Add class cycle detection during unmarshaling (#946)

Add support for detection of class cycles during unmarshaling in
languages which do no have garbage collection: C++, Swift, and Objective-C.
A `MarshalException` is thrown when a cycle is detected.

The property `Ice.AcceptClassCycles` can be set to a value greater than `0`
to change this behavior.

71 files changed, 477 insertions, 64 deletions

diff --git a/CHANGELOG-3.7.md b/CHANGELOG-3.7.md
index 54517319da2..b8bf240dee4 100644
--- a/CHANGELOG-3.7.md
+++ b/CHANGELOG-3.7.md
@@ -8,54 +8,69 @@ We recommend that you use the release notes as a guide for migrating your
 applications to this release, and the manual for complete details on a
 particular aspect of Ice.
 
+- [Changes in Ice 3.7.5](#changes-in-ice-375)
+  - [General Changes](#general-changes)
 - [Changes in Ice 3.7.4](#changes-in-ice-374)
-    - [General Changes](#general-changes)
-    - [C++ Changes](#c-changes)
-    - [C# Changes](#c-changes-1)
-    - [JavaScript Changes](#javascript-changes)
-    - [MATLAB Changes](#matlab-changes)
-    - [Python Changes](#python-changes)
-    - [Ruby Changes](#ruby-changes)
-    - [Swift Changes](#swift-changes)
+  - [General Changes](#general-changes-1)
+  - [C++ Changes](#c-changes)
+  - [C# Changes](#c-changes-1)
+  - [JavaScript Changes](#javascript-changes)
+  - [MATLAB Changes](#matlab-changes)
+  - [Python Changes](#python-changes)
+  - [Ruby Changes](#ruby-changes)
+  - [Swift Changes](#swift-changes)
 - [Changes in Ice 3.7.3](#changes-in-ice-373)
-    - [General Changes](#general-changes-1)
-    - [C++ Changes](#c-changes-2)
-    - [C# Changes](#c-changes-3)
-    - [Java Changes](#java-changes)
-    - [JavaScript Changes](#javascript-changes-1)
-    - [MATLAB Changes](#matlab-changes-1)
-    - [Python Changes](#python-changes-1)
+  - [General Changes](#general-changes-2)
+  - [C++ Changes](#c-changes-2)
+  - [C# Changes](#c-changes-3)
+  - [Java Changes](#java-changes)
+  - [JavaScript Changes](#javascript-changes-1)
+  - [MATLAB Changes](#matlab-changes-1)
+  - [Python Changes](#python-changes-1)
 - [Changes in Ice 3.7.2](#changes-in-ice-372)
-    - [General Changes](#general-changes-2)
-    - [C++ Changes](#c-changes-4)
-    - [C# Changes](#c-changes-5)
-    - [Java Changes](#java-changes-1)
-    - [JavaScript Changes](#javascript-changes-2)
-    - [MATLAB Changes](#matlab-changes-2)
-    - [Objective-C Changes](#objective-c-changes)
-    - [PHP Changes](#php-changes)
-    - [Python Changes](#python-changes-2)
+  - [General Changes](#general-changes-3)
+  - [C++ Changes](#c-changes-4)
+  - [C# Changes](#c-changes-5)
+  - [Java Changes](#java-changes-1)
+  - [JavaScript Changes](#javascript-changes-2)
+  - [MATLAB Changes](#matlab-changes-2)
+  - [Objective-C Changes](#objective-c-changes)
+  - [PHP Changes](#php-changes)
+  - [Python Changes](#python-changes-2)
 - [Changes in Ice 3.7.1](#changes-in-ice-371)
-    - [General Changes](#general-changes-3)
-    - [C++ Changes](#c-changes-6)
-    - [C# Changes](#c-changes-7)
-    - [Java Changes](#java-changes-2)
-    - [JavaScript Changes](#javascript-changes-3)
-    - [MATLAB Changes](#matlab-changes-3)
-    - [Objective-C Changes](#objective-c-changes-1)
-    - [PHP Changes](#php-changes-1)
-    - [Python Changes](#python-changes-3)
-    - [Ruby Changes](#ruby-changes-1)
+  - [General Changes](#general-changes-4)
+  - [C++ Changes](#c-changes-6)
+  - [C# Changes](#c-changes-7)
+  - [Java Changes](#java-changes-2)
+  - [JavaScript Changes](#javascript-changes-3)
+  - [MATLAB Changes](#matlab-changes-3)
+  - [Objective-C Changes](#objective-c-changes-1)
+  - [PHP Changes](#php-changes-1)
+  - [Python Changes](#python-changes-3)
+  - [Ruby Changes](#ruby-changes-1)
 - [Changes in Ice 3.7.0](#changes-in-ice-370)
-    - [General Changes](#general-changes-4)
-    - [C++ Changes](#c-changes-8)
-    - [C# Changes](#c-changes-9)
-    - [Java Changes](#java-changes-3)
-    - [JavaScript Changes](#javascript-changes-4)
-    - [Objective-C Changes](#objective-c-changes-2)
-    - [PHP Changes](#php-changes-2)
-    - [Python Changes](#python-changes-4)
-    - [Ruby Changes](#ruby-changes-2)
+  - [General Changes](#general-changes-5)
+  - [C++ Changes](#c-changes-8)
+  - [C# Changes](#c-changes-9)
+  - [Java Changes](#java-changes-3)
+  - [JavaScript Changes](#javascript-changes-4)
+  - [Objective-C Changes](#objective-c-changes-2)
+  - [PHP Changes](#php-changes-2)
+  - [Python Changes](#python-changes-4)
+  - [Ruby Changes](#ruby-changes-2)
+
+# Changes in Ice 3.7.5
+
+These are the changes since Ice 3.7.4.
+
+## General Changes
+
+- Add support for detection of class cycles during unmarshaling in
+  languages which do not have garbage collection: C++, Swift, and Objective-C.
+  A `MarshalException` is thrown when a cycle is detected.
+
+  The property `Ice.AcceptClassCycles` can be set to a value greater than `0`
+  to change this behavior.
 
 # Changes in Ice 3.7.4
 
diff --git a/config/PropertyNames.xml b/config/PropertyNames.xml
index f98f28f834e..d494f75490c 100644
--- a/config/PropertyNames.xml
+++ b/config/PropertyNames.xml
@@ -300,6 +300,7 @@ generated from the section label.
     </class>
 
     <section name="Ice">
+        <property name="AcceptClassCycles" />
         <property name="ACM.Client" deprecated="true"/>
         <property name="ACM.Server" deprecated="true"/>
         <property name="ACM" class="acm"/>
diff --git a/cpp/src/Ice/InputStream.cpp b/cpp/src/Ice/InputStream.cpp
index d0c4c91d9fc..3c4686e979d 100644
--- a/cpp/src/Ice/InputStream.cpp
+++ b/cpp/src/Ice/InputStream.cpp
@@ -1819,12 +1819,17 @@ Ice::InputStream::EncapsDecoder::addPatchEntry(Int index, PatchFunc patchFunc, v
     assert(index > 0);
 
     //
-    // Check if we already unmarshaled the object. If that's the case,
-    // just patch the object smart pointer and we're done.
+    // Check if we already unmarshaled the object. If that's the case, just patch the object smart pointer
+    // and we're done. A null value indicates we've encountered a cycle and Ice.AllowClassCycles is false.
     //
     IndexToPtrMap::iterator p = _unmarshaledMap.find(index);
     if(p != _unmarshaledMap.end())
     {
+        if (p->second == ICE_NULLPTR)
+        {
+            assert(!_stream->_instance->acceptClassCycles());
+            throw MarshalException(__FILE__, __LINE__, "cycle detected during Value unmarshaling");
+        }
         (*patchFunc)(patchAddr, p->second);
         return;
     }
@@ -1862,7 +1867,10 @@ Ice::InputStream::EncapsDecoder::unmarshal(Int index, const Ice::ValuePtr& v)
     // Add the object to the map of unmarshaled instances, this must
     // be done before reading the instances (for circular references).
     //
-    _unmarshaledMap.insert(make_pair(index, v));
+    // If circular references are not allowed we insert null (for cycle detection) and add
+    // the object to the map once it has been fully unmarshaled.
+    //
+    _unmarshaledMap.insert(make_pair(index, _stream->_instance->acceptClassCycles() ? v : Ice::ValuePtr()));
 
