Avoid missed wakeup with fiber scheduler and Fiber.blocking. (#6588)

* Ensure that blocked fibers don't prevent valid wakeups.

2 files changed, 53 insertions, 0 deletions

diff --git a/test/fiber/scheduler.rb b/test/fiber/scheduler.rb
index 3fd41ef6f1..204a297133 100644
--- a/test/fiber/scheduler.rb
+++ b/test/fiber/scheduler.rb
@@ -350,3 +350,14 @@ class SleepingUnblockScheduler < Scheduler
     sleep(0.1)
   end
 end
+
+class SleepingBlockingScheduler < Scheduler
+  def kernel_sleep(duration = nil)
+    # Deliberaly sleep in a blocking state which can trigger a deadlock if the implementation is not correct.
+    Fiber.blocking{sleep 0.0001}
+
+    self.block(:sleep, duration)
+
+    return true
+  end
+end
diff --git a/test/fiber/test_scheduler.rb b/test/fiber/test_scheduler.rb
index 5a24bff04f..300d30ad63 100644
--- a/test/fiber/test_scheduler.rb
+++ b/test/fiber/test_scheduler.rb
@@ -138,4 +138,46 @@ class TestFiberScheduler < Test::Unit::TestCase
       Object.send(:remove_const, :TestFiberSchedulerAutoload)
     end
   end
+
+  def test_deadlock
+    mutex = Thread::Mutex.new
+    condition = Thread::ConditionVariable.new
+    q = 0.0001
+
+    signaller = Thread.new do
+      loop do
+        mutex.synchronize do
+          condition.signal
+        end
+        sleep q
+      end
+    end
+
+    i = 0
+
+    thread = Thread.new do
+      scheduler = SleepingBlockingScheduler.new
+      Fiber.set_scheduler scheduler
+
+      Fiber.schedule do
+        10.times do
+          mutex.synchronize do
+            condition.wait(mutex)
+            sleep q
+            i += 1
+          end
+        end
+      end
+    end
+
+    # Wait for 10 seconds at most... if it doesn't finish, it's deadlocked.
+    thread.join(10)
+
+    # If it's deadlocked, it will never finish, so this will be 0.
+    assert_equal 10, i
+  ensure
+    # Make sure the threads are dead...
+    thread.kill
+    signaller.kill
+  end
 end