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.

This reverts commit 6505c77501f1924571b2fe620c5c7b31ede0cd22.

2 files changed, 53 insertions, 2 deletions

diff --git a/test/fiber/scheduler.rb b/test/fiber/scheduler.rb
index 0553e386f5..df82c2ff93 100644
--- a/test/fiber/scheduler.rb
+++ b/test/fiber/scheduler.rb
@@ -100,8 +100,10 @@ class Scheduler
 
     self.run
   ensure
-    @urgent.each(&:close)
-    @urgent = nil
+    if @urgent
+      @urgent.each(&:close)
+      @urgent = nil
+    end
 
     @closed = true
 
@@ -196,3 +198,13 @@ 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
diff --git a/test/fiber/test_thread.rb b/test/fiber/test_thread.rb
index 843604b5f1..5c25c43de2 100644
--- a/test/fiber/test_thread.rb
+++ b/test/fiber/test_thread.rb
@@ -20,6 +20,31 @@ class TestFiberThread < Test::Unit::TestCase
     assert_equal :done, thread.value
   end
 
+  def test_thread_join_implicit
+    sleeping = false
+    finished = false
+
+    thread = Thread.new do
+      scheduler = Scheduler.new
+      Fiber.set_scheduler scheduler
+
+      Fiber.schedule do
+        sleeping = true
+        sleep(0.1)
+        finished = true
+      end
+
+      :done
+    end
+
+    Thread.pass until sleeping
+
+    thread.join
+
+    assert_equal :done, thread.value
+    assert finished, "Scheduler thread's task should be finished!"
+  end
+
   def test_thread_join_blocking
     thread = Thread.new do
       scheduler = Scheduler.new
@@ -66,4 +91,18 @@ 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