     //
     // Read the object.
@@ -1915,6 +1923,13 @@ Ice::InputStream::EncapsDecoder::unmarshal(Int index, const Ice::ValuePtr& v)
             _valueList.clear();
         }
     }
+
+    if(!_stream->_instance->acceptClassCycles())
+    {
+        // This class has been fully unmarshaled without creating any cycles
+        // It can be added to the map now.
+        _unmarshaledMap[index] = v;
+    }
 }
 
 void
diff --git a/cpp/src/Ice/Instance.cpp b/cpp/src/Ice/Instance.cpp
index e0c06b52a9b..7a1b5952157 100644
--- a/cpp/src/Ice/Instance.cpp
+++ b/cpp/src/Ice/Instance.cpp
@@ -950,6 +950,7 @@ IceInternal::Instance::Instance(const CommunicatorPtr& communicator, const Initi
     _classGraphDepthMax(0),
     _collectObjects(false),
     _toStringMode(ICE_ENUM(ToStringMode, Unicode)),
+    _acceptClassCycles(false),
     _implicitContext(0),
     _stringConverter(Ice::getProcessStringConverter()),
     _wstringConverter(Ice::getProcessWstringConverter()),
@@ -1227,6 +1228,8 @@ IceInternal::Instance::Instance(const CommunicatorPtr& communicator, const Initi
             throw InitializationException(__FILE__, __LINE__, "The value for Ice.ToStringMode must be Unicode, ASCII or Compat");
         }
 
+        const_cast<bool&>(_acceptClassCycles) = _initData.properties->getPropertyAsInt("Ice.AcceptClassCycles") > 0;
+
         const_cast<ImplicitContextIPtr&>(_implicitContext) =
             ImplicitContextI::create(_initData.properties->getProperty("Ice.ImplicitContext"));
 
diff --git a/cpp/src/Ice/Instance.h b/cpp/src/Ice/Instance.h
index 70e9c249e42..150167190d8 100644
--- a/cpp/src/Ice/Instance.h
+++ b/cpp/src/Ice/Instance.h
@@ -106,6 +106,7 @@ public:
     size_t classGraphDepthMax() const { return _classGraphDepthMax; }
     bool collectObjects() const { return _collectObjects; }
     Ice::ToStringMode toStringMode() const { return _toStringMode; }
+    bool acceptClassCycles() const { return _acceptClassCycles; }
     const ACMConfig& clientACM() const;
     const ACMConfig& serverACM() const;
 
@@ -175,6 +176,7 @@ private:
     const size_t _classGraphDepthMax; // Immutable, not reset by destroy().
     const bool _collectObjects; // Immutable, not reset by destroy().
     const Ice::ToStringMode _toStringMode; // Immutable, not reset by destroy()
+    const bool _acceptClassCycles; // Immutable, not reset by destroy()
     ACMConfig _clientACM;
     ACMConfig _serverACM;
     RouterManagerPtr _routerManager;
diff --git a/cpp/src/Ice/PropertyNames.cpp b/cpp/src/Ice/PropertyNames.cpp
index 9d332d8618a..da4d0af3534 100644
--- a/cpp/src/Ice/PropertyNames.cpp
+++ b/cpp/src/Ice/PropertyNames.cpp
@@ -1,7 +1,7 @@
 //
 // Copyright (c) ZeroC, Inc. All rights reserved.
 //
-// Generated by makeprops.py from file ./config/PropertyNames.xml, Fri Sep  6 18:11:04 2019
+// Generated by makeprops.py from file ./config/PropertyNames.xml, Thu Jul  2 14:55:02 2020
 
 // IMPORTANT: Do not edit this file -- any edits made here will be lost!
 
@@ -9,6 +9,7 @@
 
 const IceInternal::Property IcePropsData[] =
 {
+    IceInternal::Property("Ice.AcceptClassCycles", false, 0),
     IceInternal::Property("Ice.ACM.Client", true, 0),
     IceInternal::Property("Ice.ACM.Server", true, 0),
     IceInternal::Property("Ice.ACM.Timeout", false, 0),
diff --git a/cpp/src/Ice/PropertyNames.h b/cpp/src/Ice/PropertyNames.h
index 9c057af4afe..8f65258d805 100644
--- a/cpp/src/Ice/PropertyNames.h
+++ b/cpp/src/Ice/PropertyNames.h
@@ -1,7 +1,7 @@
 //
 // Copyright (c) ZeroC, Inc. All rights reserved.
 //
-// Generated by makeprops.py from file ./config/PropertyNames.xml, Fri Sep  6 18:11:04 2019
+// Generated by makeprops.py from file ./config/PropertyNames.xml, Thu Jul  2 14:55:02 2020
 
 // IMPORTANT: Do not edit this file -- any edits made here will be lost!
 
diff --git a/cpp/test/Ice/objects/AllTests.cpp b/cpp/test/Ice/objects/AllTests.cpp
index 65134a2c49e..c84b73d51a9 100644
--- a/cpp/test/Ice/objects/AllTests.cpp
+++ b/cpp/test/Ice/objects/AllTests.cpp
@@ -604,5 +604,25 @@ allTests(Test::TestHelper* helper)
     }
     cout << "ok" << endl;
 
+    cout << "testing sending class cycle... " << flush;
+    {
+        RecursivePtr rec = ICE_MAKE_SHARED(Recursive);
+        rec->v = rec;
+        bool acceptsCycles = initial->acceptsClassCycles();
+        try
+        {
+            initial->setCycle(rec);
+            test(acceptsCycles);
+        }
+        catch(const Ice::UnknownLocalException&)
+        {
+            // expected when the remote server does not accept cycles
+            // and throws a MarshalException
+            test(!acceptsCycles);
+        }
+        rec->v = ICE_NULLPTR;
+    }
+    cout << "ok" << endl;
+
     return initial;
 }
diff --git a/cpp/test/Ice/objects/Client.cpp b/cpp/test/Ice/objects/Client.cpp
index 4d5dc868b83..a27c51e17f6 100644
--- a/cpp/test/Ice/objects/Client.cpp
+++ b/cpp/test/Ice/objects/Client.cpp
@@ -118,7 +118,7 @@ void
 Client::run(int argc, char** argv)
 {
     Ice::PropertiesPtr properties = createTestProperties(argc, argv);
-
+    properties->setProperty("Ice.AcceptClassCycles", "1");
 #ifndef ICE_CPP11_MAPPING
     properties->setProperty("Ice.CollectObjects", "1");
 #endif
diff --git a/cpp/test/Ice/objects/Collocated.cpp b/cpp/test/Ice/objects/Collocated.cpp
index eb4434fbe39..0633cfa5d13 100644
--- a/cpp/test/Ice/objects/Collocated.cpp
+++ b/cpp/test/Ice/objects/Collocated.cpp
@@ -110,6 +110,7 @@ void
 Collocated::run(int argc, char** argv)
 {
     Ice::PropertiesPtr properties = createTestProperties(argc, argv);
+    properties->setProperty("Ice.AcceptClassCycles", "1");
 #ifndef ICE_CPP11_MAPPING
     properties->setProperty("Ice.CollectObjects", "1");
 #endif
diff --git a/cpp/test/Ice/objects/Test.ice b/cpp/test/Ice/objects/Test.ice
index 38214e39951..a89b9beb0e8 100644
--- a/cpp/test/Ice/objects/Test.ice
+++ b/cpp/test/Ice/objects/Test.ice
@@ -222,6 +222,9 @@ interface Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/cpp/test/Ice/objects/TestI.cpp b/cpp/test/Ice/objects/TestI.cpp
index 9cfd4c395eb..abf4154e59e 100644
--- a/cpp/test/Ice/objects/TestI.cpp
+++ b/cpp/test/Ice/objects/TestI.cpp
@@ -187,6 +187,20 @@ InitialI::supportsClassGraphDepthMax(const Ice::Current&)
     return true;
 }
 
