From: eregontp@...
Date: 2019-05-15T09:10:13+00:00
Subject: [ruby-core:92654] [Ruby trunk Feature#15778] Expose an API to pry-open the stack frames in Ruby

Issue #15778 has been updated by Eregon (Benoit Daloze).


One thing we can do in any case for TruffleRuby is implementing the `debug_inspector` C API.

However, that doesn't let JRuby implement it, and hiding APIs in C doesn't seem a good way to communicate "should only be used for debugging".

As the debug_inspector README example shows, the API would be much more natural under Thread::Backtrace::Location, and easier to use for debuggers.
The current C-API feels suboptimal/hard-to-use by manually merging Kernel#backtrace_locations and passing indices to the debug inspector.
It's also inefficient, by walking N + N*(N+1)/2 frames instead of just N with the proposed Ruby API.

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Feature #15778: Expose an API to pry-open the stack frames in Ruby
https://bugs.ruby-lang.org/issues/15778#change-78017

* Author: gsamokovarov (Genadi Samokovarov)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Target version: 
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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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