airbrake-ruby,notifier: use the Null Pattern for notifiers

Fixes airbrake/airbrake#713
(undefined method `build_notice' for nil:NilClass)

It becomes really tedious to keep track of whether a notifier is
configured or not. Since we introduced `Airbrake.[]` to retrieve
notifiers, all integrations have to add an additional check for it:

```rb
if notifier = Airbrake[:default]
  notifier.notify(...)
end
```

With the current change this check is not needed anymore.

CHANGELOG.md

@@ -3,6 +3,10 @@ Airbrake Ruby Changelog
 
 ### master
 
+* Return `Airbrake::NilNotifier` when no notifiers are configured and
+  `Airbrake#[]` is called
+  ([#169](https://github.com/airbrake/airbrake-ruby/pull/169))
+
 ### [v2.0.0][v2.0.0] (March 21, 2017)
 
 * **IMPORTANT:** Removed the `component/action` API deprecated

README.md

@@ -409,6 +409,9 @@ Retrieves a configured notifier.
 Airbrake[:my_notifier] #=> Airbrake::Notifier
 ```
 
+If the notifier is not configured, returns an instance of
+`Airbrake::NilNotifier` (a no-op version of `Airbrake::Notifier`).
+
 #### Airbrake.notify
 
 Sends an exception to Airbrake asynchronously.

lib/airbrake-ruby.rb

@@ -73,8 +73,9 @@ module Airbrake
 
   ##
   # A Hash that holds all notifiers. The keys of the Hash are notifier
-  # names, the values are Airbrake::Notifier instances.
-  @notifiers = {}
+  # names, the values are Airbrake::Notifier instances. If a notifier is not
+  # assigned to the hash, then it returns a null object (NilNotifier).
+  @notifiers = Hash.new(NilNotifier.new)
 
   class << self
     ##
@@ -146,7 +147,7 @@ def configure(notifier_name = :default)
     # @return [Airbrake::Promise]
     # @see .notify_sync
     def notify(exception, params = {})
-      @notifiers[:default] && @notifiers[:default].notify(exception, params)
+      @notifiers[:default].notify(exception, params)
     end
 
     ##
@@ -159,7 +160,7 @@ def notify(exception, params = {})
     # @return [Hash{String=>String}] the reponse from the server
     # @see .notify
     def notify_sync(exception, params = {})
-      @notifiers[:default] && @notifiers[:default].notify_sync(exception, params)
+      @notifiers[:default].notify_sync(exception, params)
     end
 
     ##
@@ -189,7 +190,7 @@ def notify_sync(exception, params = {})
     # @return [void]
     # @note Once a filter was added, there's no way to delete it
     def add_filter(filter = nil, &block)
-      @notifiers[:default] && @notifiers[:default].add_filter(filter, &block)
+      @notifiers[:default].add_filter(filter, &block)
     end
 
     ##
@@ -208,7 +209,7 @@ def add_filter(filter = nil, &block)
     # @return [Airbrake::Notice] the notice built with help of the given
     #   arguments
     def build_notice(exception, params = {})
-      @notifiers[:default] && @notifiers[:default].build_notice(exception, params)
+      @notifiers[:default].build_notice(exception, params)
     end
 
     ##
@@ -222,7 +223,7 @@ def build_notice(exception, params = {})
     #
     # @return [void]
     def close
-      @notifiers[:default] && @notifiers[:default].close
+      @notifiers[:default].close
     end
 
     ##
@@ -237,7 +238,7 @@ def close
     # @option deploy_params [Symbol] :version
     # @return [void]
     def create_deploy(deploy_params)
-      @notifiers[:default] && @notifiers[:default].create_deploy(deploy_params)
+      @notifiers[:default].create_deploy(deploy_params)
     end
   end
 end

lib/airbrake-ruby/notifier.rb

@@ -159,4 +159,23 @@ def add_filters_for_config_keys
       add_filter(Filters::KeysWhitelist.new(@config.logger, *@config.whitelist_keys))
     end
   end
+
+  ##
+  # NilNotifier is a no-op notifier, which mimics +Airbrake::Notifier+ and
+  # serves only for the purpose of making the library API easier to use.
+  #
+  # @since 2.1.0
+  class NilNotifier
+    def notify(_exception, _params = {}); end
+
+    def notify_sync(_exception, _params); end
+
+    def add_filter(_filter = nil, &_block); end
+
+    def build_notice(_exception, _params); end
+
+    def close; end
+
+    def create_deploy(_deploy_params); end
+  end
 end

spec/airbrake_spec.rb

@@ -17,12 +17,18 @@
   end
 
   after do
-    described_class.instance_variable_set(:@notifiers, {})
+    described_class.instance_variable_set(
+      :@notifiers,
+      Hash.new(Airbrake::NilNotifier.new)
+    )
   end
 
   shared_examples 'non-configured notifier handling' do |method|
     it "returns nil if there is no configured notifier when using #{method}" do
-      described_class.instance_variable_set(:@notifiers, {})
+      described_class.instance_variable_set(
+        :@notifiers,
+        Hash.new(Airbrake::NilNotifier.new)
+      )
       expect(described_class.__send__(method, 'bingo')).to be_nil
     end
   end