Fix random failure related to migration environment

[prathamesh-sonpatki]

• Reference: https://travis-ci.org/rails/rails/jobs/189764676

• Reproduction command:

  MTB_VERBOSE=2 bundle exec minitest_bisect --seed 33328 -Itest "test/cases/migration_test.rb" "test/cases/tasks/database_tasks_test.rb"

• You need to also add minitest_bisect gem to the Gemfile to reproduce
  this failure.

r? @schneems

[kamipo]

I was also affected and deleted environment = "foofoo" record in 13803a7.

[prathamesh-sonpatki]

@kamipo I tried your patch locally.

This is the output on master with my reproduction script: https://gist.github.com/prathamesh-sonpatki/2da52556999822ddb919393d6ca83c57.

Very first failure: https://gist.github.com/prathamesh-sonpatki/2da52556999822ddb919393d6ca83c57#file-gistfile1-txt-L16

This is the output with your patch: https://gist.github.com/prathamesh-sonpatki/5fe2db5b07384be5036f829d7c5c10ab

First failure is: https://gist.github.com/prathamesh-sonpatki/5fe2db5b07384be5036f829d7c5c10ab#file-gistfile1-txt-L21

I think deleting the contents of the "metadata" table is not the right solution as the error about missing environment might occur. What do you think?

[kamipo]

Ah... I missed requiring environment setting 😢
https://github.com/rails/rails/blob/v5.0.1/activerecord/lib/active_record/migration.rb#L1256-L1257

You are right, thank you for your time!