Fix potential hang when joining threads.

If the thread termination invokes user code after `th->status` becomes
`THREAD_KILLED`, and the user unblock function causes that `th->status` to
become something else (e.g. `THREAD_RUNNING`), threads waiting in
`thread_join_sleep` will hang forever. We move the unblock function call
to before the thread status is updated, and allow threads to join as soon
as `th->value` becomes defined.

test/fiber/scheduler.rb

@@ -112,8 +112,10 @@ class Scheduler
 
     self.run
   ensure
-    @urgent.each(&:close)
-    @urgent = nil
+    if @urgent
+      @urgent.each(&:close)
+      @urgent = nil
+    end
 
     @closed = true
 
@@ -240,3 +242,13 @@ class BrokenUnblockScheduler < Scheduler
     raise "Broken unblock!"
   end
 end
+
+class SleepingUnblockScheduler < Scheduler
+  # This method is invoked when the thread is exiting.
+  def unblock(blocker, fiber)
+    super
+
+    # This changes the current thread state to `THREAD_RUNNING` which causes `thread_join_sleep` to hang.
+    sleep(0.1)
+  end
+end

test/fiber/test_thread.rb

@@ -66,4 +66,18 @@ class TestFiberThread < Test::Unit::TestCase
       thread.join
     end
   end
+
+  def test_thread_join_hang
+    thread = Thread.new do
+      scheduler = SleepingUnblockScheduler.new
+
+      Fiber.set_scheduler scheduler
+
+      Fiber.schedule do
+        Thread.new{sleep(0.01)}.value
+      end
+    end
+
+    thread.join
+  end
 end