Fix RuntimeError: can't add a new key into hash during iteration issu…

[pytong]

AR-JDBC's version used:
52.1

version of Rails / ActiveRecord you're running with:
5.2.1

JRuby version (you might include your Java version as well) - jruby -v
jruby-9.2.0.0

A way to reproduce :)
Start a console with rails c and run this line within the console

100.times { Thread.new { ActiveRecord::Base.connection.execute "SELECT SLEEP(1)" } }

Solution:
Implemented the fix based on this link:
https://stackoverflow.com/a/27643890

include any (related) back-traces (or Java stack-traces) you've seen in the logs

                             []= at org/jruby/RubyHash.java:1033
                mysql_connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-jdbc-adapter-52.1-java/lib/arjdbc/mysql/connection_methods.rb:8
                  new_connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:809
         checkout_new_connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:853
  try_to_checkout_new_connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:832
              acquire_connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:793
                        checkout at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:521
                      connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:380
             retrieve_connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:1008
             retrieve_connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_handling.rb:118
                      connection at /Users/ptong/.rvm/gems/jruby-9.2.0.0/gems/activerecord-5.2.1/lib/active_record/connection_handling.rb:90
               block in evaluate at (irb):1```

[kares]

we didn't look into support for running AR-JDBC with DB2 systems, however we welcome the effort.
... you should change the branch from master to target 50-stable so it gets fixed for all 5x releases

[rdubya]

The stack trace says the issue is in mysql, but the change is to the db2 file. Which system are you using?