+void
+InitialI::setCycle(ICE_IN(RecursivePtr) r, const Ice::Current&)
+{
+    // break the cycle
+    assert(r);
+    r->v = ICE_NULLPTR;
+}
+
+bool
+InitialI::acceptsClassCycles(const Ice::Current& c)
+{
+    return c.adapter->getCommunicator()->getProperties()->getPropertyAsInt("Ice.AcceptClassCycles") > 0;
+}
+
 #ifdef ICE_CPP11_MAPPING
 InitialI::GetMBMarshaledResult
 InitialI::getMB(const Ice::Current& current)
diff --git a/cpp/test/Ice/objects/TestI.h b/cpp/test/Ice/objects/TestI.h
index 3b265a19664..78874dbb91b 100644
--- a/cpp/test/Ice/objects/TestI.h
+++ b/cpp/test/Ice/objects/TestI.h
@@ -93,6 +93,9 @@ public:
     virtual void setRecursive(ICE_IN(Test::RecursivePtr), const Ice::Current&);
     virtual bool supportsClassGraphDepthMax(const Ice::Current&);
 
+    virtual void setCycle(ICE_IN(Test::RecursivePtr), const Ice::Current&);
+    virtual bool acceptsClassCycles(const Ice::Current&);
+
 #ifdef ICE_CPP11_MAPPING
     virtual GetMBMarshaledResult getMB(const Ice::Current&);
     virtual void getAMDMBAsync(std::function<void(const GetAMDMBMarshaledResult&)>,
diff --git a/cpp/test/Ice/optional/AllTests.cpp b/cpp/test/Ice/optional/AllTests.cpp
index 7b00e0a421c..369d2228cc6 100644
--- a/cpp/test/Ice/optional/AllTests.cpp
+++ b/cpp/test/Ice/optional/AllTests.cpp
@@ -587,6 +587,7 @@ allTests(Test::TestHelper* helper, bool)
 
     mo1->k = mo1;
     MultiOptionalPtr mo5 = ICE_DYNAMIC_CAST(MultiOptional, initial->pingPong(mo1));
+
     test(mo5->a == mo1->a);
     test(mo5->b == mo1->b);
     test(mo5->c == mo1->c);
diff --git a/cpp/test/Ice/optional/Client.cpp b/cpp/test/Ice/optional/Client.cpp
index 1f2bb2edb45..4806e2f02cf 100644
--- a/cpp/test/Ice/optional/Client.cpp
+++ b/cpp/test/Ice/optional/Client.cpp
@@ -20,6 +20,7 @@ void
 Client::run(int argc, char** argv)
 {
     Ice::PropertiesPtr properties = createTestProperties(argc, argv);
+    properties->setProperty("Ice.AcceptClassCycles", "1");
 #ifndef ICE_CPP11_MAPPING
     properties->setProperty("Ice.CollectObjects", "1");
 #endif
diff --git a/cpp/test/Ice/optional/Server.cpp b/cpp/test/Ice/optional/Server.cpp
index f526d0355d9..a84d6694b7e 100644
--- a/cpp/test/Ice/optional/Server.cpp
+++ b/cpp/test/Ice/optional/Server.cpp
@@ -20,6 +20,7 @@ void
 Server::run(int argc, char** argv)
 {
     Ice::PropertiesPtr properties = createTestProperties(argc, argv);
+    properties->setProperty("Ice.AcceptClassCycles", "1");
 #ifndef ICE_CPP11_MAPPING
     properties->setProperty("Ice.CollectObjects", "1");
 #endif
diff --git a/cpp/test/Ice/optional/ServerAMD.cpp b/cpp/test/Ice/optional/ServerAMD.cpp
index bdc012694ce..f9d16fed20f 100644
--- a/cpp/test/Ice/optional/ServerAMD.cpp
+++ b/cpp/test/Ice/optional/ServerAMD.cpp
@@ -19,6 +19,7 @@ void
 ServerAMD::run(int argc, char** argv)
 {
     Ice::PropertiesPtr properties = createTestProperties(argc, argv);
+    properties->setProperty("Ice.AcceptClassCycles", "1");
 #ifndef ICE_CPP11_MAPPING
     properties->setProperty("Ice.CollectObjects", "1");
 #endif
diff --git a/cpp/test/Ice/slicing/objects/Client.cpp b/cpp/test/Ice/slicing/objects/Client.cpp
index 537d7e9d216..b9f217e495d 100644
--- a/cpp/test/Ice/slicing/objects/Client.cpp
+++ b/cpp/test/Ice/slicing/objects/Client.cpp
@@ -23,6 +23,7 @@ Client::run(int argc, char** argv)
     //
     // For this test, we enable object collection.
     //
+    properties->setProperty("Ice.AcceptClassCycles", "1");
     properties->setProperty("Ice.CollectObjects", "1");
 
     Ice::CommunicatorHolder communicator = initialize(argc, argv, properties);
diff --git a/cpp/test/Ice/slicing/objects/Server.cpp b/cpp/test/Ice/slicing/objects/Server.cpp
index f1cd93e6ae3..b7ca13be31b 100644
--- a/cpp/test/Ice/slicing/objects/Server.cpp
+++ b/cpp/test/Ice/slicing/objects/Server.cpp
@@ -19,6 +19,7 @@ void
 Server::run(int argc, char** argv)
 {
     Ice::PropertiesPtr properties = createTestProperties(argc, argv);
+    properties->setProperty("Ice.AcceptClassCycles", "1");
 #ifndef ICE_CPP11_MAPPING
     properties->setProperty("Ice.CollectObjects", "1");
 #endif
diff --git a/cpp/test/Ice/slicing/objects/ServerAMD.cpp b/cpp/test/Ice/slicing/objects/ServerAMD.cpp
index ab4df8a0f5e..e7e60f6998f 100644
--- a/cpp/test/Ice/slicing/objects/ServerAMD.cpp
+++ b/cpp/test/Ice/slicing/objects/ServerAMD.cpp
@@ -19,6 +19,7 @@ void
 ServerAMD::run(int argc, char** argv)
 {
     Ice::PropertiesPtr properties = createTestProperties(argc, argv);
+    properties->setProperty("Ice.AcceptClassCycles", "1");
 #ifndef ICE_CPP11_MAPPING
     properties->setProperty("Ice.CollectObjects", "1");
 #endif
diff --git a/cpp/test/Ice/stream/Client.cpp b/cpp/test/Ice/stream/Client.cpp
index 23a2be692df..879b93ffb61 100644
--- a/cpp/test/Ice/stream/Client.cpp
+++ b/cpp/test/Ice/stream/Client.cpp
@@ -1365,7 +1365,9 @@ public:
 void
 Client::run(int argc, char** argv)
 {
-    Ice::CommunicatorHolder communicator = initialize(argc, argv);
+    Ice::PropertiesPtr properties = createTestProperties(argc, argv);
+    properties->setProperty("Ice.AcceptClassCycles", "1");
+    Ice::CommunicatorHolder communicator = initialize(argc, argv, properties);
     void allTests(Test::TestHelper*);
     allTests(this);
 }
diff --git a/csharp/src/Ice/PropertyNames.cs b/csharp/src/Ice/PropertyNames.cs
index a4b0440ead6..d8926852333 100644
--- a/csharp/src/Ice/PropertyNames.cs
+++ b/csharp/src/Ice/PropertyNames.cs
@@ -1,7 +1,7 @@
 //
 // Copyright (c) ZeroC, Inc. All rights reserved.
 //
-// Generated by makeprops.py from file ./config/PropertyNames.xml, Fri Sep  6 18:11:04 2019
+// Generated by makeprops.py from file ./config/PropertyNames.xml, Thu Jul  2 14:55:02 2020
 
 // IMPORTANT: Do not edit this file -- any edits made here will be lost!
 
