Avoid repeated calls to respond_to?

The common case is that the class will implement `succ`, so catch
NoMethodError's instead to avoid the repeated calls to `respond_to?`.

Also add test coverage for advancing cursors.

Author: djmb

activejob/lib/active_job/continuation/step.rb

@@ -48,8 +48,14 @@ def set!(cursor)
       # An UnadvanceableCursorError error will be raised if the cursor does not implement +succ+.
       def advance!(from: nil)
         from = cursor if from.nil?
-        raise UnadvanceableCursorError, "Cursor class '#{from.class}' does not implement succ, " unless from.respond_to?(:succ)
-        set! from.succ
+
+        begin
+          to = from.succ
+        rescue NoMethodError
+          raise UnadvanceableCursorError, "Cursor class '#{from.class}' does not implement 'succ'"
+        end
+
+        set! to
       end
 
       # Has this step been resumed from a previous job execution?

activejob/test/cases/continuation_test.rb

@@ -18,6 +18,10 @@
 
 return unless adapter_is?(:test)
 
+class ContinuableJob < ActiveJob::Base
+  include ActiveJob::Continuable
+end
+
 class ActiveJob::TestContinuation < ActiveSupport::TestCase
   include ActiveJob::Continuation::TestHelper
   include ActiveSupport::Testing::Stream
@@ -251,6 +255,38 @@ class ActiveJob::TestContinuation < ActiveSupport::TestCase
     assert_equal "Step 'unexpected' found, expected to resume from 'iterating'", exception.message
   end
 
+  class ContinuableAdvancingJob < ContinuableJob
+    def perform(start_from, advance_from = nil)
+      step :test_step, start: start_from do |step|
+        step.advance! from: advance_from
+      end
+    end
+  end
+
+  test "cursor must implement succ to advance" do
+    perform_enqueued_jobs do
+      assert_raises ActiveJob::Continuation::UnadvanceableCursorError do
+        ContinuableAdvancingJob.perform_later(nil)
+      end
+
+      assert_raises ActiveJob::Continuation::UnadvanceableCursorError do
+        ContinuableAdvancingJob.perform_later(1.1)
+      end
+
+      assert_raises ActiveJob::Continuation::UnadvanceableCursorError do
+        ContinuableAdvancingJob.perform_later(nil, 1.1)
+      end
+
+      assert_nothing_raised do
+        ContinuableAdvancingJob.perform_later(1)
+      end
+
+      assert_nothing_raised do
+        ContinuableAdvancingJob.perform_later(nil, 1)
+      end
+    end
+  end
+
   test "deserializes a job with no continuation" do
     ContinuableDeletingJob.items = 10.times.map { |i| "item_#{i}" }
     ContinuableDeletingJob.perform_later