8284165: Add pid to process reaper thread name

Reviewed-by: rriggs

src/java.base/share/classes/java/lang/ProcessHandleImpl.java

@@ -140,6 +140,9 @@ final class ProcessHandleImpl implements ProcessHandle {
                 processReaperExecutor.execute(new Runnable() {
                     // Use inner class to avoid lambda stack overhead
                     public void run() {
+                        String threadName = Thread.currentThread().getName();
+                        Thread.currentThread().setName("process reaper (pid " + pid + ")");
+                        try {
                             int exitValue = waitForProcessExit0(pid, shouldReap);
                             if (exitValue == NOT_A_CHILD) {
                                 // pid not alive or not a child of this process
@@ -167,6 +170,10 @@ final class ProcessHandleImpl implements ProcessHandle {
                             newCompletion.complete(exitValue);
                             // remove from cache afterwards
                             completions.remove(pid, newCompletion);
+                        } finally {
+                            // Restore thread name
+                            Thread.currentThread().setName(threadName);
+                        }
                     }
                 });
             }

test/jdk/java/lang/ProcessBuilder/ProcessReaperCCL.java

@@ -57,7 +57,7 @@ public class ProcessReaperCCL {
 
         // Verify all "Process Reaper" threads have a null CCL
         for (Thread th : Thread.getAllStackTraces().keySet()) {
-            if ("process reaper".equals(th.getName())) {
+            if (th.getName().startsWith("process reaper")) {
                 Assert.assertEquals(th.getContextClassLoader(), null, "CCL not null");
             }
         }

test/jdk/java/util/concurrent/Phaser/Basic.java

@@ -440,7 +440,7 @@ public class Basic {
             if ("Finalizer".equals(name)
                 && info.getLockName().startsWith("java.lang.ref.ReferenceQueue$Lock"))
                 continue;
-            if ("process reaper".equals(name))
+            if (name.startsWith("process reaper"))
                 continue;
             if (name != null && name.startsWith("ForkJoinPool.commonPool-worker"))
                 continue;