@@ -11,6 +11,7 @@ namespace IceInternal
     {
         public static Property[] IceProps =
         {
+             new Property(@"^Ice\.AcceptClassCycles$", false, null),
              new Property(@"^Ice\.ACM\.Client$", true, null),
              new Property(@"^Ice\.ACM\.Server$", true, null),
              new Property(@"^Ice\.ACM\.Timeout$", false, null),
diff --git a/csharp/test/Ice/objects/AllTests.cs b/csharp/test/Ice/objects/AllTests.cs
index d0cfb08c6ba..abfa3785dc0 100644
--- a/csharp/test/Ice/objects/AllTests.cs
+++ b/csharp/test/Ice/objects/AllTests.cs
@@ -538,6 +538,25 @@ namespace Ice
                     }
                     output.WriteLine("ok");
 
+                    output.Write("testing sending class cycle... ");
+                    output.Flush();
+                    {
+                        var rec = new Test.Recursive();
+                        rec.v = rec;
+                        var acceptsCycles = initial.acceptsClassCycles();
+                        try
+                        {
+                            initial.setCycle(rec);
+                            test(acceptsCycles);
+                        }
+                        catch(Ice.UnknownLocalException)
+                        {
+                            test(!acceptsCycles);
+                        }
+
+                    }
+                    output.WriteLine("ok");
+
                     return initial;
                 }
             }
diff --git a/csharp/test/Ice/objects/InitialI.cs b/csharp/test/Ice/objects/InitialI.cs
index 19e34a228d8..0aa1a0451f6 100644
--- a/csharp/test/Ice/objects/InitialI.cs
+++ b/csharp/test/Ice/objects/InitialI.cs
@@ -123,6 +123,15 @@ namespace Ice
                 return true;
             }
 
+            public override void setCycle(Test.Recursive r, Ice.Current current)
+            {
+            }
+
+            public override bool acceptsClassCycles(Ice.Current current)
+            {
+                return true;
+            }
+
             public override Test.D1 getD1(Test.D1 d1, Ice.Current current)
             {
                 return d1;
diff --git a/csharp/test/Ice/objects/Test.ice b/csharp/test/Ice/objects/Test.ice
index 9fda575741c..9bcea5b2eaa 100644
--- a/csharp/test/Ice/objects/Test.ice
+++ b/csharp/test/Ice/objects/Test.ice
@@ -216,6 +216,9 @@ interface Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/java-compat/src/Ice/src/main/java/IceInternal/PropertyNames.java b/java-compat/src/Ice/src/main/java/IceInternal/PropertyNames.java
index 36daa5282b8..b974493637b 100644
--- a/java-compat/src/Ice/src/main/java/IceInternal/PropertyNames.java
+++ b/java-compat/src/Ice/src/main/java/IceInternal/PropertyNames.java
@@ -1,7 +1,7 @@
 //
 // Copyright (c) ZeroC, Inc. All rights reserved.
 //
-// Generated by makeprops.py from file ./config/PropertyNames.xml, Fri Sep  6 18:11:04 2019
+// Generated by makeprops.py from file ./config/PropertyNames.xml, Thu Jul  2 14:55:02 2020
 
 // IMPORTANT: Do not edit this file -- any edits made here will be lost!
 
@@ -11,6 +11,7 @@ public final class PropertyNames
 {
     public static final Property IceProps[] =
     {
+        new Property("Ice\\.AcceptClassCycles", false, null),
         new Property("Ice\\.ACM\\.Client", true, null),
         new Property("Ice\\.ACM\\.Server", true, null),
         new Property("Ice\\.ACM\\.Timeout", false, null),
diff --git a/java-compat/test/src/main/java/test/Ice/objects/AllTests.java b/java-compat/test/src/main/java/test/Ice/objects/AllTests.java
index 20e6c0972ee..82087aeccf4 100644
--- a/java-compat/test/src/main/java/test/Ice/objects/AllTests.java
+++ b/java-compat/test/src/main/java/test/Ice/objects/AllTests.java
@@ -445,6 +445,24 @@ public class AllTests
         }
         out.println("ok");
 
+        out.print("testing sending class cycle... ");
+        out.flush();
+        {
+            Recursive rec = new Recursive();
+            rec.v = rec;
+            boolean acceptsCycles = initial.acceptsClassCycles();
+            try
+            {
+                initial.setCycle(rec);
+                test(acceptsCycles);
+            }
+            catch(Ice.UnknownLocalException ex)
+            {
+                test(!acceptsCycles);
+            }
+        }
+        out.println("ok");
+
         return initial;
     }
 }
diff --git a/java-compat/test/src/main/java/test/Ice/objects/InitialI.java b/java-compat/test/src/main/java/test/Ice/objects/InitialI.java
index 1a48cf0f7bf..1b2a76804c3 100644
--- a/java-compat/test/src/main/java/test/Ice/objects/InitialI.java
+++ b/java-compat/test/src/main/java/test/Ice/objects/InitialI.java
@@ -148,6 +148,17 @@ public final class InitialI extends Initial
     }
 
     @Override
+    public void setCycle(Recursive r, Ice.Current current)
+    {
+    }
+
+    @Override
+    public boolean acceptsClassCycles(Ice.Current current)
+    {
+        return true;
+    }
+
+    @Override
     public B
     getMB(Ice.Current current)
     {
diff --git a/java-compat/test/src/main/java/test/Ice/objects/Test.ice b/java-compat/test/src/main/java/test/Ice/objects/Test.ice
index 9af55ef027c..9a572e054ef 100644
--- a/java-compat/test/src/main/java/test/Ice/objects/Test.ice
+++ b/java-compat/test/src/main/java/test/Ice/objects/Test.ice
@@ -219,6 +219,9 @@ class Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/java/src/Ice/src/main/java/com/zeroc/IceInternal/PropertyNames.java b/java/src/Ice/src/main/java/com/zeroc/IceInternal/PropertyNames.java
index 60078543e6f..3ba05d0bde6 100644
--- a/java/src/Ice/src/main/java/com/zeroc/IceInternal/PropertyNames.java
+++ b/java/src/Ice/src/main/java/com/zeroc/IceInternal/PropertyNames.java
@@ -1,7 +1,7 @@
 //
 // Copyright (c) ZeroC, Inc. All rights reserved.
 //
-// Generated by makeprops.py from file ./config/PropertyNames.xml, Fri Sep  6 18:11:04 2019
+// Generated by makeprops.py from file ./config/PropertyNames.xml, Thu Jul  2 14:55:02 2020
 
 // IMPORTANT: Do not edit this file -- any edits made here will be lost!
 
@@ -11,6 +11,7 @@ public final class PropertyNames
 {
     public static final Property IceProps[] =
     {
+        new Property("Ice\\.AcceptClassCycles", false, null),
         new Property("Ice\\.ACM\\.Client", true, null),
         new Property("Ice\\.ACM\\.Server", true, null),
         new Property("Ice\\.ACM\\.Timeout", false, null),
diff --git a/java/test/src/main/java/test/Ice/objects/AllTests.java b/java/test/src/main/java/test/Ice/objects/AllTests.java
index aa11250a6fa..d161292e807 100644
--- a/java/test/src/main/java/test/Ice/objects/AllTests.java
+++ b/java/test/src/main/java/test/Ice/objects/AllTests.java
@@ -435,6 +435,24 @@ public class AllTests
         }
         out.println("ok");
 
+        out.print("testing sending class cycle... ");
+        out.flush();
+        {
+            Recursive rec = new Recursive();
+            rec.v = rec;
+            boolean acceptsCycles = initial.acceptsClassCycles();
+            try
+            {
+                initial.setCycle(rec);
+                test(acceptsCycles);
+            }
+            catch(com.zeroc.Ice.UnknownLocalException ex)
+            {
+                test(!acceptsCycles);
+            }
+        }
+        out.println("ok");
+
         return initial;
     }
 }
diff --git a/java/test/src/main/java/test/Ice/objects/InitialI.java b/java/test/src/main/java/test/Ice/objects/InitialI.java
index 7760480f1e9..125427eb06a 100644
--- a/java/test/src/main/java/test/Ice/objects/InitialI.java
+++ b/java/test/src/main/java/test/Ice/objects/InitialI.java
@@ -112,6 +112,17 @@ public final class InitialI implements Initial
     }
 
