/resources/arbitrary/demo2/lorem1.txt

@@ -0,0 +1 @@
+Lorem ipsum dolor sit amet, consectetur adipiscing elit.

src/test/resources/arbitrary/demo2/lorem2.txt

@@ -0,0 +1 @@
+Quisque viverra neque quis eros dapibus

src/test/resources/arbitrary/demo2/lorem3.txt

@@ -0,0 +1 @@
+Sed ac magna pretium, suscipit mauris sed, ultrices nunc.