Is this logging appropriate?

[jesseduffield]

I'm seeing some warnings in my tests about tasks that 'may have ended with unhandled exception.'

I can't tell if this is async being overly cautious, or if I'm actually doing something wrong. As far as I can tell, this code is correct:

Code:

    def async_map(items:, concurrency_limit: Float::INFINITY, &block)
      result = []

      Async do
        semaphore = Async::Semaphore.new(concurrency_limit)

        tasks = items.map do |item|
          semaphore.async do
            yield(item)
          end
        end

        result = tasks.map(&:wait)
      end.wait

      return result
    end

Spec:

require "rails_helper"

RSpec.describe AsyncUtils do
  describe ".async_map" do
    subject { described_class.async_map(items:, concurrency_limit:, &block) }

    context "when the block raises an error" do
      let(:block) { ->(_) { raise "test error" } }
      let(:items) { [1, 2, 3] }
      let(:concurrency_limit) { Float::INFINITY }

      it "propagates the error" do
        expect { subject }.to raise_error("test error")
      end
    end
  end
end

Test output:

    when the block raises an error
  0.0s     warn: Async::Task [oid=0x7b34] [ec=0x7b48] [pid=150195] [2025-04-09 11:52:45 +0000]
               | Task may have ended with unhandled exception.
               |   RuntimeError: test error
               |   → spec/services/async_utils_spec.rb:40 in `block (5 levels) in <top (required)>'
               |     app/services/async_utils.rb:20 in `block (3 levels) in async_map'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/semaphore.rb:68 in `block in async'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/task.rb:200 in `block in run'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/task.rb:438 in `block in schedule'
  0.0s     warn: Async::Task [oid=0x7b5c] [ec=0x7b70] [pid=150195] [2025-04-09 11:52:45 +0000]
               | Task may have ended with unhandled exception.
               |   RuntimeError: test error
               |   → spec/services/async_utils_spec.rb:40 in `block (5 levels) in <top (required)>'
               |     app/services/async_utils.rb:20 in `block (3 levels) in async_map'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/semaphore.rb:68 in `block in async'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/task.rb:200 in `block in run'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/task.rb:438 in `block in schedule'
  0.0s     warn: Async::Task [oid=0x7b84] [ec=0x7b98] [pid=150195] [2025-04-09 11:52:45 +0000]
               | Task may have ended with unhandled exception.
               |   RuntimeError: test error
               |   → spec/services/async_utils_spec.rb:40 in `block (5 levels) in <top (required)>'
               |     app/services/async_utils.rb:20 in `block (3 levels) in async_map'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/semaphore.rb:68 in `block in async'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/task.rb:200 in `block in run'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/task.rb:438 in `block in schedule'
  0.0s     warn: Async::Task [oid=0x7bac] [ec=0x7bc0] [pid=150195] [2025-04-09 11:52:45 +0000]
               | Task may have ended with unhandled exception.
               |   RuntimeError: test error
               |   → spec/services/async_utils_spec.rb:40 in `block (5 levels) in <top (required)>'
               |     app/services/async_utils.rb:20 in `block (3 levels) in async_map'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/semaphore.rb:68 in `block in async'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/task.rb:200 in `block in run'
               |     /api-bundle/ruby/3.3.0/gems/async-2.23.1/lib/async/task.rb:438 in `block in schedule'

So my question is: should I just disable these noisy logs, or do they necessarily point to actual problems in my code? Thanks.

[ioquatix]

You should check out this PR: socketry/async-await#10, basically add finished: false which indicates that while we aren't waiting yet, we will eventually wait.

finished: nil (the default) means unspecified and will log an exception because we don't know if it is going to be handled or not. Unfortunately Async cannot read the future.

[jesseduffield]

Thanks for the quick response. I am a little confused at having to add finished: false: I would assume that the only reason for using async would be to have multiple tasks run concurrently and then be waited later. Could you provide an example of where the default is preferable to finished: false?

Also looking at your async_map method, I see that the outer block is a Sync and not an Async (followed by a .wait). Is that the idiomatic approach to take?