From aeb58ab70470b7f395a1e77b10c9b7a73955dad8 Mon Sep 17 00:00:00 2001
From: Matthew Draper <matthew@trebex.net>
Date: Mon, 1 Feb 2016 23:41:07 +1030
Subject: Block new share attempts if there's an exclusive waiter

---
 .../lib/active_support/concurrency/share_lock.rb     | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

(limited to 'activesupport/lib/active_support')

diff --git a/activesupport/lib/active_support/concurrency/share_lock.rb b/activesupport/lib/active_support/concurrency/share_lock.rb
index ca48164c54..1537f2898f 100644
--- a/activesupport/lib/active_support/concurrency/share_lock.rb
+++ b/activesupport/lib/active_support/concurrency/share_lock.rb
@@ -48,14 +48,14 @@ module ActiveSupport
       def start_exclusive(purpose: nil, compatible: [], no_wait: false)
         synchronize do
           unless @exclusive_thread == Thread.current
-            if busy?(purpose)
+            if busy_for_exclusive?(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) }
+                @cv.wait_while { busy_for_exclusive?(purpose) }
               ensure
                 @waiting.delete Thread.current
                 @sharing[Thread.current] = loose_shares if loose_shares
@@ -83,10 +83,10 @@ module ActiveSupport
         end
       end
 
-      def start_sharing
+      def start_sharing(purpose: :share)
         synchronize do
-          if @exclusive_thread && @exclusive_thread != Thread.current
-            @cv.wait_while { @exclusive_thread }
+          if busy_for_sharing?(purpose)
+            @cv.wait_while { busy_for_sharing?(purpose) }
           end
           @sharing[Thread.current] += 1
         end
@@ -132,11 +132,15 @@ module ActiveSupport
       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) } ||
+      def busy_for_exclusive?(purpose)
+        busy_for_sharing?(purpose) ||
           @sharing.size > (@sharing[Thread.current] > 0 ? 1 : 0)
       end
+
+      def busy_for_sharing?(purpose)
+        (@exclusive_thread && @exclusive_thread != Thread.current) ||
+          @waiting.any? { |k, v| k != Thread.current && !v.include?(purpose) }
+      end
     end
   end
 end
-- 
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