[JENKINS-49054] Work around XStream bug by looking up list type before trying to unmarshal elements

core/src/main/java/hudson/util/DescribableList.java

@@ -270,10 +270,9 @@ public void marshal(Object source, HierarchicalStreamWriter writer, MarshallingC
         }
 
         public Object unmarshal(HierarchicalStreamReader reader, UnmarshallingContext context) {
-            CopyOnWriteList core = copyOnWriteListConverter.unmarshal(reader, context);
-
             try {
-                DescribableList r = (DescribableList)context.getRequiredType().newInstance();
+                DescribableList r = (DescribableList) context.getRequiredType().asSubclass(DescribableList.class).newInstance();
+                CopyOnWriteList core = copyOnWriteListConverter.unmarshal(reader, context);
                 r.data.replaceBy(core);
                 return r;
             } catch (InstantiationException e) {

core/src/test/java/hudson/util/DescribableListTest.java

@@ -0,0 +1,99 @@
+/*
+ * The MIT License
+ *
+ * Copyright 2018 CloudBees, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package hudson.util;
+
+import com.thoughtworks.xstream.converters.basic.AbstractSingleValueConverter;
+import hudson.model.Describable;
+import hudson.model.Descriptor;
+import org.junit.Test;
+import static org.junit.Assert.*;
+import org.jvnet.hudson.test.Issue;
+
+public class DescribableListTest {
+
+    @Issue("JENKINS-49054")
+    @Test
+    public void exceptionDuringUnmarshalling() throws Exception {
+        Data data = new Data();
+        data.list.add(new Datum(1));
+        data.list.add(new Datum(2));
+        data.list.add(new Datum(3));
+        XStream2 xs = new XStream2();
+        xs.addCriticalField(Data.class, "list");
+        String xml = xs.toXML(data);
+        data = (Data) xs.fromXML(xml);
+        assertEquals("[1, 3]", data.toString());
+    }
+
+    private static final class Data {
+
+        final DescribableList<Datum,Descriptor<Datum>> list = new DescribableList<>();
+
+        @Override
+        public String toString() {
+            return list.toString();
+        }
+
+    }
+
+    private static final class Datum implements Describable<Datum> {
+
+        final int val;
+
+        Datum(int val) {
+            this.val = val;
+        }
+
+        @Override
+        public Descriptor<Datum> getDescriptor() {
+            return new Descriptor<Datum>(Datum.class) {};
+        }
+
+        @Override
+        public String toString() {
+            return Integer.toString(val);
+        }
+
+        public static final class ConverterImpl extends AbstractSingleValueConverter {
+
+            @Override
+            public boolean canConvert(Class type) {
+                return type == Datum.class;
+            }
+
+            @Override
+            public Object fromString(String str) {
+                int val = Integer.parseInt(str);
+                if (val == 2) {
+                    throw new IllegalStateException("oops");
+                }
+                return new Datum(val);
+            }
+
+        }
+
+    }
+
+}