[#3900] Implement better formatting for nested Records

jOOQ/src/main/java/org/jooq/impl/AbstractRecord.java

@@ -51,6 +51,7 @@ import static org.jooq.impl.Utils.hasColumnAnnotations;
 import static org.jooq.impl.Utils.indexOrFail;
 import static org.jooq.impl.Utils.resetChangedOnNotNull;
 import static org.jooq.impl.Utils.settings;
+import static org.jooq.impl.Utils.ThreadGuard.Guard.RECORD_TOSTRING;
 
 import java.lang.reflect.Method;
 import java.sql.ResultSet;
@@ -95,6 +96,8 @@ import org.jooq.Table;
 import org.jooq.UniqueKey;
 import org.jooq.exception.InvalidResultException;
 import org.jooq.exception.MappingException;
+import org.jooq.impl.Utils.ThreadGuard;
+import org.jooq.impl.Utils.ThreadGuard.GuardedOperation;
 import org.jooq.tools.Convert;
 import org.jooq.tools.StringUtils;
 
@@ -970,9 +973,20 @@ abstract class AbstractRecord extends AbstractStore implements Record {
 
     @Override
     public String toString() {
-        Result<AbstractRecord> result = new ResultImpl<AbstractRecord>(configuration(), fields.fields.fields);
-        result.add(this);
-        return result.toString();
+        // [#3900] Nested records should generate different toString() behaviour
+        return ThreadGuard.run(RECORD_TOSTRING, new GuardedOperation<String>() {
+            @Override
+            public String unguarded() {
+                Result<AbstractRecord> result = new ResultImpl<AbstractRecord>(configuration(), fields.fields.fields);
+                result.add(AbstractRecord.this);
+                return result.toString();
+            }
+
+            @Override
+            public String guarded() {
+                return valuesRow().toString();
+            }
+        });
     }
 
     @Override

jOOQ/src/main/java/org/jooq/impl/Utils.java

@@ -1845,6 +1845,58 @@ final class Utils {
     // XXX: [#2965] Reflection cache
     // ------------------------------------------------------------------------
 
+    /**
+     * This API acts as a "guard" to prevent the same code from being executed
+     * recursively within the same thread.
+     */
+    static class ThreadGuard {
+
+        /**
+         * The type of guard.
+         */
+        static enum Guard {
+            RECORD_TOSTRING;
+
+            ThreadLocal<Object> tl = new ThreadLocal<Object>();
+        }
+
+        /**
+         * A guarded operation.
+         */
+        static interface GuardedOperation<V> {
+
+            /**
+             * This callback is executed only once on the current stack.
+             */
+            V unguarded();
+
+            /**
+             * This callback is executed if {@link #unguarded()} has already been executed on the current stack.
+             */
+            V guarded();
+        }
+
+        /**
+         * Run an operation using a guard.
+         */
+        static final <V> V run(Guard guard, GuardedOperation<V> operation) {
+            boolean unguarded = (guard.tl.get() != null);
+            if (unguarded)
+                guard.tl.set(Guard.class);
+
+            try {
+                if (unguarded)
+                    return operation.unguarded();
+                else
+                    return operation.guarded();
+            }
+            finally {
+                if (unguarded)
+                    guard.tl.remove();
+            }
+        }
+    }
+
     /**
      * [#2965] This is a {@link Configuration}-based cache that can cache reflection information and other things
      */