VMProcess: Poll faster for reapable chidren if there are threads waiting

If there are threads currently waiting in waitFor() or destroy(),
have the reaper thread switch to a shorter delay when polling for
reapable children.

By default, set this "fast polling" delay to 100 ms. This can be
controlled via the gnu.lang.process.fastPolling system property.

Closes #23 (BZ#111587)

Author: guillerodriguez

vm/reference/java/lang/VMProcess.java

@@ -79,6 +79,9 @@ final class VMProcess extends Process
   // New processes waiting to be spawned by processThread.
   static final LinkedList workList = new LinkedList();
 
+  // Number of threads waiting in waitFor() or destroy()
+  static int waiters;
+
   // Return values set by nativeReap() when a child is reaped.
   // These are only accessed by processThread so no locking required.
   static long reapedPid;
@@ -109,6 +112,18 @@ private static class ProcessThread extends Thread
     // Max time (in ms) we'll delay before trying to reap another child.
     private static final int MAX_REAP_DELAY = 1000;
 
+    // Default polling delay (in ms) in "fast polling mode", i.e. when
+    // there are threads waiting in waitFor() or destroy().
+    // Can be changed using the gnu.lang.process.fastPolling system property.
+    private static final int DEFAULT_FAST_POLLING_DELAY = 100;
+
+    // Actual polling delay in fast polling mode
+    private static final int fastPollingDelay;
+    static
+    {
+      fastPollingDelay = Integer.getInteger("gnu.lang.process.fastPolling", DEFAULT_FAST_POLLING_DELAY);
+    }
+
     // Processes created but not yet terminated; maps Long(pid) -> VMProcess
     // Only used in run() and spawn() method from this Thread, so no locking.
     private final HashMap activeMap = new HashMap();
@@ -180,7 +195,7 @@ public void run()
 
               try
                 {
-                  workList.wait(MAX_REAP_DELAY);
+                  workList.wait(waiters > 0 ? fastPollingDelay : MAX_REAP_DELAY);
                 }
               catch (InterruptedException e)
                 {
@@ -330,15 +345,35 @@ public InputStream getErrorStream()
     return stderr;
   }
 
-  public synchronized int waitFor() throws InterruptedException
+  private void addWaiter()
   {
     synchronized (workList)
       {
+        waiters++;
         workList.notify();
       }
+  }
+
+  private void removeWaiter()
+  {
+    synchronized (workList)
+      {
+        waiters--;
+      }
+  }
 
-    while (state != TERMINATED)
-      wait();
+  public synchronized int waitFor() throws InterruptedException
+  {
+    try
+      {
+        addWaiter();
+        while (state != TERMINATED)
+          wait();
+      }
+    finally
+      {
+        removeWaiter();
+      }
     return exitValue;
   }
 
@@ -356,12 +391,9 @@ public synchronized void destroy()
 
     nativeKill(pid);
 
-    synchronized (workList)
-      {
-        workList.notify();
-      }
-
+    addWaiter();
     waitForStateUninterruptibly(TERMINATED, false);
+    removeWaiter();
   }
 
   private void waitForStateUninterruptibly(int state, boolean eq)