Line breakpoints aren't shared between processes

[st0012]

Your environment

• ruby -v: 3.1.0
• rdbg -v: 1.6.0

Describe the bug

When the debuggee application has multiple processes, line breakpoints added to 1 of them won't be shared to other processes.
This will cause inconsistent breakpoint stop because only that process will stop and others won't.

To Reproduce

This issue should happen to any application that:

• Has multiple processes
• Uses line breakpoints

But reproducing it with VSCode is easier:

1. Create a new rails application

2. In config/puma.rb, set

   threads 1, 1 # limit the threads number to 1 will increase the reproduction chance
   
   # other configs
   
   workers 5 # increase this number will also help reproduction

3. Start the server with bundle exec rdbg -c -- bundle exec rails s

4. Set a breakpoint

5. Attach the debugger

6. Sending requests to the Rails app

7. The breakpoint should stop at some requests but not all of them

Expected behavior

The breakpoints should stop at every request

Additional context

@ko1 mentioned that:

• Resolving this will require rewriting a big part of the process handling mechanism
• Possible workarounds: 1) Use only 1 process or 2) For VSCode users, use launch mode instead
• There are no timeline for this issue yet (probably not before Ruby 3.2 is out)

[ko1]

For now, do not use both mode for fork_mode is only solution.