2 files changed, 146 insertions, 16 deletions
diff --git a/activesupport/lib/active_support/concurrency/latch.rb b/activesupport/lib/active_support/concurrency/latch.rb
index 1507de433e..7b8df0df04 100644
--- a/activesupport/lib/active_support/concurrency/latch.rb
+++ b/activesupport/lib/active_support/concurrency/latch.rb
@@ -1,26 +1,18 @@
-require 'thread'
-require 'monitor'
+require 'concurrent/atomics'
 
 module ActiveSupport
   module Concurrency
-    class Latch
-      def initialize(count = 1)
-        @count = count
-        @lock = Monitor.new
-        @cv = @lock.new_cond
-      end
+    class Latch < Concurrent::CountDownLatch
 
-      def release
-        @lock.synchronize do
-          @count -= 1 if @count > 0
-          @cv.broadcast if @count.zero?
-        end
+      def initialize(count = 1)
+        ActiveSupport::Deprecation.warn("ActiveSupport::Concurrency::Latch is deprecated. Please use Concurrent::CountDownLatch instead.")
+        super(count)
       end
+      
+      alias_method :release, :count_down
 
       def await
-        @lock.synchronize do
-          @cv.wait_while { @count > 0 }
-        end
+        wait(nil)
       end
     end
   end
diff --git a/activesupport/lib/active_support/concurrency/share_lock.rb b/activesupport/lib/active_support/concurrency/share_lock.rb
new file mode 100644
index 0000000000..f1c6230084
--- /dev/null
+++ b/activesupport/lib/active_support/concurrency/share_lock.rb
@@ -0,0 +1,138 @@
+require 'thread'
+require 'monitor'
+
+module ActiveSupport
+  module Concurrency
+    # A share/exclusive lock, otherwise known as a read/write lock.
+    #
+    # https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock
+    #--
+    # Note that a pending Exclusive lock attempt does not block incoming
+    # Share requests (i.e., we are "read-preferring"). That seems
+    # consistent with the behavior of +loose_upgrades+, but may be the
+    # wrong choice otherwise: it nominally reduces the possibility of
+    # deadlock by risking starvation instead.
+    class ShareLock
+      include MonitorMixin
+
+      # We track Thread objects, instead of just using counters, because
+      # we need exclusive locks to be reentrant, and we need to be able
+      # to upgrade share locks to exclusive.
+
+
+      # If +loose_upgrades+ is false (the default), then a thread that
+      # is waiting on an Exclusive lock will continue to hold any Share
+      # lock that it has already established. This is safer, but can
+      # lead to deadlock.
+      #
+      # If +loose_upgrades+ is true, a thread waiting on an Exclusive
+      # lock will temporarily relinquish its Share lock. Being less
+      # strict, this behavior prevents some classes of deadlocks. For
+      # many resources, loose upgrades are sufficient: if a thread is
+      # awaiting a lock, it is not running any other code.
+      attr_reader :loose_upgrades
+
+      def initialize(loose_upgrades = false)
+        @loose_upgrades = loose_upgrades
+
+        super()
+
+        @cv = new_cond
+
+        @sharing = Hash.new(0)
+        @exclusive_thread = nil
+        @exclusive_depth = 0
+      end
+
+      # Returns false if +no_wait+ is specified and the lock is not
+      # immediately available. Otherwise, returns true after the lock
+      # has been acquired.
+      def start_exclusive(no_wait=false)
+        synchronize do
+          unless @exclusive_thread == Thread.current
+            return false if no_wait && busy?
+
+            loose_shares = nil
+            if @loose_upgrades
+              loose_shares = @sharing.delete(Thread.current)
+            end
+
+            @cv.wait_while { busy? } if busy?
+
+            @exclusive_thread = Thread.current
+            @sharing[Thread.current] = loose_shares if loose_shares
+          end
+          @exclusive_depth += 1
+
+          true
+        end
+      end
+
+      # Relinquish the exclusive lock. Must only be called by the thread
+      # that called start_exclusive (and currently holds the lock).
+      def stop_exclusive
+        synchronize do
+          raise "invalid unlock" if @exclusive_thread != Thread.current
+
+          @exclusive_depth -= 1
+          if @exclusive_depth == 0
+            @exclusive_thread = nil
+            @cv.broadcast
+          end
+        end
+      end
+
+      def start_sharing
+        synchronize do
+          if @exclusive_thread && @exclusive_thread != Thread.current
+            @cv.wait_while { @exclusive_thread }
+          end
+          @sharing[Thread.current] += 1
+        end
+      end
+
+      def stop_sharing
+        synchronize do
+          if @sharing[Thread.current] > 1
+            @sharing[Thread.current] -= 1
+          else
+            @sharing.delete Thread.current
+            @cv.broadcast
+          end
+        end
+      end
+
+      # Execute the supplied block while holding the Exclusive lock. If
+      # +no_wait+ is set and the lock is not immediately available,
+      # returns +nil+ without yielding. Otherwise, returns the result of
+      # the block.
+      def exclusive(no_wait=false)
+        if start_exclusive(no_wait)
+          begin
+            yield
+          ensure
+            stop_exclusive
+          end
+        end
+      end
+
+      # Execute the supplied block while holding the Share lock.
+      def sharing
+        start_sharing
+        begin
+          yield
+        ensure
+          stop_sharing
+        end
+      end
+
+      private
+
+      # Must be called within synchronize
+      def busy?
+        (@exclusive_thread && @exclusive_thread != Thread.current) ||
+          @sharing.size > (@sharing[Thread.current] > 0 ? 1 : 0)
+      end
+    end
+  end
+end