1 files changed, 142 insertions, 0 deletions
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..ca48164c54
--- /dev/null
+++ b/activesupport/lib/active_support/concurrency/share_lock.rb
@@ -0,0 +1,142 @@
+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.
+
+
+      def initialize
+        super()
+
+        @cv = new_cond
+
+        @sharing = Hash.new(0)
+        @waiting = {}
+        @exclusive_thread = nil
+        @exclusive_depth = 0
+      end
+
+      # Returns false if +no_wait+ is set and the lock is not
+      # immediately available. Otherwise, returns true after the lock
+      # has been acquired.
+      #
+      # +purpose+ and +compatible+ work together; while this thread is
+      # waiting for the exclusive lock, it will yield its share (if any)
+      # to any other attempt whose +purpose+ appears in this attempt's
+      # +compatible+ list. This allows a "loose" upgrade, which, being
+      # less strict, 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. With
+      # +purpose+ matching, it is possible to yield only to other
+      # threads whose activity will not interfere.
+      def start_exclusive(purpose: nil, compatible: [], no_wait: false)
+        synchronize do
+          unless @exclusive_thread == Thread.current
+            if busy?(purpose)
+              return false if no_wait
+
+              loose_shares = @sharing.delete(Thread.current)
+              @waiting[Thread.current] = compatible 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
+          @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.
+      #
+      # See +start_exclusive+ for other options.
+      def exclusive(purpose: nil, compatible: [], no_wait: false)
+        if start_exclusive(purpose: purpose, compatible: compatible, no_wait: 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?(purpose)
+        (@exclusive_thread && @exclusive_thread != Thread.current) ||
+          @waiting.any? { |k, v| k != Thread.current && !v.include?(purpose) } ||
+          @sharing.size > (@sharing[Thread.current] > 0 ? 1 : 0)
+      end
+    end
+  end
+end