     @Override
+    public void setCycle(Recursive r, com.zeroc.Ice.Current current)
+    {
+    }
+
+    @Override
+    public boolean acceptsClassCycles(com.zeroc.Ice.Current current)
+    {
+        return true;
+    }
+
+    @Override
     public com.zeroc.Ice.Value getI(com.zeroc.Ice.Current current)
     {
         return new II();
diff --git a/java/test/src/main/java/test/Ice/objects/Test.ice b/java/test/src/main/java/test/Ice/objects/Test.ice
index 50e6248fd4a..a0862f9f4f4 100644
--- a/java/test/src/main/java/test/Ice/objects/Test.ice
+++ b/java/test/src/main/java/test/Ice/objects/Test.ice
@@ -216,6 +216,9 @@ interface Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/js/src/Ice/PropertyNames.js b/js/src/Ice/PropertyNames.js
index 4f198ec4f30..79e02bcad47 100644
--- a/js/src/Ice/PropertyNames.js
+++ b/js/src/Ice/PropertyNames.js
@@ -1,7 +1,7 @@
 //
 // Copyright (c) ZeroC, Inc. All rights reserved.
 //
-// Generated by makeprops.py from file ./config/PropertyNames.xml, Fri Sep  6 18:11:04 2019
+// Generated by makeprops.py from file ./config/PropertyNames.xml, Thu Jul  2 14:55:02 2020
 
 // IMPORTANT: Do not edit this file -- any edits made here will be lost!
 
@@ -14,6 +14,7 @@ const PropertyNames = {};
 const Property = Ice.Property;
 PropertyNames.IceProps =
 [
+    new Property("/^Ice\.AcceptClassCycles/", false, null),
     new Property("/^Ice\.ACM\.Client/", true, null),
     new Property("/^Ice\.ACM\.Server/", true, null),
     new Property("/^Ice\.ACM\.Timeout/", false, null),
diff --git a/js/test/Ice/objects/Client.js b/js/test/Ice/objects/Client.js
index 0f5e5cb730b..76697dc3d17 100644
--- a/js/test/Ice/objects/Client.js
+++ b/js/test/Ice/objects/Client.js
@@ -491,6 +491,23 @@
             }
             out.writeLine("ok");
 
+            out.write("testing sending class cycle... ");
+            {
+                const rec = new Test.Recursive();
+                rec.v = rec;
+                const acceptsCycles = await initial.acceptsClassCycles();
+                try
+                {
+                    await initial.setCycle(rec);
+                    test(acceptsCycles);
+                }
+                catch (error)
+                {
+                    test(!acceptsCycles);
+                }
+            }
+            out.writeLine("ok");
+
             await initial.shutdown();
         }
 
diff --git a/js/test/Ice/objects/InitialI.js b/js/test/Ice/objects/InitialI.js
index e6140642963..c5c3d623b40 100644
--- a/js/test/Ice/objects/InitialI.js
+++ b/js/test/Ice/objects/InitialI.js
@@ -225,6 +225,15 @@
             return false;
         }
 
+        setCycle(r, current)
+        {
+        }
+
+        acceptsClassCycles(current)
+        {
+            return true;
+        }
+
         getMB(current)
         {
             return this._b1;
diff --git a/js/test/Ice/objects/Test.ice b/js/test/Ice/objects/Test.ice
index 617deda65de..3f60df69a41 100644
--- a/js/test/Ice/objects/Test.ice
+++ b/js/test/Ice/objects/Test.ice
@@ -220,6 +220,9 @@ interface Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/matlab/src/Init.cpp b/matlab/src/Init.cpp
index fb90429390a..6147c9de1bd 100644
--- a/matlab/src/Init.cpp
+++ b/matlab/src/Init.cpp
@@ -53,6 +53,14 @@ Ice_initialize(mxArray* args, void* propsImpl, void** r)
             id.properties = deref<Ice::Properties>(propsImpl);
         }
 
+        if(!id.properties)
+        {
+            id.properties = Ice::createProperties();
+        }
+
+        // Always accept cycles in MATLAB
+        id.properties->setProperty("Ice.AcceptClassCycles", "1");
+
         *r = new shared_ptr<Ice::Communicator>(Ice::initialize(a, id));
         return createResultValue(createStringList(a));
     }
diff --git a/matlab/test/Ice/objects/AllTests.m b/matlab/test/Ice/objects/AllTests.m
index 7ad96bcb5c3..a12538eb07e 100644
--- a/matlab/test/Ice/objects/AllTests.m
+++ b/matlab/test/Ice/objects/AllTests.m
@@ -309,6 +309,18 @@ classdef AllTests
             end
             fprintf('ok\n');
 
+            fprintf('testing sending class cycle... ');
+            rec = Recursive();
+            rec.v = rec;
+            acceptsCycles = initial.acceptsClassCycles();
+            try
+                initial.setCycle(rec);
+                assert(acceptsCycles);
+            catch ex
+                assert(!acceptsCycles);
+            end
+            fprintf('ok\n');
+
             r = initial;
         end
     end
diff --git a/matlab/test/Ice/objects/Test.ice b/matlab/test/Ice/objects/Test.ice
index d05e6feff56..aac48083c68 100644
--- a/matlab/test/Ice/objects/Test.ice
+++ b/matlab/test/Ice/objects/Test.ice
@@ -216,6 +216,9 @@ interface Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/objective-c/test/Ice/objects/AllTests.m b/objective-c/test/Ice/objects/AllTests.m
index f88a3adc7c8..de23a3d1b63 100644
--- a/objective-c/test/Ice/objects/AllTests.m
+++ b/objective-c/test/Ice/objects/AllTests.m
@@ -706,5 +706,23 @@ objectsAllTests(id<ICECommunicator> communicator, BOOL __unused collocated)
         tprintf("ok\n");
     }
 
+    {
+        tprintf("testing sending class cycle... ");
+        TestObjectsRecursive* rec = [TestObjectsRecursive recursive];
+        rec.v = rec;
+        bool acceptsCycles = [initial acceptsClassCycles];
+        @try
+        {
+            [initial setCycle:rec];
+            test(acceptsCycles);
+        }
+        @catch(ICEUnknownLocalException*)
+        {
+             test(!acceptsCycles);
+        }
+        rec.v = nil;
+        tprintf("ok\n");
+    }
+
     return initial;
 }
diff --git a/objective-c/test/Ice/objects/Client.m b/objective-c/test/Ice/objects/Client.m
index fc3354caec1..243625c942b 100644
--- a/objective-c/test/Ice/objects/Client.m
+++ b/objective-c/test/Ice/objects/Client.m
@@ -121,6 +121,7 @@ main(int argc, char* argv[])
         {
             ICEInitializationData* initData = [ICEInitializationData initializationData];
             initData.properties = defaultClientProperties(&argc, argv);
+            [initData.properties setProperty:@"Ice.AcceptClassCycles" value:@"1"];
 #if TARGET_OS_IPHONE
             initData.prefixTable_ = [NSDictionary dictionaryWithObjectsAndKeys:
                                       @"TestObjects", @"::Test",
diff --git a/objective-c/test/Ice/objects/Collocated.m b/objective-c/test/Ice/objects/Collocated.m
index 26af67be66d..cf2e2068f93 100644
--- a/objective-c/test/Ice/objects/Collocated.m
+++ b/objective-c/test/Ice/objects/Collocated.m
@@ -135,6 +135,7 @@ main(int argc, char* argv[])
             ICEInitializationData* initData = [ICEInitializationData initializationData];
             initData.properties = defaultServerProperties(&argc, argv);
             [initData.properties setProperty:@"Ice.Warn.Dispatch" value:@"0"];
+            [initData.properties setProperty:@"Ice.AcceptClassCycles" value:@"1"];
 #if TARGET_OS_IPHONE
             initData.prefixTable_ = [NSDictionary dictionaryWithObjectsAndKeys:
                                       @"TestObjects", @"::Test",
diff --git a/objective-c/test/Ice/objects/ObjectsTest.ice b/objective-c/test/Ice/objects/ObjectsTest.ice
index b7919f45a9e..019db4cd751 100644
--- a/objective-c/test/Ice/objects/ObjectsTest.ice
+++ b/objective-c/test/Ice/objects/ObjectsTest.ice
@@ -225,6 +225,9 @@ class Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/objective-c/test/Ice/objects/TestI.m b/objective-c/test/Ice/objects/TestI.m
index f217e09daf4..1c0dae64e65 100644
--- a/objective-c/test/Ice/objects/TestI.m
+++ b/objective-c/test/Ice/objects/TestI.m
@@ -198,6 +198,17 @@
     return YES;
 }
 
