rails: `ThreadError: can't create Thread: Resource temporarily unavailable` after multiple calls to `Model.establish_connection`

If anyone is still facing this issue, I back-ported the fix with this money patch, since setting reaping_frequency: 0 didn’t help.

It worked for me as a temporary solution before updating application to Rails 6.

config/initializers/rails6_backports.rb

raise "Remove no-longer-needed #{__FILE__}!" if Rails::VERSION::MAJOR >= 6

require "weakref"

module ActiveRecord
  # Backport https://github.com/rails/rails/pull/36998 and https://github.com/rails/rails/pull/36999
  # to avoid `ThreadError: can't create Thread: Resource temporarily unavailable` issues
  module ConnectionAdapters
    class ConnectionPool
      class Reaper
        @mutex = Mutex.new
        @pools = {}
        @threads = {}

        class << self
          def register_pool(pool, frequency) # :nodoc:
            @mutex.synchronize do
              unless @threads[frequency]&.alive?
                @threads[frequency] = spawn_thread(frequency)
              end
              @pools[frequency] ||= []
              @pools[frequency] << WeakRef.new(pool)
            end
          end

          private
            def spawn_thread(frequency)
              Thread.new(frequency) do |t|
                running = true
                while running
                  sleep t
                  @mutex.synchronize do
                    @pools[frequency].select!(&:weakref_alive?)
                    @pools[frequency].each do |p|
                      p.reap
                      p.flush
                    rescue WeakRef::RefError
                    end

                    if @pools[frequency].empty?
                      @pools.delete(frequency)
                      @threads.delete(frequency)
                      running = false
                    end
                  end
                end
              end
            end
        end

        def run
          return unless frequency && frequency > 0
          self.class.register_pool(pool, frequency)
        end
      end

      def reap
        stale_connections = synchronize do
          return unless @connections
          @connections.select do |conn|
            conn.in_use? && !conn.owner.alive?
          end.each do |conn|
            conn.steal!
          end
        end

        stale_connections.each do |conn|
          if conn.active?
            conn.reset!
            checkin conn
          else
            remove conn
          end
        end
      end

      def flush(minimum_idle = @idle_timeout)
        return if minimum_idle.nil?

        idle_connections = synchronize do
          return unless @connections
          @connections.select do |conn|
            !conn.in_use? && conn.seconds_idle >= minimum_idle
          end.each do |conn|
            conn.lease

            @available.delete conn
            @connections.delete conn
          end
        end

        idle_connections.each do |conn|
          conn.disconnect!
        end
      end
    end
  end
end

This issue has been automatically marked as stale because it has not been commented on for at least three months. The resources of the Rails team are limited, and so we are asking for your help. If you can still reproduce this error on the 6-0-stable branch or on master, please reply with all of the information you have about it in order to keep the issue open. Thank you for all your contributions.

It is failing on the 1021th iteration for revision e559d6dcecb871c174870fdeeed1560a0353cc49 in 5-2-stable. Thank you very much @jrmhaig for the repository.

I can still reproduce this with 5-2-stable and master, failing my example test on the 2045th iteration in both cases. The top lines of the Gemfile.lock files in each case are:

# 5-2-stable
GIT
  remote: https://github.com/rails/rails.git
  revision: b667758483f1c6480ddf9c7385d41c2f1d78c054
  branch: 5-2-stable

# master
GIT
  remote: https://github.com/rails/rails.git
  revision: a04a757e5da646c6a3b81d879f5c11b1329b67d2

I have put my test example here, mainly for my own reference for the next time that @rails-bot marks this as stale.