From ca19b7f5d86aa590077766cbe8006f952b6d4296 Mon Sep 17 00:00:00 2001
From: sushant <sushant@bigbinary.com>
Date: Thu, 4 Apr 2019 10:57:57 +0530
Subject: Added 'detach_from' to 'ActiveSupport::Subscriber' to detach a
 subscriber from a namespace.

---
 activesupport/lib/active_support/subscriber.rb | 61 +++++++++++++++++++++++---
 activesupport/test/subscriber_test.rb          | 23 ++++++++++
 2 files changed, 78 insertions(+), 6 deletions(-)

diff --git a/activesupport/lib/active_support/subscriber.rb b/activesupport/lib/active_support/subscriber.rb
index f3e902f9dd..c3cd175a52 100644
--- a/activesupport/lib/active_support/subscriber.rb
+++ b/activesupport/lib/active_support/subscriber.rb
@@ -24,6 +24,10 @@ module ActiveSupport
   # After configured, whenever a "sql.active_record" notification is published,
   # it will properly dispatch the event (ActiveSupport::Notifications::Event) to
   # the +sql+ method.
+  #
+  # We can detach a subscriber as well:
+  #
+  #   ActiveRecord::StatsSubscriber.detach_from(:active_record)
   class Subscriber
     class << self
       # Attach the subscriber to a namespace.
@@ -40,6 +44,25 @@ module ActiveSupport
         end
       end
 
+      # Detach the subscriber from a namespace.
+      def detach_from(namespace, notifier = ActiveSupport::Notifications)
+        @namespace  = namespace
+        @subscriber = find_attached_subscriber
+        @notifier   = notifier
+
+        return unless subscriber
+
+        subscribers.delete(subscriber)
+
+        # Remove event subscribers of all existing methods on the class.
+        subscriber.public_methods(false).each do |event|
+          remove_event_subscriber(event)
+        end
+
+        # Reset notifier so that event subscribers will not add for new methods added to the class.
+        @notifier = nil
+      end
+
       # Adds event subscribers for all new methods added to the class.
       def method_added(event)
         # Only public methods are added as subscribers, and only if a notifier
@@ -58,15 +81,41 @@ module ActiveSupport
         attr_reader :subscriber, :notifier, :namespace
 
         def add_event_subscriber(event) # :doc:
-          return if %w{ start finish }.include?(event.to_s)
+          return if invalid_event?(event.to_s)
 
-          pattern = "#{event}.#{namespace}"
+          pattern = prepare_pattern(event)
 
           # Don't add multiple subscribers (eg. if methods are redefined).
-          return if subscriber.patterns.include?(pattern)
+          return if pattern_subscribed?(pattern)
+
+          subscriber.patterns[pattern] = notifier.subscribe(pattern, subscriber)
+        end
+
+        def remove_event_subscriber(event) # :doc:
+          return if invalid_event?(event.to_s)
+
+          pattern = prepare_pattern(event)
+
+          return unless pattern_subscribed?(pattern)
+
+          notifier.unsubscribe(subscriber.patterns[pattern])
+          subscriber.patterns.delete(pattern)
+        end
+
+        def find_attached_subscriber
+          subscribers.find { |attached_subscriber| attached_subscriber.instance_of?(self) }
+        end
+
+        def invalid_event?(event)
+          %w{ start finish }.include?(event.to_s)
+        end
+
+        def prepare_pattern(event)
+          "#{event}.#{namespace}"
+        end
 
-          subscriber.patterns << pattern
-          notifier.subscribe(pattern, subscriber)
+        def pattern_subscribed?(pattern)
+          subscriber.patterns.key?(pattern)
         end
     end
 
@@ -74,7 +123,7 @@ module ActiveSupport
 
     def initialize
       @queue_key = [self.class.name, object_id].join "-"
-      @patterns  = []
+      @patterns  = {}
       super
     end
 
diff --git a/activesupport/test/subscriber_test.rb b/activesupport/test/subscriber_test.rb
index 6b012e43af..bc8d8f1c13 100644
--- a/activesupport/test/subscriber_test.rb
+++ b/activesupport/test/subscriber_test.rb
@@ -23,6 +23,21 @@ class TestSubscriber < ActiveSupport::Subscriber
     end
 end
 
+class TestSubscriber2 < ActiveSupport::Subscriber
+  attach_to :doodle
+  detach_from :doodle
+
+  cattr_reader :events
+
+  def self.clear
+    @@events = []
+  end
+
+  def open_party(event)
+    events << event
+  end
+end
+
 # Monkey patch subscriber to test that only one subscriber per method is added.
 class TestSubscriber
   remove_method :open_party
@@ -34,6 +49,7 @@ end
 class SubscriberTest < ActiveSupport::TestCase
   def setup
     TestSubscriber.clear
+    TestSubscriber2.clear
   end
 
   def test_attaches_subscribers
@@ -53,4 +69,11 @@ class SubscriberTest < ActiveSupport::TestCase
 
     assert_equal [], TestSubscriber.events
   end
+
+  def test_detaches_subscribers
+    ActiveSupport::Notifications.instrument("open_party.doodle")
+
+    assert_equal [], TestSubscriber2.events
+    assert_equal 1, TestSubscriber.events.size
+  end
 end
-- 
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