fixed reference counting for exception objects in Py.With (#1062)

lostmsu · web-flow · commit 6c0324525f96 · 2020-02-26T11:47:24.000+01:00
PyObject(s) constructed for __exit__ method referenced existing Python objects without increasing refcount appropriately, which could lead to double-free.
diff --git a/src/runtime/Util.cs b/src/runtime/Util.cs
@@ -3,7 +3,7 @@
 
 namespace Python.Runtime
 {
-    internal class Util
+    internal static class Util
     {
         internal static Int64 ReadCLong(IntPtr tp, int offset)
         {
@@ -29,5 +29,12 @@ internal static void WriteCLong(IntPtr type, int offset, Int64 flags)
                 Marshal.WriteInt64(type, offset, flags);
             }
         }
+
+        /// <summary>
+        /// Null-coalesce: if <paramref name="primary"/> parameter is not
+        /// <see cref="IntPtr.Zero"/>, return it. Otherwise return <paramref name="fallback"/>.
+        /// </summary>
+        internal static IntPtr Coalesce(this IntPtr primary, IntPtr fallback)
+            => primary == IntPtr.Zero ? fallback : primary;
     }
-}
+}
diff --git a/src/runtime/pythonengine.cs b/src/runtime/pythonengine.cs
@@ -758,11 +758,14 @@ public static void With(PyObject obj, Action<dynamic> Body)
             catch (PythonException e)
             {
                 ex = e;
-                type = ex.PyType;
-                val = ex.PyValue;
-                traceBack = ex.PyTB;
+                type = ex.PyType.Coalesce(type);
+                val = ex.PyValue.Coalesce(val);
+                traceBack = ex.PyTB.Coalesce(traceBack);
             }
 
+            Runtime.XIncref(type);
+            Runtime.XIncref(val);
+            Runtime.XIncref(traceBack);
             var exitResult = obj.InvokeMethod("__exit__", new PyObject(type), new PyObject(val), new PyObject(traceBack));
 
             if (ex != null && !exitResult.IsTrue()) throw ex;

Original file line number	Diff line number	Diff line change
`@@ -3,7 +3,7 @@`
`3`	`3`
`4`	`4`	`namespace Python.Runtime`
`5`	`5`	`{`
`6`		`- internal class Util`
	`6`	`+ internal static class Util`
`7`	`7`	`{`
`8`	`8`	`internal static Int64 ReadCLong(IntPtr tp, int offset)`
`9`	`9`	`{`
`@@ -29,5 +29,12 @@ internal static void WriteCLong(IntPtr type, int offset, Int64 flags)`
`29`	`29`	`Marshal.WriteInt64(type, offset, flags);`
`30`	`30`	`}`
`31`	`31`	`}`
	`32`	`+`
	`33`	`+ /// <summary>`
	`34`	`+ /// Null-coalesce: if <paramref name="primary"/> parameter is not`
	`35`	`+ /// <see cref="IntPtr.Zero"/>, return it. Otherwise return <paramref name="fallback"/>.`
	`36`	`+ /// </summary>`
	`37`	`+ internal static IntPtr Coalesce(this IntPtr primary, IntPtr fallback)`
	`38`	`+ => primary == IntPtr.Zero ? fallback : primary;`
`32`	`39`	`}`
`33`		`-}`
	`40`	`+}`