Make AS::Deprecation.silence block thread-local

Previouly `ActiveSupport::Deprecation.silence { ... }` would silence
deprecations from all threads, which I think is undesirable in a
mutli-threaded environment.

This leaves `ActiveSupport::Deprecation.silenced=` as setting a global
option, which might be a little confusing, but I think that these are
used in different ways, and this best preserves how they are generally
used. `silence {}` is used to silence a small section of code,
`silenced=` is usually used to set a default in an initializer or test
helper.

I somewhat think that `ActiveSupport::Deprecation.silenced=` should be
deprecated in favour of `ActiveSupport::Deprecation.behavior = :silence`

activesupport/lib/active_support/deprecation.rb

@@ -41,6 +41,7 @@ module ActiveSupport
       # By default, warnings are not silenced and debugging is off.
       self.silenced = false
       self.debug = false
+      @silenced_thread = Concurrent::ThreadLocalVar.new(false)
     end
   end
 end

activesupport/lib/active_support/deprecation/reporting.rb

@@ -6,7 +6,7 @@ module ActiveSupport
   class Deprecation
     module Reporting
       # Whether to print a message (silent mode)
-      attr_accessor :silenced
+      attr_writer :silenced
       # Name of gem where method is deprecated
       attr_accessor :gem_name
 
@@ -33,11 +33,12 @@ module ActiveSupport
       #     ActiveSupport::Deprecation.warn('something broke!')
       #   end
       #   # => nil
-      def silence
-        old_silenced, @silenced = @silenced, true
-        yield
-      ensure
-        @silenced = old_silenced
+      def silence(&block)
+        @silenced_thread.bind(true, &block)
+      end
+
+      def silenced
+        @silenced || @silenced_thread.value
       end
 
       def deprecation_warning(deprecated_method_name, message = nil, caller_backtrace = nil)

activesupport/test/deprecation_test.rb

@@ -290,6 +290,34 @@ class DeprecationTest < ActiveSupport::TestCase
     ActiveSupport::Deprecation.silenced = false
   end
 
+  def test_silence_threaded
+    barrier = Concurrent::CyclicBarrier.new(2)
+
+    th = Thread.new do
+      ActiveSupport::Deprecation.silence do
+        barrier.wait
+        barrier.wait
+        assert_not_deprecated { ActiveSupport::Deprecation.warn "abc" }
+      end
+      assert_deprecated("abc") { ActiveSupport::Deprecation.warn "abc" }
+    end
+
+    barrier.wait
+
+    assert_deprecated("abc") { ActiveSupport::Deprecation.warn "abc" }
+
+    ActiveSupport::Deprecation.silence do
+      assert_not_deprecated { ActiveSupport::Deprecation.warn "abc" }
+    end
+
+    assert_deprecated("abc") { ActiveSupport::Deprecation.warn "abc" }
+
+    barrier.wait
+    th.join
+  ensure
+    th.kill
+  end
+
   def test_deprecation_without_explanation
     assert_deprecated { @dtc.a }
     assert_deprecated { @dtc.b }