Fix #437

Directory.Build.props

@@ -71,6 +71,7 @@
     <PlatformTarget>AnyCPU</PlatformTarget>
     <Prefer32Bit>false</Prefer32Bit>
     <RunCodeAnalysis Condition="'$(Configuration)' == 'FxCop'">true</RunCodeAnalysis>
+    <LangVersion>latest</LangVersion>
   </PropertyGroup>
 
   <!-- Release -->

Src/IronPython.Modules/_weakref.cs

@@ -322,15 +322,15 @@ private weakproxy(CodeContext/*!*/ context, object target, object callback) {
             /// <summary>
             /// gets the object or throws a reference exception
             /// </summary>
-            object GetObject() {
+            private object GetObject() {
                 object res;
                 if (!TryGetObject(out res)) {
                     throw PythonOps.ReferenceError("weakly referenced object no longer exists");
                 }
                 return res;
             }
 
-            bool TryGetObject(out object result) {
+            private bool TryGetObject(out object result) {
                 result = _target.Target;
                 if (result == null) return false;
                 GC.KeepAlive(this);
@@ -754,7 +754,7 @@ public object __nonzero__() {
             }
         }
 
-        static class WeakRefHelpers {
+        private static class WeakRefHelpers {
             public static WeakRefTracker InitializeWeakRef(PythonContext context, object self, object target, object callback) {
                 IWeakReferenceable iwr = ConvertToWeakReferenceable(context, target);
 
@@ -764,14 +764,17 @@ public static WeakRefTracker InitializeWeakRef(PythonContext context, object sel
                         throw PythonOps.TypeError("cannot create weak reference to '{0}' object", PythonOps.GetPythonTypeName(target));
                 }
 
-                wrt.ChainCallback(callback,self);
+                if (callback != null || !wrt.Contains(callback, self)) {
+                    wrt.ChainCallback(callback, self);
+                }
+
                 return wrt;
             }
         }
     }
 
     [PythonType("wrapper_descriptor")]
-    class SlotWrapper : PythonTypeSlot, ICodeFormattable {
+    internal class SlotWrapper : PythonTypeSlot, ICodeFormattable {
         private readonly string _name;
         private readonly PythonType _type;
 
@@ -817,8 +820,8 @@ internal override bool TryGetValue(CodeContext context, object instance, PythonT
 
     [PythonType("method-wrapper")]
     public class GenericMethodWrapper {
-        string name;
-        IProxyObject target;
+        private readonly string name;
+        private readonly IProxyObject target;
 
         public GenericMethodWrapper(string methodName, IProxyObject proxyTarget) {
             name = methodName;

Src/IronPython/Runtime/WeakRef.cs

@@ -6,14 +6,12 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Dynamic;
+using System.Linq;
 using System.Threading;
 
-using Microsoft.Scripting.Actions;
 using Microsoft.Scripting.Runtime;
 using Microsoft.Scripting.Utils;
 
-using IronPython.Runtime.Binding;
 using IronPython.Runtime.Operations;
 
 namespace IronPython.Runtime {
@@ -25,10 +23,10 @@ namespace IronPython.Runtime {
     /// is held in objects that implement IWeakReferenceable.
     /// </summary>
     public class WeakRefTracker {
-        struct CallbackInfo {
-            readonly object _callback;
-            readonly WeakHandle _longRef;
-            readonly WeakHandle _shortRef;
+        private readonly struct CallbackInfo {
+            private readonly object _callback;
+            private readonly WeakHandle _longRef;
+            private readonly WeakHandle _shortRef;
 
             public CallbackInfo(object callback, object weakRef) {
                 _callback = callback;
@@ -127,6 +125,15 @@ public void RemoveHandler(object o) {
 
         }
 
+        internal bool Contains(object callback, object weakref) {
+            _lock.EnterReadLock();
+            try {
+                return _callbacks.Any(o => o.Callback == callback && o.WeakRef == weakref);
+            } finally {
+                _lock.ExitReadLock();
+            }
+        }
+
         public object GetHandlerCallback(int index) {
             _lock.EnterReadLock();
             try {

Tests/test_regressions.py

@@ -1168,6 +1168,14 @@ def test_ipy2_gh25(self):
         else:
             self.assertTrue(hasattr(os, 'startfile'))
 
+    def test_ipy2_gh437(self):
+        """https://github.com/IronLanguages/ironpython2/issues/437"""
+        import weakref
+        class SomeWeakReferenceableObject(object): pass
+
+        o = SomeWeakReferenceableObject()
+        x = [weakref.ref(o) for i in range(10)]
+        self.assertEqual(weakref.getweakrefcount(o), 1)
 
 
 run_test(__name__)