From 4fc2ea11811c50fb1f7c5baba6d4c20532a5a00c Mon Sep 17 00:00:00 2001
From: Peter McCracken <peter.mccracken@shopify.com>
Date: Wed, 10 May 2017 13:30:41 -0400
Subject: [PATCH] handle loops in the cause chain in
 Rescuable#rescue_with_handler

---
 activesupport/lib/active_support/rescuable.rb | 10 ++++--
 activesupport/test/rescuable_test.rb          | 34 ++++++++++++++++++-
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/activesupport/lib/active_support/rescuable.rb b/activesupport/lib/active_support/rescuable.rb
index ee6592fb5a..12ec8bf1b8 100644
--- a/activesupport/lib/active_support/rescuable.rb
+++ b/activesupport/lib/active_support/rescuable.rb
@@ -84,12 +84,18 @@ def rescue_from(*klasses, with: nil, &block)
       #     end
       #
       # Returns the exception if it was handled and +nil+ if it was not.
-      def rescue_with_handler(exception, object: self)
+      def rescue_with_handler(exception, object: self, visited_exceptions: [])
+        visited_exceptions << exception
+
         if handler = handler_for_rescue(exception, object: object)
           handler.call exception
           exception
         elsif exception
-          rescue_with_handler(exception.cause, object: object)
+          if visited_exceptions.include?(exception.cause)
+            nil
+          else
+            rescue_with_handler(exception.cause, object: object, visited_exceptions: visited_exceptions)
+          end
         end
       end
 
diff --git a/activesupport/test/rescuable_test.rb b/activesupport/test/rescuable_test.rb
index f7eb047d44..3bdd1651e7 100644
--- a/activesupport/test/rescuable_test.rb
+++ b/activesupport/test/rescuable_test.rb
@@ -43,7 +43,9 @@ class NuclearExplosion < StandardError; end
   def dispatch(method)
     send(method)
   rescue Exception => e
-    rescue_with_handler(e)
+    unless rescue_with_handler(e)
+      @result = "unhandled"
+    end
   end
 
   def attack
@@ -58,6 +60,26 @@ def ronanize
     raise MadRonon.new("dex")
   end
 
+  def crash
+    raise "unhandled RuntimeError"
+  end
+
+  def looped_crash
+    ex1 = StandardError.new("error 1")
+    ex2 = StandardError.new("error 2")
+    begin
+      begin
+        raise ex1
+      rescue
+        # sets the cause on ex2 to be ex1
+        raise ex2
+      end
+    rescue
+      # sets the cause on ex1 to be ex2
+      raise ex1
+    end
+  end
+
   def fall_back_to_cause
     # This exception is the cause and has a handler.
     ronanize
@@ -139,4 +161,14 @@ def test_rescue_falls_back_to_exception_cause
     @stargate.dispatch :fall_back_to_cause
     assert_equal "dex", @stargate.result
   end
+
+  def test_unhandled_exceptions
+    @stargate.dispatch(:crash)
+    assert_equal "unhandled", @stargate.result
+  end
+
+  def test_rescue_handles_loops_in_exception_cause_chain
+    @stargate.dispatch :looped_crash
+    assert_equal "unhandled", @stargate.result
+  end
 end