Feature #10211: Implement Signal.current_trap(sig)

• Status: Feedback
• Assignee: -
• Target version: -

[kyrylo (Kyrylo Silin)]

Here's pseudocode by ko1:

def trap2(sig)
  previous_proc = Signal.current_trap(sig)
  Signal.trap(sig){
    previous_proc.call if previous_proc
    yield
  }
end
 
trap2(:INT){...}

Motivation

I'm developing a gem that allows using multiple callbacks for a trap: [[https://github.com/kyrylo/multitrap]]
It's pretty simple (and slightly broken). The problem is that if you earlier had defined traps and then
required my library, it would discard your previously defined callbacks.
The library overrides Signal.trap and stores callbacks in a hash. However, it stores only new callbacks.
I cannot access previously defined callbacks for signals. They are stored in GET_VM()->trap_list, which
isn't exposed neither to Ruby nor to the C extension API. I know when you define a trap, it returns a proc.
However, nobody typicaly stores it, so there's no way to access it. So if my gem loads after this assignment,
I'm unable to capture that proc, hence I always overwrite previous "traps". This library might be useful if you
want to define a trap that conflicts with some other gem you depend on, which defines its own trap for the
same signal.

So I need some way to access the callbacks.

[ko1 (Koichi Sasada)]

• Description updated (diff)

[akr (Akira Tanaka)]

Any problem with the return value of trap?

% ruby -e '
def trap2(sig)
  previous_proc = Signal.trap(sig){
    previous_proc.call if previous_proc
    yield
  }
end

trap(:INT) { p 1 }
trap2(:INT) { p 2 }
Process.kill :INT, $$
'
1
2

[akr (Akira Tanaka)]

• Status changed from Open to Feedback

[kyrylo (Kyrylo Silin)]

Akira Tanaka wrote:

Any problem with the return value of trap?

There's no problem with it by itself. However, there are some libraries like RSpec that use trap
and not store its return value: https://github.com/rspec/rspec-core/blob/d06ffe85b4513dba296abc653090191806963da0/lib/rspec/core/runner.rb#L150

There's no way to access that proc (unless you monkey-patch the method
it is defined in, but it's crazy :).

[kyrylo (Kyrylo Silin)]

Well, even monkey-patching won't help.

[nobu (Nobuyoshi Nakada)]

I can't get your point.
Do you want to get the previous trap defined before RSpec is loaded?

[deivid (David Rodríguez)]

I happen to need something like this too.

I'd like to be able to override RSpec's trap somehow. For example, if someone types something in byebug or pry's prompt, we surely don't want to quit rspec but only empty the input written so far by the user and print another prompt.

[kyrylo (Kyrylo Silin)]

Nobuyoshi Nakada wrote:

I can't get your point.
Do you want to get the previous trap defined before RSpec is loaded?

I want to get the trap defined by RSpec (and probably other gems loaded
before Multitrap that define various trap callbacks for other signals).

If I define a new trap('INT') within my RSpec tests, it overwrites the
RSpec one.

[deivid (David Rodríguez)]

Disregard my previous message, I thought all traps were executed from last to first, but only last one is. I guess that's why you're implementing Multitrap, right Kyrylo?

[kyrylo (Kyrylo Silin)]

David Rodríguez wrote:

Disregard my previous message, I thought all traps were executed from last to first, but only last one is. I guess that's why you're implementing Multitrap, right Kyrylo?

Exactly :)

[kyrylo (Kyrylo Silin)]

I think we can close this. Thanks everyone for help! I finally figured out how to do that without modifying Ruby.

I updated https://github.com/kyrylo/multitrap