+-(void) setCycle:(TestObjectsRecursive*)r current:(ICECurrent*)__unused current
+{
+    // break the cycle
+    r.v = nil;
+}
+
+-(BOOL) acceptsClassCycles:(ICECurrent*) current
+{
+    return [[[[current adapter] getCommunicator] getProperties] getPropertyAsInt:@"Ice.AcceptClassCycles"] > 0;
+}
+
 -(TestObjectsB*) getMB:(ICECurrent*)__unused current
 {
     return _b1;
diff --git a/objective-c/test/Ice/slicing/objects/Client.m b/objective-c/test/Ice/slicing/objects/Client.m
index 689c70963d1..857a18b5a9f 100644
--- a/objective-c/test/Ice/slicing/objects/Client.m
+++ b/objective-c/test/Ice/slicing/objects/Client.m
@@ -38,6 +38,7 @@ main(int argc, char* argv[])
         {
             ICEInitializationData* initData = [ICEInitializationData initializationData];
             initData.properties = defaultClientProperties(&argc, argv);
+            [initData.properties setProperty:@"Ice.AcceptClassCycles" value:@"1"];
 #if TARGET_OS_IPHONE
             initData.prefixTable_ = [NSDictionary dictionaryWithObjectsAndKeys:
                                       @"TestSlicingObjectsClient", @"::Test",
diff --git a/objective-c/test/Ice/slicing/objects/Server.m b/objective-c/test/Ice/slicing/objects/Server.m
index a75e72ea65e..f7dbd8b8365 100644
--- a/objective-c/test/Ice/slicing/objects/Server.m
+++ b/objective-c/test/Ice/slicing/objects/Server.m
@@ -47,6 +47,7 @@ main(int argc, char* argv[])
         {
             ICEInitializationData* initData = [ICEInitializationData initializationData];
             initData.properties = defaultServerProperties(&argc, argv);
+            [initData.properties setProperty:@"Ice.AcceptClassCycles" value:@"1"];
 #if TARGET_OS_IPHONE
             initData.prefixTable_ = [NSDictionary dictionaryWithObjectsAndKeys:
                                       @"TestSlicingObjectsServer", @"::Test",
diff --git a/objective-c/test/Ice/stream/Client.m b/objective-c/test/Ice/stream/Client.m
index b51a3b43f29..8f22ca293a3 100644
--- a/objective-c/test/Ice/stream/Client.m
+++ b/objective-c/test/Ice/stream/Client.m
@@ -932,6 +932,7 @@ main(int argc, char* argv[])
 
             ICEInitializationData* initData = [ICEInitializationData initializationData];
             initData.properties = defaultClientProperties(&argc, argv);
+            [initData.properties setProperty:@"Ice.AcceptClassCycles" value:@"1"];
 #if TARGET_OS_IPHONE
             initData.prefixTable_ = [NSDictionary dictionaryWithObjectsAndKeys:
                                       @"TestStream", @"::Test",
diff --git a/php/src/php5/Communicator.cpp b/php/src/php5/Communicator.cpp
index a7d185e9ccc..41b91656238 100644
--- a/php/src/php5/Communicator.cpp
+++ b/php/src/php5/Communicator.cpp
@@ -1234,6 +1234,14 @@ ZEND_FUNCTION(Ice_initialize)
     initData.compactIdResolver = new IdResolver(TSRMLS_C);
     initData.valueFactoryManager = new ValueFactoryManager;
 
+    if(!initData.properties)
+    {
+        initData.properties = Ice::createProperties();
+    }
+
+    // Always accept cycles in PHP
+    initData.properties->setProperty("Ice.AcceptClassCycles", "1");
+
     CommunicatorInfoIPtr info = initializeCommunicator(return_value, seq, zvargs != 0, initData TSRMLS_CC);
     if(!info)
     {
diff --git a/php/src/php7/Communicator.cpp b/php/src/php7/Communicator.cpp
index 3a0f53ca70b..4143ec65a4c 100644
--- a/php/src/php7/Communicator.cpp
+++ b/php/src/php7/Communicator.cpp
@@ -1245,6 +1245,14 @@ ZEND_FUNCTION(Ice_initialize)
     initData.compactIdResolver = new IdResolver();
     initData.valueFactoryManager = new ValueFactoryManager;
 
+    if(!initData.properties)
+    {
+        initData.properties = Ice::createProperties();
+    }
+
+    // Always accept cycles in PHP
+    initData.properties->setProperty("Ice.AcceptClassCycles", "1");
+
     CommunicatorInfoIPtr info = initializeCommunicator(return_value, seq, zvargs != 0, initData);
     if(!info)
     {
diff --git a/php/test/Ice/objects/Client.php b/php/test/Ice/objects/Client.php
index cc6e62bb838..9355713801b 100644
--- a/php/test/Ice/objects/Client.php
+++ b/php/test/Ice/objects/Client.php
@@ -595,6 +595,23 @@ function allTests($helper)
     }
     echo "ok\n";
 
+    echo "testing sending class cycle...";
+    $rec = new Test_Recursive();
+    $rec->v = $rec;
+    $acceptsCycles = $initial->acceptsClassCycles();
+    try
+    {
+        $initial->setCycle($rec);
+        test($acceptsCycles);
+    }
+    catch(Exception $ex)
+    {
+        $ule = $NS ? "Ice\\UnknownLocalException" : "Ice_UnknownLocalException";
+        test($ex instanceof $ule);
+        test(!$acceptsCycles);
+    }
+    echo "ok\n";
+
     return $initial;
 }
 
diff --git a/php/test/Ice/objects/Test.ice b/php/test/Ice/objects/Test.ice
index 81fc72314b2..cf407b7cf90 100644
--- a/php/test/Ice/objects/Test.ice
+++ b/php/test/Ice/objects/Test.ice
@@ -173,6 +173,9 @@ interface Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/python/modules/IcePy/Communicator.cpp b/python/modules/IcePy/Communicator.cpp
index 7d7931e5f09..6ad3b3c2cf1 100644
--- a/python/modules/IcePy/Communicator.cpp
+++ b/python/modules/IcePy/Communicator.cpp
@@ -286,6 +286,9 @@ communicatorInit(CommunicatorObject* self, PyObject* args, PyObject* /*kwds*/)
 
     data.compactIdResolver = new IdResolver;
 
+    // Always accept cycles in Python
+    data.properties->setProperty("Ice.AcceptClassCycles", "1");
+
     Ice::CommunicatorPtr communicator;
     try
     {
diff --git a/python/test/Ice/admin/AllTests.py b/python/test/Ice/admin/AllTests.py
index 1e8e1cb72d9..a6ac1fcb2f8 100644
--- a/python/test/Ice/admin/AllTests.py
+++ b/python/test/Ice/admin/AllTests.py
@@ -183,12 +183,14 @@ def allTests(helper, communicator):
     # Test: PropertiesAdmin::getProperties()
     #
     pd = pa.getPropertiesForPrefix("")
-    test(len(pd) == 5)
+    test(len(pd) == 6)
     test(pd["Ice.Admin.Endpoints"] == "tcp -h 127.0.0.1")
     test(pd["Ice.Admin.InstanceName"] == "Test")
     test(pd["Prop1"] == "1")
     test(pd["Prop2"] == "2")
     test(pd["Prop3"] == "3")
+    # Ice for Python always sets Ice.AcceptClassCycles
+    test(pd["Ice.AcceptClassCycles"] == "1")
 
     changes = {}
 
diff --git a/python/test/Ice/objects/AllTests.py b/python/test/Ice/objects/AllTests.py
index 13dcef18c27..0cdcbd7d48d 100644
--- a/python/test/Ice/objects/AllTests.py
+++ b/python/test/Ice/objects/AllTests.py
@@ -357,4 +357,16 @@ def allTests(helper, communicator):
         test(f32.f2.ice_getIdentity().name == "F22")
     print("ok")
 
