URLClassLoader support for PluginFirstClassLoader (#5970)

Co-authored-by: Jesse Glick <jglick@cloudbees.com>

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

@@ -0,0 +1,74 @@
+package hudson.util;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import org.junit.Test;
+
+public class CompoundEnumerationTest {
+
+    @Test
+    public void smokes() {
+        assertEquals(
+                rangeClosed(1, 12),
+                Collections.list(
+                        new CompoundEnumeration<>(
+                                Collections.enumeration(rangeClosed(1, 4)),
+                                Collections.enumeration(rangeClosed(5, 8)),
+                                Collections.enumeration(rangeClosed(9, 12)))));
+    }
+
+    @Test
+    public void empty() {
+        assertEquals(Collections.emptyList(), Collections.list(new CompoundEnumeration<>()));
+        assertEquals(
+                Collections.emptyList(),
+                Collections.list(new CompoundEnumeration<>(Collections.emptyEnumeration())));
+        assertEquals(
+                Collections.emptyList(),
+                Collections.list(
+                        new CompoundEnumeration<>(
+                                Collections.emptyEnumeration(), Collections.emptyEnumeration())));
+        assertEquals(
+                Collections.emptyList(),
+                Collections.list(
+                        new CompoundEnumeration<>(
+                                Collections.emptyEnumeration(),
+                                Collections.emptyEnumeration(),
+                                Collections.emptyEnumeration())));
+    }
+
+    @Test
+    public void gaps() {
+        assertEquals(
+                rangeClosed(1, 8),
+                Collections.list(
+                        new CompoundEnumeration<>(
+                                Collections.emptyEnumeration(),
+                                Collections.enumeration(rangeClosed(1, 4)),
+                                Collections.enumeration(rangeClosed(5, 8)))));
+        assertEquals(
+                rangeClosed(1, 8),
+                Collections.list(
+                        new CompoundEnumeration<>(
+                                Collections.enumeration(rangeClosed(1, 4)),
+                                Collections.emptyEnumeration(),
+                                Collections.enumeration(rangeClosed(5, 8)))));
+        assertEquals(
+                rangeClosed(1, 8),
+                Collections.list(
+                        new CompoundEnumeration<>(
+                                Collections.enumeration(rangeClosed(1, 4)),
+                                Collections.enumeration(rangeClosed(5, 8)),
+                                Collections.emptyEnumeration())));
+    }
+
+    private static List<Integer> rangeClosed(int startInclusive, int endInclusive) {
+        return IntStream.rangeClosed(startInclusive, endInclusive)
+                .boxed()
+                .collect(Collectors.toList());
+    }
+}