Don't crash when printing a recursive structure

doc/source/commands/serialization.rst

@@ -25,3 +25,14 @@ They allow to transform data structures into JSON objects and store them in a fi
 It serializes messages currently stored on message queues to ConfigNode (KSP data format) and adds them to KSP save files.
 
 It is **important** to remember that any data that you supply to :ref:`WRITEJSON` and :meth:`Connection:SENDMESSAGE` must be serializable.
+
+.. note::
+
+  It's not possible to serialize structures that loop on themselves. Only `Directed Acyclical Graphs <https://en.wikipedia.org/wiki/Directed_acyclic_graph>`_ are supported.
+
+  An example of a looping structure is::
+
+      SET a TO LIST().
+	  SET b TO LIST(a).
+	  a:ADD(b).
+	  WRITEJSON(a, "test"). // <-- This will fail

src/kOS.Safe/Serialization/SafeSerializationMgr.cs

@@ -36,32 +36,49 @@ public static bool IsPrimitiveStructure(object serialized)
             return serialized is PrimitiveStructure;
         }
 
-        private object DumpValue(object value, bool includeType)
+        private object DumpValue(object value, bool includeType, bool allowTruncatedRecursion, List<object> seenList)
         {
             var valueDumper = value as IDumper;
 
+            if (!(value is string) && seenList.Contains(value))
+            {
+                if (!allowTruncatedRecursion)
+                    throw new KOSSerializationException("Trying to serialize a structure that loops on itself. Only Directed Acyclical Graphs are supported.");
+                return "...recurse...";
+            }
+
             if (valueDumper != null) {
-                return Dump(valueDumper, includeType);
+                return Dump(valueDumper, includeType, allowTruncatedRecursion, seenList);
             } else if (value is Dump) {
                 return value;
             } else if (value is List<object>) {
-                return (value as List<object>).Select((v) => DumpValue(v, includeType)).ToList();
+                var nextSeenList = new List<object>(seenList);
+                nextSeenList.Add(value);
+                return (value as List<object>).Select((v) => DumpValue(v, includeType, allowTruncatedRecursion, nextSeenList)).ToList();
             } else if (IsSerializablePrimitive(value)) {
                 return Structure.ToPrimitive(value);
             } else {
                 return value.ToString();
             }
         }
 
-        public Dump Dump(IDumper dumper, bool includeType = true)
+        public Dump Dump(IDumper dumper, bool includeType = true, bool allowTruncatedRecursion = false, List<object> seenList = null)
         {
+            // We want to allow DAG-like structure serialization (so nodes can occur in the output more than once.
+            // Cyclical graphs crash us with a stackoverflow however. To protect from this we check wether we're already
+            // in the list of objects that are between us and the root.
+            if (seenList == null)
+                seenList = new List<object>();
+            seenList = new List<object>(seenList);
+            seenList.Add(dumper);
+
             var dump = dumper.Dump();
 
             List<object> keys = new List<object>(dump.Keys);
 
             foreach (object key in keys)
             {
-                dump[key] = DumpValue(dump[key], includeType);
+                dump[key] = DumpValue(dump[key], includeType, allowTruncatedRecursion, seenList);
             }
 
             if (includeType)
@@ -72,9 +89,9 @@ public Dump Dump(IDumper dumper, bool includeType = true)
             return dump;
         }
 
-        public string Serialize(IDumper serialized, IFormatWriter formatter, bool includeType = true)
+        public string Serialize(IDumper serialized, IFormatWriter formatter, bool includeType = true, bool allowTruncatedRecursion = false)
         {
-            return formatter.Write(Dump(serialized, includeType));
+            return formatter.Write(Dump(serialized, includeType, allowTruncatedRecursion));
         }
 
         public object CreateValue(object value)
@@ -221,7 +238,7 @@ public IDumper Deserialize(string input, IFormatReader formatter)
 
         public string ToString(IDumper dumper)
         {
-            return Serialize(dumper, TerminalFormatter.Instance, false);
+            return Serialize(dumper, TerminalFormatter.Instance, false, true);
         }
     }
 }