+    sys.stdout.write("testing sending class cycle... ")
+    sys.stdout.flush()
+    rec = Test.Recursive()
+    rec.v = rec
+    acceptsCycles = initial.acceptsClassCycles()
+    try:
+        initial.setCycle(rec)
+        test(acceptsCycles)
+    except Ice.UnknownLocalException:
+        test(not acceptsCycles)
+    print("ok")
+
     return initial
diff --git a/python/test/Ice/objects/Test.ice b/python/test/Ice/objects/Test.ice
index e6e4828ffb9..0acc2fd52a5 100644
--- a/python/test/Ice/objects/Test.ice
+++ b/python/test/Ice/objects/Test.ice
@@ -216,6 +216,9 @@ interface Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/python/test/Ice/objects/TestI.py b/python/test/Ice/objects/TestI.py
index 7bebf279f22..82d868f0143 100644
--- a/python/test/Ice/objects/TestI.py
+++ b/python/test/Ice/objects/TestI.py
@@ -126,6 +126,12 @@ class InitialI(Test.Initial):
     def supportsClassGraphDepthMax(self, current):
         return True
 
+    def setCycle(self, r, current):
+        pass
+
+    def acceptsClassCycles(self, current):
+        return True
+
     def getMB(self, current):
         return Test.Initial.GetMBMarshaledResult(self._b1, current)
 
diff --git a/ruby/src/IceRuby/Communicator.cpp b/ruby/src/IceRuby/Communicator.cpp
index 6050352e5d1..29f4f9e387e 100644
--- a/ruby/src/IceRuby/Communicator.cpp
+++ b/ruby/src/IceRuby/Communicator.cpp
@@ -207,6 +207,9 @@ IceRuby_initialize(int argc, VALUE* argv, VALUE /*self*/)
             data.properties = Ice::createProperties(seq, data.properties);
         }
 
+        // Always accept cycles in Ruby
+        data.properties->setProperty("Ice.AcceptClassCycles", "1");
+
         //
         // Remaining command line options are passed to the communicator
         // as an argument vector in case they contain plugin properties.
diff --git a/ruby/test/Ice/objects/AllTests.rb b/ruby/test/Ice/objects/AllTests.rb
index 65d1faa0fcf..1a01c9b5864 100644
--- a/ruby/test/Ice/objects/AllTests.rb
+++ b/ruby/test/Ice/objects/AllTests.rb
@@ -404,5 +404,19 @@ def allTests(helper, communicator)
         test(f32.f2.ice_getIdentity().name == "F22")
     end
     puts "ok"
+
+    print "testing sending class cycle... "
+    STDOUT.flush
+    rec = Test::Recursive.new
+    rec.v = rec
+    acceptsCycles = initial.acceptsClassCycles()
+    begin
+        initial.setCycle(rec)
+        test(acceptsCycles)
+    rescue Ice::UnknownLocalException => ex
+        test(!acceptsCycles)
+    end
+    puts "ok"
+
     return initial
 end
diff --git a/ruby/test/Ice/objects/Test.ice b/ruby/test/Ice/objects/Test.ice
index b4e0f3fd690..af4ab4786c9 100644
--- a/ruby/test/Ice/objects/Test.ice
+++ b/ruby/test/Ice/objects/Test.ice
@@ -186,6 +186,9 @@ class Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/swift/src/Ice/CommunicatorI.swift b/swift/src/Ice/CommunicatorI.swift
index 13a68d4cce8..0f930e325e3 100644
--- a/swift/src/Ice/CommunicatorI.swift
+++ b/swift/src/Ice/CommunicatorI.swift
@@ -11,6 +11,7 @@ class CommunicatorI: LocalObject<ICECommunicator>, Communicator {
     let initData: InitializationData
     let classGraphDepthMax: Int32
     let traceSlicing: Bool
+    let acceptClassCycles: Bool
 
     init(handle: ICECommunicator, initData: InitializationData) {
         defaultsAndOverrides = DefaultsAndOverrides(handle: handle)
@@ -22,6 +23,7 @@ class CommunicatorI: LocalObject<ICECommunicator>, Communicator {
             classGraphDepthMax = num
         }
         traceSlicing = initData.properties!.getPropertyAsIntWithDefault(key: "Ice.Trace.Slicing", value: 0) > 0
+        acceptClassCycles = initData.properties!.getPropertyAsIntWithDefault(key: "Ice.AcceptClassCycles", value: 0) > 0
 
         super.init(handle: handle)
     }
diff --git a/swift/src/Ice/InputStream.swift b/swift/src/Ice/InputStream.swift
index c92fe9efbf3..57d354869a6 100644
--- a/swift/src/Ice/InputStream.swift
+++ b/swift/src/Ice/InputStream.swift
@@ -14,6 +14,7 @@ public class InputStream {
     private(set) var communicator: Communicator
     private let encoding: EncodingVersion
     private let traceSlicing: Bool
+    fileprivate let acceptClassCycles: Bool
 
     private var encaps: Encaps!
 
@@ -43,6 +44,7 @@ public class InputStream {
         self.encoding = encoding
         classGraphDepthMax = (communicator as! CommunicatorI).classGraphDepthMax
         traceSlicing = (communicator as! CommunicatorI).traceSlicing
+        acceptClassCycles = (communicator as! CommunicatorI).acceptClassCycles
     }
 
     /// Reads an encapsulation from the stream.
@@ -887,7 +889,7 @@ private protocol EncapsDecoder: AnyObject {
     // Encapsulation attributes for value unmarshaling.
     //
     var patchMap: [Int32: [PatchEntry]] { get set }
-    var unmarshaledMap: [Int32: Value] { get set }
+    var unmarshaledMap: [Int32: Value?] { get set }
     var typeIdMap: [Int32: String] { get set }
     var typeIdIndex: Int32 { get set }
     var valueList: [Value] { get set }
@@ -985,10 +987,14 @@ extension EncapsDecoder {
         precondition(index > 0, "invalid index")
 
         //
-        // Check if we have already unmarshalled the instance. If that's the case,
-        // just invoke the callback and we're done.
+        // Check if we already unmarshaled the object. If that's the case, just patch the object smart pointer
+        // and we're done. A null value indicates we've encountered a cycle and Ice.AllowClassCycles is false.
         //
-        if let obj: Value = unmarshaledMap[index] {
+        if let optObj = unmarshaledMap[index] {
+            guard let obj = optObj else {
+                assert(!stream.acceptClassCycles)
+                throw MarshalException(reason: "cycle detected during Value unmarshaling")
+            }
             try cb(obj)
             return
         }
@@ -1008,7 +1014,11 @@ extension EncapsDecoder {
         // Add the instance to the map of unmarshaled instances, this must
         // be done before reading the instances (for circular references).
         //
-        unmarshaledMap[index] = v
+        // If circular references are not allowed we insert null (for cycle detection) and add
+        // the object to the map once it has been fully unmarshaled.
+        //
+        unmarshaledMap[index] = stream.acceptClassCycles ? v : (nil as Value?)
+        assert(unmarshaledMap[index] != nil)
 
         //
         // Read the instance.
@@ -1053,6 +1063,12 @@ extension EncapsDecoder {
                 valueList.removeAll()
             }
         }
+
+        if !stream.acceptClassCycles {
+            // This class has been fully unmarshaled without creating any cycles
+            // It can be added to the map now.
+            unmarshaledMap[index] = v
+        }
     }
 }
 
