Handle thread death during lock acquisition

Specifically, clean up if the thread is killed while it's blocked
awaiting the lock... if we get killed on some other arbitrary line, the
result remains quite undefined.

activesupport/lib/active_support/concurrency/share_lock.rb

@@ -54,10 +54,12 @@ module ActiveSupport
               loose_shares = @sharing.delete(Thread.current)
               @waiting[Thread.current] = compatible if loose_shares
 
-              @cv.wait_while { busy?(purpose) }
-
-              @waiting.delete Thread.current
-              @sharing[Thread.current] = loose_shares if loose_shares
+              begin
+                @cv.wait_while { busy?(purpose) }
+              ensure
+                @waiting.delete Thread.current
+                @sharing[Thread.current] = loose_shares if loose_shares
+              end
             end
             @exclusive_thread = Thread.current
           end

activesupport/test/share_lock_test.rb

@@ -81,6 +81,27 @@ class ShareLockTest < ActiveSupport::TestCase
     end
   end
 
+  def test_killed_thread_loses_lock
+    with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+      thread = Thread.new do
+        @lock.sharing do
+          @lock.exclusive {}
+        end
+      end
+
+      assert_threads_stuck thread
+      thread.kill
+
+      sharing_thread_release_latch.count_down
+
+      thread = Thread.new do
+        @lock.exclusive {}
+      end
+
+      assert_threads_not_stuck thread
+    end
+  end
+
   def test_exclusive_conflicting_purpose
     [true, false].each do |use_upgrading|
       with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|