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Feature #15778

Expose an API to pry-open the stack frames in Ruby

Added by gsamokovarov (Genadi Samokovarov) over 1 year ago. Updated about 1 year ago.

Status:
Open
Priority:
Normal
Target version:
-
[ruby-core:92338]

Description

Hello,

I'm the maintainer of the web-console (https://github.com/rails/web-console/) gem, where one of our features is to jump between the frames in which an error occurred. To accomplish this, I currently use the Debug Inspector CRuby API. I think we should expose this functionality in Rubyland, so tools like web-console don't need to resort to C code for this. This also makes it quite harder for me to support different implementations like JRuby or TruffleRuby as everyone is having a different way to create Ruby Binding objects that represent the frames.

Here the API ideas:

Add Thread::Backtrace::Location#binding method that can create a binding for a specific caller of the current frame. We can reuse the existing Kernel.caller_locations method to generate the array of Thread::Backtrace::Location objects. We can optionally have the Kernel.caller_locations(debug: true) argument if we cannot generate the bindings lazily on the VM that can instruct the VM to do the slower operation.

  • Thread::Backtrace::Location#binding returns Binding|nil. Nil result may mean that the current location is a C frame or a JITted/optimized frame and we cannot debug it.

We can also expose the DebugInspector API directly, as done in the https://github.com/banister/debug_inspector gem, but for tools like web-console, we'd need to map the bindings with the backtrace, as we cannot generate Bindings for every frame (C frames) and this needs to be done in application code, so I think the Thread::Backtrace::Location#binding is the better API for Ruby-land.

Such API can help us eventually write most of our debuggers in Ruby as right now we don't have a way to do Post-Mortem debugging without native code or even start our debuggers without monkey-patching Binding.

I have presented this idea in a RubyKaigi's 2019 talk called "Writing Debuggers in Plain Ruby", you can check-out the slides for more context: http://kaigi-debuggers-in-ruby.herokuapp.com.

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