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