Fix [JENKINS-49206] by ensuring Timer threads get standard classloader (#3272)

* Fix [JENKINS-49206] by ensuring Timer threads get standard classloader

* Refactor to better fit in with existing ThreadFactor utils

* Fix javadocs

core/src/test/java/jenkins/util/TimerTest.java

@@ -1,12 +1,18 @@
 package jenkins.util;
 
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 
+import groovy.lang.GroovyClassLoader;
 import hudson.triggers.SafeTimerTask;
+import org.junit.Assert;
 import org.junit.Test;
 import org.jvnet.hudson.test.Issue;
 
+import java.net.URLClassLoader;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 
 public class TimerTest {
@@ -44,4 +50,52 @@ public class TimerTest {
         }
 
     }
+
+    /**
+     * Launch two tasks which can only complete
+     * by running doRun() concurrently.
+     */
+    @Test
+    @Issue("JENKINS-49206")
+    public void timerBogusClassloader() throws Exception {
+        final int threadCount = 10;  // Twice Timer pool size to ensure we end up creating at least one new thread
+        final CountDownLatch startLatch = new CountDownLatch(threadCount);
+
+        final ClassLoader[] contextClassloaders = new ClassLoader[threadCount];
+        ScheduledFuture[] futures = new ScheduledFuture[threadCount];
+        final ClassLoader bogusClassloader = new GroovyClassLoader();
+
+        Runnable timerTest = new Runnable() {
+            @Override
+            public void run() {
+                ClassLoader cl = Thread.currentThread().getContextClassLoader();
+                Thread.currentThread().setContextClassLoader(bogusClassloader);
+                ScheduledExecutorService exec = Timer.get();
+                for (int i=0; i<threadCount; i++) {
+                    final int j = i;
+                    futures[j] = exec.schedule(new Runnable() {
+                        @Override
+                        public void run() {
+                            try {
+                                startLatch.countDown();
+                                contextClassloaders[j] = Thread.currentThread().getContextClassLoader();
+                            } catch (Exception ex) {
+                                throw  new RuntimeException(ex);
+                            }
+                        }
+                    }, 0, TimeUnit.SECONDS);
+                }
+                Thread.currentThread().setContextClassLoader(cl);
+            }
+        };
+
+        Thread t = new Thread(timerTest);
+        t.run();
+        t.join(1000L);
+
+        for (int i=0; i<threadCount; i++) {
+            futures[i].get();
+            Assert.assertEquals(Timer.class.getClassLoader(), contextClassloaders[i]);
+        }
+    }
 }