Dump more details when two objects at the same locations are read

lib/java/persistance/src/main/java/org/enso/persist/PerGenerator.java

@@ -13,8 +13,6 @@
 import java.util.logging.Logger;
 
 final class PerGenerator {
-  private static final Logger LOG = Logger.getLogger(Persistance.class.getPackageName());
-
   static final byte[] HEADER = new byte[] {0x0a, 0x0d, 0x03, 0x0f};
   private final OutputStream main;
   private final Map<Object, Integer> knownObjects = new IdentityHashMap<>();
@@ -33,7 +31,7 @@ private PerGenerator(
   }
 
   static byte[] writeObject(Object obj, Function<Object, Object> writeReplace) throws IOException {
-    var histogram = LOG.isLoggable(Level.FINE) ? new Histogram() : null;
+    var histogram = PerUtils.LOG.isLoggable(Level.FINE) ? new Histogram() : null;
 
     var out = new ByteArrayOutputStream();
     var data = new DataOutputStream(out);
@@ -50,7 +48,7 @@ static byte[] writeObject(Object obj, Function<Object, Object> writeReplace) thr
     arr[11] = (byte) (at & 0xff);
 
     if (histogram != null) {
-      histogram.dump(LOG, arr.length);
+      histogram.dump(PerUtils.LOG, arr.length);
     }
     return arr;
   }

lib/java/persistance/src/main/java/org/enso/persist/PerInputImpl.java

@@ -7,7 +7,9 @@
 import java.nio.ByteBuffer;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Objects;
 import java.util.function.Function;
+import java.util.logging.Level;
 import org.enso.persist.Persistance.Input;
 import org.enso.persist.Persistance.Reference;
 
@@ -202,19 +204,29 @@ static Object readIndirect(InputCache cache, PerMap map, Input in) throws IOExce
     res = cache.readResolve().apply(res);
     var prev = cache.cache().put(at, res);
     if (prev != null) {
-      throw raise(
-          RuntimeException.class,
-          new IOException(
-              "Adding at "
-                  + at
-                  + " object: "
-                  + res.getClass().getName()
-                  + " but there already is "
-                  + prev.getClass().getName()));
+      var bothObjectsAreTheSame = Objects.equals(res, prev);
+      var sb = new StringBuilder();
+      sb.append("Adding at ").append(at).append(" object:\n  ");
+      dumpObject(sb, res);
+      sb.append("\nbut there already is:\n  ");
+      dumpObject(sb, prev);
+      sb.append("\nare they equal: ").append(bothObjectsAreTheSame);
+      var ex = new IOException(sb.toString());
+      if (bothObjectsAreTheSame) {
+        PerUtils.LOG.log(Level.WARNING, sb.toString(), ex);
+      } else {
+        throw raise(RuntimeException.class, ex);
+      }
     }
     return res;
   }
 
+  private static void dumpObject(StringBuilder sb, Object obj) {
+    sb.append(obj.getClass().getName())
+        .append("@")
+        .append(Integer.toHexString(System.identityHashCode(obj)));
+  }
+
   @SuppressWarnings("unchecked")
   static <T> Reference<T> readIndirectAsReference(
       InputCache buffer, PerMap map, Input in, Class<T> clazz) throws IOException {

lib/java/persistance/src/main/java/org/enso/persist/PerUtils.java

@@ -1,8 +1,12 @@
 package org.enso.persist;
 
+import java.util.logging.Logger;
+
 final class PerUtils {
   private PerUtils() {}
 
+  static final Logger LOG = Logger.getLogger(Persistance.class.getPackageName());
+
   @SuppressWarnings("unchecked")
   static <E extends Throwable> E raise(Class<E> clazz, Throwable t) throws E {
     throw (E) t;