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Stale connection in `DatabaseCleaner::ActiveRecord::Truncation#connection`

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
postgresql, rails, ruby
Domain
database, testing

Research direction

Start at DatabaseCleaner::ActiveRecord::Truncation#clean and the strategy's @connection initialization described in the issue. Reproduce the connection-pool scenario with the provided RSpec example, then verify that repeated cleaning obtains a usable connection and that the tests no longer raise ActiveRecord::ConnectionNotEstablished.

Written by the indexing model from the issue text.

Description

Summary

Each instance of the DatabaseCleaner::ActiveRecord::Truncation strategy initializes its @connection field only once, using the ActiveRecord default connection. When the connection is first requested, ActiveRecord checks it out of the connection pool. Various hooks (e.g. ActiveRecord::TestFixtures.teardown_fixtures) can return the connection to the pool, but Truncation is unaware of this and holds onto the connection object.

If another thread checks the same connection out of the pool and calls disconnect!, then when Truncation tries to use the connection to clean the database, it will find that the connection is closed and raise an error.

Steps to reproduce:

  1. In a Rails/PostgreSQL project using RSpec, configure DatabaseCleaner as follows:

    RSpec.configure do |config|
  config.before(:suite) do
    DatabaseCleaner.strategy = :truncation
  end

  config.around do |example|
    DatabaseCleaner.cleaning do
      example.run
    end
  end
end

  2. Write a test that, in a background thread, checks a connection out of the pool, removes it from the pool, and disconnects it, e.g.

     describe 'connection pooling' do
   def do_disconnect
     Thread.new do
       connection_pool = ActiveRecord::Base.connection_pool
       connection = connection_pool.checkout.tap do |conn|
         connection_pool.remove(conn)
       end
       begin
         connection.execute('SELECT 1')
       ensure
         connection.disconnect!
       end
     end
   end
 
   5.times do |i|
     it "test #{i}" do
       ActiveRecord::Base.connection.execute('SELECT 1')
       sleep(0.5)
       do_disconnect if i % 2 == 0
     end
   end
 end

    (This example is obviously quite contrived; I ran into the problem in a more realistic situation. See discussion in https://github.com/bensheldon/good_job/issues/849.)

Expected

  • Tests pass.

Actual

  • First couple of tests pass
  • Subsequent tests fail with ActiveRecord::ConnectionNotEstablished: connection is closed raised from DatabaseCleaner.cleaning via Truncation#clean

Workaround

Instead of using DatabaseCleaner.cleaning in an around block, explicitly call DatabaseCleaner.clean_with(:truncation) in an after(:each) block:

RSpec.configure do |config|
  config.after(:each) do
    DatabaseCleaner.clean_with(:truncation)
  end
end

Proposed fix

Get a fresh connection in each call to Truncation.clean -- I simulated this with a prepended module and it seems to work:

module Cleaninator
  def clean
    @connection = nil
    super
  end
end

class DatabaseCleaner::ActiveRecord::Truncation
  prepend Cleaninator
end
Dominant language
Ruby
Stars
76
Forks
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