clean up OS resources immediately upon thread exit

Previously, we waited until the next GC to do this, but that can be
too long for workloads which create a lot of short-lived threads but
don't do much allocation.

src/machine.cpp

@@ -45,7 +45,11 @@ void
 join(Thread* t, Thread* o)
 {
   if (t != o) {
-    o->systemThread->join();
+    if (o->state != Thread::ZombieState
+        and o->state != Thread::JoinedState)
+    {
+      o->systemThread->join();
+    }
     o->state = Thread::JoinedState;
   }
 }
@@ -2281,6 +2285,12 @@ Thread::exit()
     } else {
       threadPeer(this, javaThread) = 0;
       enter(this, Thread::ZombieState);
+
+      lock->dispose();
+      lock = 0;
+
+      systemThread->dispose();
+      systemThread = 0;
     }
   }
 }
@@ -2288,7 +2298,9 @@ Thread::exit()
 void
 Thread::dispose()
 {
-  lock->dispose();
+  if (lock) {
+    lock->dispose();
+  }
 
   if (systemThread) {
     systemThread->dispose();

src/machine.h

@@ -2746,7 +2746,11 @@ notifyAll(Thread* t, object o)
 inline void
 interrupt(Thread*, Thread* target)
 {
-  target->systemThread->interrupt();
+  if (target->state != Thread::ZombieState
+      and target->state != Thread::JoinedState)
+  {
+    target->systemThread->interrupt();
+  }
 }
 
 inline void