Make 129-ThreadGetId more robust.

Make sure we get the heap task daemon, to avoid a race with the runtime
creating the stack trace, and then updating the peer.

test: 129-ThreadGetId
bug: 28261069
Change-Id: I739ab6cd0180e2be07b7cecac6ad8a905a7c9cd0

1 files changed, 27 insertions, 2 deletions

diff --git a/test/129-ThreadGetId/src/Main.java b/test/129-ThreadGetId/src/Main.java
index 5aefd17f0e..6ba01ff0b5 100644
--- a/test/129-ThreadGetId/src/Main.java
+++ b/test/129-ThreadGetId/src/Main.java
@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import java.lang.reflect.Field;
 import java.util.Map;
 
 public class Main implements Runnable {
@@ -33,14 +34,38 @@ public class Main implements Runnable {
         System.out.println("Finishing");
     }
 
-    static void test_getStackTraces() {
+    static Thread getHeapTaskDaemon() throws Exception {
+        Field f = ThreadGroup.class.getDeclaredField("systemThreadGroup");
+        f.setAccessible(true);
+        ThreadGroup systemThreadGroup = (ThreadGroup) f.get(null);
+
+        while (true) {
+            int activeCount = systemThreadGroup.activeCount();
+            Thread[] array = new Thread[activeCount];
+            systemThreadGroup.enumerate(array);
+            for (Thread thread : array) {
+               if (thread.getName().equals("HeapTaskDaemon") &&
+                   thread.getState() != Thread.State.NEW) {
+                  return thread;
+                }
+            }
+            // Yield to eventually get the daemon started.
+            Thread.sleep(10);
+        }
+    }
+
+    static void test_getStackTraces() throws Exception {
+        Thread heapDaemon = getHeapTaskDaemon();
+
+        // Force a GC to ensure the daemon truly started.
+        Runtime.getRuntime().gc();
         // Check all the current threads for positive IDs.
         Map<Thread, StackTraceElement[]> map = Thread.getAllStackTraces();
         for (Map.Entry<Thread, StackTraceElement[]> pair : map.entrySet()) {
             Thread thread = pair.getKey();
             // Expect empty stack trace since we do not support suspending the GC thread for
             // obtaining stack traces. See b/28261069.
-            if (thread.getName().equals("HeapTaskDaemon")) {
+            if (thread == heapDaemon) {
                 System.out.println(thread.getName() + " depth " + pair.getValue().length); 
             }
         }