Make backtrace truncation much more robust.

This is necessary for the changes in rspec/rspec-support#210.

That adds an additional stack frame to the parent exception
(for the lambda that wraps `raise`) and our truncation wasn’t
working properly with it. Really, the old way just happened to
work. This is much more robust.

lib/rspec/core/formatters/exception_presenter.rb

@@ -228,6 +228,8 @@ def multiple_failure_sumarizer(exception, prior_detail_formatter, color)
           end
 
           def sub_failure_list_formatter(exception, message_color)
+            common_backtrace_truncater = CommonBacktraceTruncater.new(exception)
+
             lambda do |failure_number, colorizer, indentation|
               exception.all_exceptions.each_with_index.map do |failure, index|
                 options = with_multiple_error_options_as_needed(
@@ -238,9 +240,7 @@ def sub_failure_list_formatter(exception, message_color)
                   :skip_shared_group_trace => true
                 )
 
-                failure = failure.dup
-                failure.set_backtrace(failure.backtrace[0..-exception.backtrace.size])
-
+                failure   = common_backtrace_truncater.with_truncated_backtrace(failure)
                 presenter = ExceptionPresenter.new(failure, @example, options)
                 presenter.fully_formatted("#{failure_number}.#{index + 1}", colorizer)
               end.join
@@ -255,6 +255,29 @@ def self.format_backtrace(*)
               []
             end
           end
+
+          # @private
+          class CommonBacktraceTruncater
+            def initialize(parent)
+              @parent = parent
+            end
+
+            def with_truncated_backtrace(child)
+              child_bt  = child.backtrace
+              parent_bt = @parent.backtrace
+              return child if child_bt.nil? || child_bt.empty? || parent_bt.nil?
+
+              index_before_first_common_frame = -1.downto(-child_bt.size).find do |index|
+                parent_bt[index] != child_bt[index]
+              end
+
+              return child if index_before_first_common_frame == -1
+
+              child = child.dup
+              child.set_backtrace(child_bt[0..index_before_first_common_frame])
+              child
+            end
+          end
         end
 
         # @private

spec/rspec/core/formatters/exception_presenter_spec.rb

@@ -194,4 +194,116 @@ def read_failed_line
       end
     end
   end
+
+  RSpec.describe Formatters::ExceptionPresenter::Factory::CommonBacktraceTruncater do
+    def truncate(parent, child)
+      described_class.new(parent).with_truncated_backtrace(child)
+    end
+
+    def exception_with(backtrace)
+      exception = Exception.new
+      exception.set_backtrace(backtrace)
+      exception
+    end
+
+    it 'returns an exception with the common part truncated' do
+      parent = exception_with %w[ foo.rb:1 bar.rb:2 car.rb:7 ]
+      child  = exception_with %w[ file_1.rb:3 file_1.rb:9 foo.rb:1 bar.rb:2 car.rb:7 ]
+
+      truncated = truncate(parent, child)
+
+      expect(truncated.backtrace).to eq %w[ file_1.rb:3 file_1.rb:9 ]
+    end
+
+    it 'ignores excess lines in the top of the parent trace that the child does not have' do
+      parent = exception_with %w[ foo.rb:1 foo.rb:2 foo.rb:3 bar.rb:2 car.rb:7 ]
+      child  = exception_with %w[ file_1.rb:3 file_1.rb:9 bar.rb:2 car.rb:7 ]
+
+      truncated = truncate(parent, child)
+
+      expect(truncated.backtrace).to eq %w[ file_1.rb:3 file_1.rb:9 ]
+    end
+
+    it 'does not truncate anything if the parent has excess lines at the bottom of the trace' do
+      parent = exception_with %w[ foo.rb:1 bar.rb:2 car.rb:7 bazz.rb:9 ]
+      child  = exception_with %w[ file_1.rb:3 file_1.rb:9 foo.rb:1 bar.rb:2 car.rb:7 ]
+
+      truncated = truncate(parent, child)
+
+      expect(truncated.backtrace).to eq %w[ file_1.rb:3 file_1.rb:9 foo.rb:1 bar.rb:2 car.rb:7 ]
+    end
+
+    it 'does not mutate the provided exception' do
+      parent = exception_with %w[ foo.rb:1 bar.rb:2 car.rb:7 ]
+      child  = exception_with %w[ file_1.rb:3 file_1.rb:9 foo.rb:1 bar.rb:2 car.rb:7 ]
+
+      expect { truncate(parent, child) }.not_to change(child, :backtrace)
+    end
+
+    it 'returns an exception with all the same attributes (except backtrace) as the provided one' do
+      parent = exception_with %w[ foo.rb:1 bar.rb:2 car.rb:7 ]
+
+      my_custom_exception_class = Class.new(StandardError) do
+        attr_accessor :foo, :bar
+      end
+
+      child = my_custom_exception_class.new("Some Message")
+      child.foo = 13
+      child.bar = 20
+      child.set_backtrace(%w[ foo.rb:1 ])
+
+      truncated = truncate(parent, child)
+
+      expect(truncated).to have_attributes(
+        :message => "Some Message",
+        :foo => 13,
+        :bar => 20
+      )
+    end
+
+    it 'handles child exceptions that have a blank array for the backtrace' do
+      parent = exception_with %w[ foo.rb:1 bar.rb:2 car.rb:7 ]
+      child  = exception_with %w[ ]
+
+      truncated = truncate(parent, child)
+
+      expect(truncated.backtrace).to eq %w[ ]
+    end
+
+    it 'handles child exceptions that have `nil` for the backtrace' do
+      parent = exception_with %w[ foo.rb:1 bar.rb:2 car.rb:7 ]
+      child  = Exception.new
+
+      truncated = truncate(parent, child)
+
+      expect(truncated.backtrace).to be_nil
+    end
+
+    it 'handles parent exceptions that have a blank array for the backtrace' do
+      parent = exception_with %w[ ]
+      child  = exception_with %w[ foo.rb:1 ]
+
+      truncated = truncate(parent, child)
+
+      expect(truncated.backtrace).to eq %w[ foo.rb:1 ]
+    end
+
+    it 'handles parent exceptions that have `nil` for the backtrace' do
+      parent = Exception.new
+      child  = exception_with %w[ foo.rb:1 ]
+
+      truncated = truncate(parent, child)
+
+      expect(truncated.backtrace).to eq %w[ foo.rb:1 ]
+    end
+
+    it 'returns the original exception object (not a dup) when there is no need to update the backtrace' do
+      parent = exception_with %w[ bar.rb:1 ]
+      child  = exception_with %w[ foo.rb:1 ]
+
+      truncated = truncate(parent, child)
+
+      expect(truncated).to be child
+    end
+  end
 end