Debugging Async tasks yields `RuntimeError: running is given, but running`

Reproduce the example with Ruby 3.1.0, rdbg 1.4.0, and async 2.0.0, using breakpoints on both puts calls. Start with debug/thread_client.rb around set_mode and wait_next_action_, then follow the breakpoint path through debug/breakpoint.rb. Done means the same asynchronous debugging flow continues without raising RuntimeError: running is given, but running.

Your environment

• ruby -v: ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin20]
• rdbg -v: rdbg 1.4.0

Describe the bug

Debugging programs with the Async library often leads to RuntimeError: running is given, but running

To Reproduce

Install async 2.0.0:

gem install async

Create ruby script async_debug_test_case.rb as follows:

require 'async'

Async do |t|
  t.async do
    puts "1\n"
  end
  t.async do
    puts "2\n"
  end
end

Run rdbg, set breakpoints on both putss, and continue until error is raised. See output below:

rdbg scratch/async_debug_test_case.rb 
[1, 10] in scratch/async_debug_test_case.rb
=>   1| require 'async'
     2| 
     3| Async do |t|
     4|   t.async do
     5|     puts "1\n"
     6|   end
     7|   t.async do
     8|     puts "2\n"
     9|   end
=>#0    <main> at scratch/async_debug_test_case.rb:1
(rdbg) b 5    # break command
#0  BP - Line  /Users/machty/code/exc/rails/scratch/async_debug_test_case.rb:5 (line)
(rdbg) b 8    # break command
#1  BP - Line  /Users/machty/code/exc/rails/scratch/async_debug_test_case.rb:8 (line)
(rdbg) c    # continue command
[1, 10] in scratch/async_debug_test_case.rb
     1| require 'async'
     2| 
     3| Async do |t|
     4|   t.async do
=>   5|     puts "1\n"
     6|   end
     7|   t.async do
     8|     puts "2\n"
    10| end
=>#0    block in <main> (2 levels) at scratch/async_debug_test_case.rb:5
  #1    block in schedule at ~/.gem/ruby/3.1.0/gems/async-2.0.0/lib/async/task.rb:258

Stop by #0  BP - Line  /Users/machty/code/exc/rails/scratch/async_debug_test_case.rb:5 (line)
(rdbg) c    # continue command
[3, 11] in scratch/async_debug_test_case.rb
     3| Async do |t|
     4|   t.async do
     5|     puts "1\n"
     6|   end
     7|   t.async do
=>   8|     puts "2\n"
     9|   end
    10| end
    11| 
=>#0    block in <main> (2 levels) at scratch/async_debug_test_case.rb:8
  #1    block in schedule at ~/.gem/ruby/3.1.0/gems/async-2.0.0/lib/async/task.rb:258

Stop by #1  BP - Line  /Users/machty/code/exc/rails/scratch/async_debug_test_case.rb:8 (line)
1
(rdbg) c    # continue command
["DEBUGGER Exception: /Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:986",
 #<RuntimeError: running is given, but running>,
 ["/Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:115:in `set_mode'",
  "/Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:698:in `wait_next_action_'",
  "/Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:676:in `wait_next_action'",
  "/Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:284:in `suspend'",
  "/Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:217:in `on_breakpoint'",
  "/Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/breakpoint.rb:65:in `suspend'",
  "/Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/breakpoint.rb:157:in `block in setup'",
  "scratch/async_debug_test_case.rb:8:in `block (2 levels) in <main>'",
  "/Users/machty/.gem/ruby/3.1.0/gems/async-2.0.0/lib/async/task.rb:258:in `block in schedule'"]]
  0.0s    error: Async::Task [oid=0x5c8] [ec=0x5dc] [pid=34553] [2022-01-12 04:52:04 -0400]
               |   RuntimeError: running is given, but running
               |   → /Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:115 in `set_mode'
               |     /Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:698 in `wait_next_action_'
               |     /Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:676 in `wait_next_action'
               |     /Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:284 in `suspend'
               |     /Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/thread_client.rb:217 in `on_breakpoint'
               |     /Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/breakpoint.rb:65 in `suspend'
               |     /Users/machty/.rubies/ruby-3.1.0/lib/ruby/gems/3.1.0/gems/debug-1.4.0/lib/debug/breakpoint.rb:157 in `block in setup'
               |     scratch/async_debug_test_case.rb:8 in `block (2 levels) in <main>'
               |     /Users/machty/.gem/ruby/3.1.0/gems/async-2.0.0/lib/async/task.rb:258 in `block in schedule'

Expected behavior

Debugging should not crash.

Additional context

I can't replicate this issue using pure Fibers, so I suspect this has something to do with Ruby's new Fiber Scheduler interface and the way the Async library uses it. Note that simpler test cases using a single test cases involving one task don't seem to cause the rror.