resolve waiting counter problem

if a thread is in the wait state, but transitions into
a killed state the waiting counter will not be decremented
leaving the pool to believe there is a thread available
and waiting to complete work when that is not the case.
Introduce a begin..ensure block so that any thread exception
raised while in the wait state properly balances the
waiting counter.

lib/puma/thread_pool.rb

@@ -122,8 +122,11 @@ module Puma
                 @out_of_band_pending = false
               end
               not_full.signal
-              not_empty.wait mutex
-              @waiting -= 1
+              begin
+                not_empty.wait mutex
+              ensure
+                @waiting -= 1
+              end
             end
 
             work = todo.shift

test/test_thread_pool.rb

@@ -266,4 +266,24 @@ class TestThreadPool < Minitest::Test
     assert_equal 2, rescued.length
     refute rescued.compact.any?(&:alive?)
   end
+
+  def test_correct_waiting_count_for_killed_threads
+    pool = new_pool(1, 1) { |_| }
+
+    pause
+
+    # simulate our waiting worker thread getting killed for whatever reason
+    pool.instance_eval { @workers[0].kill }
+
+    pause
+
+    pool.reap
+
+    pause
+
+    pool << 0
+
+    pause
+    assert_equal 0, pool.backlog
+  end
 end