From 04b4a0666bdf900f9a4e797022dfe0197eebef6a Mon Sep 17 00:00:00 2001
From: Matthew Draper <matthew@trebex.net>
Date: Fri, 10 Jun 2016 19:25:40 +0930
Subject: Provide a middleware to debug misbehaving locks

Only intended to be enabled when in use; by necessity, it sits above any
reasonable access control.
---
 .../lib/active_support/concurrency/share_lock.rb   | 50 +++++++++++++++++++---
 1 file changed, 45 insertions(+), 5 deletions(-)

(limited to 'activesupport/lib/active_support/concurrency')

diff --git a/activesupport/lib/active_support/concurrency/share_lock.rb b/activesupport/lib/active_support/concurrency/share_lock.rb
index 89e63aefd4..cf632ea7b0 100644
--- a/activesupport/lib/active_support/concurrency/share_lock.rb
+++ b/activesupport/lib/active_support/concurrency/share_lock.rb
@@ -14,6 +14,38 @@ module ActiveSupport
       # to upgrade share locks to exclusive.
 
 
+      def raw_state # :nodoc:
+        synchronize do
+          threads = @sleeping.keys | @sharing.keys | @waiting.keys
+          threads |= [@exclusive_thread] if @exclusive_thread
+
+          data = {}
+
+          threads.each do |thread|
+            purpose, compatible = @waiting[thread]
+
+            data[thread] = {
+              thread: thread,
+              sharing: @sharing[thread],
+              exclusive: @exclusive_thread == thread,
+              purpose: purpose,
+              compatible: compatible,
+              waiting: !!@waiting[thread],
+              sleeper: @sleeping[thread],
+            }
+          end
+
+          # NB: Yields while holding our *internal* synchronize lock,
+          # which is supposed to be used only for a few instructions at
+          # a time. This allows the caller to inspect additional state
+          # without things changing out from underneath, but would have
+          # disastrous effects upon normal operation. Fortunately, this
+          # method is only intended to be called when things have
+          # already gone wrong.
+          yield data
+        end
+      end
+
       def initialize
         super()
 
@@ -21,6 +53,7 @@ module ActiveSupport
 
         @sharing = Hash.new(0)
         @waiting = {}
+        @sleeping = {}
         @exclusive_thread = nil
         @exclusive_depth = 0
       end
@@ -46,7 +79,7 @@ module ActiveSupport
               return false if no_wait
 
               yield_shares(purpose: purpose, compatible: compatible, block_share: true) do
-                @cv.wait_while { busy_for_exclusive?(purpose) }
+                wait_for(:start_exclusive) { busy_for_exclusive?(purpose) }
               end
             end
             @exclusive_thread = Thread.current
@@ -69,7 +102,7 @@ module ActiveSupport
 
             if eligible_waiters?(compatible)
               yield_shares(compatible: compatible, block_share: true) do
-                @cv.wait_while { @exclusive_thread || eligible_waiters?(compatible) }
+                wait_for(:stop_exclusive) { @exclusive_thread || eligible_waiters?(compatible) }
               end
             end
             @cv.broadcast
@@ -84,11 +117,11 @@ module ActiveSupport
           elsif @waiting[Thread.current]
             # We're nested inside a +yield_shares+ call: we'll resume as
             # soon as there isn't an exclusive lock in our way
-            @cv.wait_while { @exclusive_thread }
+            wait_for(:start_sharing) { @exclusive_thread }
           else
             # This is an initial / outermost share call: any outstanding
             # requests for an exclusive lock get to go first
-            @cv.wait_while { busy_for_sharing?(false) }
+            wait_for(:start_sharing) { busy_for_sharing?(false) }
           end
           @sharing[Thread.current] += 1
         end
@@ -153,7 +186,7 @@ module ActiveSupport
           yield
         ensure
           synchronize do
-            @cv.wait_while { @exclusive_thread && @exclusive_thread != Thread.current }
+            wait_for(:yield_shares) { @exclusive_thread && @exclusive_thread != Thread.current }
 
             if previous_wait
               @waiting[Thread.current] = previous_wait
@@ -181,6 +214,13 @@ module ActiveSupport
       def eligible_waiters?(compatible)
         @waiting.any? { |t, (p, _)| compatible.include?(p) && @waiting.all? { |t2, (_, c2)| t == t2 || c2.include?(p) } }
       end
+
+      def wait_for(method)
+        @sleeping[Thread.current] = method
+        @cv.wait_while { yield }
+      ensure
+        @sleeping.delete Thread.current
+      end
     end
   end
 end
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