Use ActiveJob 5.2 retry logic for old jobs

Rails 6 introduces retries per-exception, instead of a global count of
retries. Because ActiveJob 5.2 doesn't serialize the execution count
per-exception, when ActiveJob 6.0 picks up an "old" job it can't know
the exception count in the new format.

This can also be an issue if AJ 6.0 serializes a new job with
exception_executions which is later picked up by AJ 5.2, which would
clear exception_executions (since it has no knowledge of it).

Previously we handled this by resetting exception_executions, if it
wasn't defined on a job, which could result in the worst case retrying
the job 2x the times we should.

This commit changes how we handle loading a legacy job: instead of
resetting exception_executions, we instead will always use the global
executions count.

This way, jobs which only have one retry_on (and didn't have a behaviour
change in AJ 6) are backwards-and-forwards-compatible with counts
respected exactly.

Jobs with multiple retry_on will revert to the AJ5.2 behaviour if they
were ever run under AJ5.2.

1 files changed, 25 insertions, 0 deletions

diff --git a/activejob/test/cases/exceptions_test.rb b/activejob/test/cases/exceptions_test.rb
index 840f4d40b5..1f07b7b294 100644
--- a/activejob/test/cases/exceptions_test.rb
+++ b/activejob/test/cases/exceptions_test.rb
@@ -179,6 +179,31 @@ class ExceptionsTest < ActiveSupport::TestCase
     assert_equal ["Raised ActiveJob::DeserializationError for the 5 time"], JobBuffer.values
   end
 
+  test "running a job enqueued by AJ 5.2" do
+    job = RetryJob.new("DefaultsError", 6)
+    job.exception_executions = nil # This is how jobs from Rails 5.2 will look
+
+    assert_raises DefaultsError do
+      job.enqueue
+    end
+
+    assert_equal 5, JobBuffer.values.count
+  end
+
+  test "running a job enqueued and attempted under AJ 5.2" do
+    job = RetryJob.new("DefaultsError", 6)
+
+    # Fake 4 previous executions under AJ 5.2
+    job.exception_executions = nil
+    job.executions = 4
+
+    assert_raises DefaultsError do
+      job.enqueue
+    end
+
+    assert_equal ["Raised DefaultsError for the 5th time"], JobBuffer.values
+  end
+
   private
     def adapter_skips_scheduling?(queue_adapter)
       [