ruby-concurrency/concurrent-ruby · error · ArgumentError

#{@fallback_policy} is not a valid fallback policy

Error message

#{@fallback_policy} is not a valid fallback policy

What it means

On MRI, ThreadPoolExecutor validates opts[:fallback_policy] (default :abort) against FALLBACK_POLICIES (:abort, :discard, :caller_runs) and raises ArgumentError otherwise. The policy governs tasks posted after shutdown; Symbols are required, so Strings and typos fail.

Source

Thrown at lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb:155

    # @!macro thread_pool_executor_method_prune_pool
    def prune_pool
      deprecated "#prune_pool has no effect and will be removed in next the release, see https://github.com/ruby-concurrency/concurrent-ruby/pull/1082."
    end

    private

    # @!visibility private
    def ns_initialize(opts)
      @min_length      = opts.fetch(:min_threads, DEFAULT_MIN_POOL_SIZE).to_i
      @max_length      = opts.fetch(:max_threads, DEFAULT_MAX_POOL_SIZE).to_i
      @idletime        = opts.fetch(:idletime, DEFAULT_THREAD_IDLETIMEOUT).to_i
      @max_queue       = opts.fetch(:max_queue, DEFAULT_MAX_QUEUE_SIZE).to_i
      @synchronous     = opts.fetch(:synchronous, DEFAULT_SYNCHRONOUS)
      @fallback_policy = opts.fetch(:fallback_policy, :abort)

      raise ArgumentError.new("`synchronous` cannot be set unless `max_queue` is 0") if @synchronous && @max_queue > 0
      raise ArgumentError.new("#{@fallback_policy} is not a valid fallback policy") unless FALLBACK_POLICIES.include?(@fallback_policy)
      raise ArgumentError.new("`max_threads` cannot be less than #{DEFAULT_MIN_POOL_SIZE}") if @max_length < DEFAULT_MIN_POOL_SIZE
      raise ArgumentError.new("`max_threads` cannot be greater than #{DEFAULT_MAX_POOL_SIZE}") if @max_length > DEFAULT_MAX_POOL_SIZE
      raise ArgumentError.new("`min_threads` cannot be less than #{DEFAULT_MIN_POOL_SIZE}") if @min_length < DEFAULT_MIN_POOL_SIZE
      raise ArgumentError.new("`min_threads` cannot be more than `max_threads`") if min_length > max_length

      @pool                 = [] # all workers
      @ready                = [] # used as a stash (most idle worker is at the start)
      @queue                = [] # used as queue
      # @ready or @queue is empty at all times
      @scheduled_task_count = 0
      @completed_task_count = 0
      @largest_length       = 0
      @workers_counter      = 0
      @ruby_pid             = $$ # detects if Ruby has forked
    end

    # @!visibility private
    def ns_limited_queue?

View on GitHub (pinned to 0b88d5ff75)

Solutions

1. Use exactly :abort, :discard or :caller_runs as Symbols
2. Normalize strings early: policy = raw.to_s.downcase.to_sym
3. Validate against Concurrent::AbstractExecutorService::FALLBACK_POLICIES before constructing

Example fix

# before
Concurrent::ThreadPoolExecutor.new(fallback_policy: 'discard')
# after
Concurrent::ThreadPoolExecutor.new(fallback_policy: :discard)

ALLOWED = Concurrent::AbstractExecutorService::FALLBACK_POLICIES
policy = cfg.fetch(:fallback_policy, :abort).to_s.to_sym
raise ArgumentError, "fallback_policy must be one of #{ALLOWED}" unless ALLOWED.include?(policy)
Concurrent::ThreadPoolExecutor.new(fallback_policy: policy)

def valid_fallback_policy?(v)
  Concurrent::AbstractExecutorService::FALLBACK_POLICIES.include?(v)
end

begin
  Concurrent::ThreadPoolExecutor.new(fallback_policy: policy)
rescue ArgumentError => e
  raise unless e.message.end_with?('is not a valid fallback policy')
  Concurrent::ThreadPoolExecutor.new(fallback_policy: :abort)
end

When it happens

Trigger: Concurrent::ThreadPoolExecutor.new(fallback_policy: 'discard') on MRI; fallback_policy: :ignore (typo/invalid); values loaded from YAML, JSON or ENV that arrive as strings.

Common situations: Serialized config producing Strings; policy names copied from other libraries; per-environment defaults where one environment has a typo.

Related errors

• number of threads must be greater than zero
• `synchronous` cannot be set unless `max_queue` is 0
• `max_threads` cannot be less than #{DEFAULT_MIN_POOL_SIZE}
• `max_threads` cannot be greater than #{DEFAULT_MAX_POOL_SIZE
• `min_threads` cannot be less than #{DEFAULT_MIN_POOL_SIZE}

AI-assisted analysis of ruby-concurrency/concurrent-ruby@0b88d5ff75 (2026-08-21). Data as JSON: /api/errors/5961729f8abead81. Report an issue: GitHub.