ruby on rails - RSpec fails to run --bisect, with error: undefined method `run' for #<RSpec::Core::Bisect::Server> -

i in process of upgrading large rails application rails 3 --> rails 4 (or maybe 5).

i @ point have rails v4.0.13 application, running on ruby v2.1.5. test suite running on rspec v3.5.2, , green (woohoo!), unfortunately intermittent failures.

i'd ideally track down cause of these failures before upgrading further, using rspec --bisect.

however, when run rspec --bisect (with or without other arguments), following error:

bisect started using options: "" running suite find failures...  bisect failed! failed results spec run. spec run output:  (druby://127.0.0.1:33858) /home/tom/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/drb/drb.rb:1588:in `perform_without_block': undefined method `run' #<rspec::core::bisect::server:0x00000001e71430> (nomethoderror)         (druby://127.0.0.1:33858) /home/tom/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/drb/drb.rb:1548:in `perform'         (druby://127.0.0.1:33858) /home/tom/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/drb/drb.rb:1626:in `block (2 levels) in main_loop'         (druby://127.0.0.1:33858) /home/tom/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/drb/drb.rb:1622:in `loop'         (druby://127.0.0.1:33858) /home/tom/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/drb/drb.rb:1622:in `block in main_loop'         /home/tom/.rvm/gems/ruby-2.1.5@<project-name>/gems/rspec-core-3.5.2/lib/rspec/core/drb.rb:23:in `run'         /home/tom/.rvm/gems/ruby-2.1.5@<project-name>/gems/rspec-core-3.5.2/lib/rspec/core/invocations.rb:19:in `call'         /home/tom/.rvm/gems/ruby-2.1.5@<project-name>/gems/rspec-core-3.5.2/lib/rspec/core/runner.rb:69:in `run'         /home/tom/.rvm/gems/ruby-2.1.5@<project-name>/gems/rspec-core-3.5.2/lib/rspec/core/runner.rb:45:in `invoke'         /home/tom/.rvm/gems/ruby-2.1.5@<project-name>/gems/rspec-core-3.5.2/exe/rspec:4:in `<main>' 

has got clue causing this, or how may able track down cause?

some things i've tried far, no avail:

• downgrade rspec lower version (>= 3.3.0, since that's when --bisect added).
• upgrade ruby higher version (namely 2.3.1). (this causes few test failures i'm yet resolve, still doesn't affect --bisect issue).
• clean system rvm gemset empty <project_name>; gem install bunder; bundle, purge pre-upgrade library code lying around.

it's worth noting have other rails projects on computer, running --bisect works fine - don't think can system library compatibility issue. guess must project configuration -- what?! (i can't see application back-trace clues??)

edit: without finding resolution yet, i've continued upgrading application. running rails v4.2.7 , ruby v2.3.1, specs passing (but still ordering issue). error message when running rspec --bisect remains unchanged.

this long shot, happen have .rspec file?

i able recreate similar error on empty rails project adding spork to. reason running rspec --bisect --drb provokes issue.

the spork documentation suggests adding --drb straight .rspec file. make sure has been removed , when run rspec --bisect should work.