Add RSpec::Support.failure_notifier.

This is intended to support the new `aggregate_failures`
feature discussed in rspec/rspec-expectations#733. I
put the `failure_notifier` here so that rspec-mocks can
use it as well -- that way, when we are aggregating failures,
we can aggregate rspec-mocks failures as well.

Author: myronmarston

lib/rspec/support.rb

@@ -73,6 +73,31 @@ def self.method_handle_for(object, method_name)
       end
     end
 
+    # A single thread local variable so we don't excessively pollute that namespace.
+    def self.thread_local_data
+      Thread.current[:__rspec] ||= {}
+    end
+
+    def self.failure_notifier=(callable)
+      thread_local_data[:failure_notifier] = callable
+    end
+
+    def self.failure_notifier
+      thread_local_data[:failure_notifier] ||= method(:raise)
+    end
+
+    def self.notify_failure(failure)
+      failure_notifier.call(failure)
+    end
+
+    def self.with_failure_notifier(callable)
+      orig_notifier = failure_notifier
+      self.failure_notifier = callable
+      yield
+    ensure
+      self.failure_notifier = orig_notifier
+    end
+
     # The Differ is only needed when a a spec fails with a diffable failure.
     # In the more common case of all specs passing or the only failures being
     # non-diffable, we can avoid the extra cost of loading the differ, diff-lcs,

spec/rspec/support_spec.rb

@@ -117,5 +117,68 @@ def method_missing(name, *args, &block)
         end
       end
     end
+
+    describe "failure notification" do
+      before { @failure_notifier = RSpec::Support.failure_notifier }
+      after  { RSpec::Support.failure_notifier = @failure_notifier }
+      let(:error) { NotImplementedError.new("some message") }
+      let(:failures) { [] }
+      let(:append_to_failures_array_notifier) { failures.method(:<<) }
+
+      def notify(failure)
+        RSpec::Support.notify_failure(failure)
+      end
+
+      def append_to_failures_array_instead_of_raising
+        avoid_raising_errors.and change { failures }.from([]).to([error])
+      end
+
+      def raise_instead_of_appending_to_failures_array
+        raise_error(error).and avoid_changing { failures }
+      end
+
+      it "defaults to raising the provided exception" do
+        expect { notify(error) }.to raise_error(error)
+      end
+
+      it "can be set to another callable" do
+        RSpec::Support.failure_notifier = append_to_failures_array_notifier
+
+        expect {
+          notify(error)
+        }.to append_to_failures_array_instead_of_raising
+      end
+
+      it "isolates notifier changes to the current thread" do
+        RSpec::Support.failure_notifier = append_to_failures_array_notifier
+
+        Thread.new do
+          expect { notify(error) }.to raise_instead_of_appending_to_failures_array
+        end.join
+      end
+
+      it "can be changed for the duration of a block" do
+        yielded = false
+
+        RSpec::Support.with_failure_notifier(append_to_failures_array_notifier) do
+          yielded = true
+          expect {
+            notify(error)
+          }.to append_to_failures_array_instead_of_raising
+        end
+
+        expect(yielded).to be true
+      end
+
+      it "resets the notifier back to what it originally was when the block completes, even if an error was raised" do
+        expect {
+          RSpec::Support.with_failure_notifier(append_to_failures_array_notifier) do
+            raise "boom"
+          end
+        }.to raise_error("boom")
+
+        expect { notify(error) }.to raise_instead_of_appending_to_failures_array
+      end
+    end
   end
 end

spec/spec_helper.rb

@@ -1,2 +1,5 @@
 require 'rspec/support/spec'
 RSpec::Support::Spec.setup_simplecov
+
+RSpec::Matchers.define_negated_matcher :avoid_raising_errors, :raise_error
+RSpec::Matchers.define_negated_matcher :avoid_changing, :change