@@ -1061,7 +1077,7 @@ private class EncapsDecoder10: EncapsDecoder {
     unowned let stream: InputStream
     let valueFactoryManager: ValueFactoryManager
     lazy var patchMap = [Int32: [PatchEntry]]()
-    lazy var unmarshaledMap = [Int32: Value]()
+    lazy var unmarshaledMap = [Int32: Value?]()
     lazy var typeIdMap = [Int32: String]()
     var typeIdIndex: Int32 = 0
     lazy var valueList = [Value]()
@@ -1325,7 +1341,7 @@ private class EncapsDecoder11: EncapsDecoder {
     unowned let stream: InputStream
     let valueFactoryManager: ValueFactoryManager
     lazy var patchMap = [Int32: [PatchEntry]]()
-    lazy var unmarshaledMap = [Int32: Value]()
+    lazy var unmarshaledMap = [Int32: Value?]()
     lazy var typeIdMap = [Int32: String]()
     var typeIdIndex: Int32 = 0
     lazy var valueList = [Value]()
diff --git a/swift/test/Ice/objects/AllTests.swift b/swift/test/Ice/objects/AllTests.swift
index 393b9bf6031..9efa1f8fa1e 100644
--- a/swift/test/Ice/objects/AllTests.swift
+++ b/swift/test/Ice/objects/AllTests.swift
@@ -313,5 +313,20 @@ func allTests(_ helper: TestHelper) throws -> InitialPrx {
     }
     output.writeLine("ok")
 
+    output.write("testing sending class cycle... ")
+    do {
+        let rec = Recursive(v: nil)
+        rec.v = rec
+        let acceptsCycles = try initial.acceptsClassCycles()
+        do {
+            try initial.setCycle(rec)
+            try test(acceptsCycles)
+        } catch is Ice.UnknownLocalException {
+            try test(!acceptsCycles)
+        }
+        rec.v = nil
+    }
+    output.writeLine("ok")
+
     return initial
 }
diff --git a/swift/test/Ice/objects/Client.swift b/swift/test/Ice/objects/Client.swift
index 7fd5decd901..5f242a0b70e 100644
--- a/swift/test/Ice/objects/Client.swift
+++ b/swift/test/Ice/objects/Client.swift
@@ -9,6 +9,7 @@ import TestCommon
 public class Client: TestHelperI {
     public override func run(args: [String]) throws {
         let properties = try createTestProperties(args)
+        properties.setProperty(key: "Ice.AcceptClassCycles", value: "1")
         var initData = InitializationData()
         initData.properties = properties
         initData.classResolverPrefix = ["IceObjects"]
diff --git a/swift/test/Ice/objects/Collocated.swift b/swift/test/Ice/objects/Collocated.swift
index 611bc9745da..1a8f85e70b5 100644
--- a/swift/test/Ice/objects/Collocated.swift
+++ b/swift/test/Ice/objects/Collocated.swift
@@ -8,6 +8,7 @@ import TestCommon
 class Collocated: TestHelperI {
     public override func run(args: [String]) throws {
         let properties = try createTestProperties(args)
+        properties.setProperty(key: "Ice.AcceptClassCycles", value: "1")
         properties.setProperty(key: "Ice.Warn.Dispatch", value: "0")
         var initData = Ice.InitializationData()
         initData.properties = properties
diff --git a/swift/test/Ice/objects/Test.ice b/swift/test/Ice/objects/Test.ice
index f7e681f216b..db85cb340c0 100644
--- a/swift/test/Ice/objects/Test.ice
+++ b/swift/test/Ice/objects/Test.ice
@@ -222,6 +222,9 @@ interface Initial
     void setRecursive(Recursive p);
     bool supportsClassGraphDepthMax();
 
+    void setCycle(Recursive r);
+    bool acceptsClassCycles();
+
     ["marshaled-result"] B getMB();
     ["amd", "marshaled-result"] B getAMDMB();
 
diff --git a/swift/test/Ice/objects/TestI.swift b/swift/test/Ice/objects/TestI.swift
index 31af1d0c95a..f7e5cb73bae 100644
--- a/swift/test/Ice/objects/TestI.swift
+++ b/swift/test/Ice/objects/TestI.swift
@@ -174,6 +174,18 @@ class InitialI: Initial {
         return true
     }
 
+    func setCycle(r: Recursive?, current _: Ice.Current) {
+        precondition(r != nil)
+        precondition(r!.v === r)
+        // break the cycle
+        r!.v = nil
+    }
+
+    func acceptsClassCycles(current: Ice.Current) throws -> Bool {
+        let properties = current.adapter!.getCommunicator().getProperties()
+        return properties.getPropertyAsIntWithDefault(key: "Ice.AcceptClassCycles", value: 0) > 0
+    }
+
     func getD1(d1: D1?, current _: Ice.Current) throws -> D1? {
         return d1
     }
diff --git a/swift/test/Ice/slicing/objects/Client.swift b/swift/test/Ice/slicing/objects/Client.swift
index 580a37a371e..203d07afabb 100644
--- a/swift/test/Ice/slicing/objects/Client.swift
+++ b/swift/test/Ice/slicing/objects/Client.swift
@@ -34,8 +34,10 @@ class PNodeI: PNode {
 
 public class Client: TestHelperI {
     public override func run(args: [String]) throws {
+        let properties = try createTestProperties(args)
+        properties.setProperty(key: "Ice.AcceptClassCycles", value: "1")
         var initData = InitializationData()
-        initData.properties = try createTestProperties(args)
+        initData.properties = properties
         initData.classResolverPrefix = ["IceSlicingObjects", "IceSlicingObjectsClient"]
         let communicator = try initialize(initData)
         defer {
diff --git a/swift/test/Ice/slicing/objects/Server.swift b/swift/test/Ice/slicing/objects/Server.swift
index faf8d5c6037..8d89d0bc01f 100644
--- a/swift/test/Ice/slicing/objects/Server.swift
+++ b/swift/test/Ice/slicing/objects/Server.swift
@@ -8,6 +8,7 @@ import TestCommon
 class Server: TestHelperI {
     public override func run(args: [String]) throws {
         let properties = try createTestProperties(args)
+        properties.setProperty(key: "Ice.AcceptClassCycles", value: "1")
         properties.setProperty(key: "Ice.Warn.Dispatch", value: "0")
         var initData = InitializationData()
         initData.properties = properties
diff --git a/swift/test/Ice/slicing/objects/ServerAMD.swift b/swift/test/Ice/slicing/objects/ServerAMD.swift
index dd2fcdae22b..d9d87fad95e 100644
--- a/swift/test/Ice/slicing/objects/ServerAMD.swift
+++ b/swift/test/Ice/slicing/objects/ServerAMD.swift
@@ -8,6 +8,7 @@ import TestCommon
 class ServerAMD: TestHelperI {
     public override func run(args: [String]) throws {
         let properties = try createTestProperties(args)
+        properties.setProperty(key: "Ice.AcceptClassCycles", value: "1")
         properties.setProperty(key: "Ice.Warn.Dispatch", value: "0")
         var initData = InitializationData()
         initData.properties = properties
diff --git a/swift/test/Ice/stream/Client.swift b/swift/test/Ice/stream/Client.swift
index 6a5bce8776e..a84378d8204 100644
--- a/swift/test/Ice/stream/Client.swift
+++ b/swift/test/Ice/stream/Client.swift
@@ -11,8 +11,10 @@ public class Client: TestHelperI {
         let writer = getWriter()
         writer.write("testing primitive types... ")
 
+        let properties = try createTestProperties(args)
+        properties.setProperty(key: "Ice.AcceptClassCycles", value: "1")
         var initData = Ice.InitializationData()
-        initData.properties = try createTestProperties(args)
+        initData.properties = properties
         initData.classResolverPrefix = ["IceStrem"]
         let communicator = try initialize(initData)
         defer {