Feature #15289: Accept "target" keyword on `TracePoint#enable`

• Status: Open
• Priority: Normal
• Assignee: ko1 (Koichi Sasada)
• Target version: 2.6

[ko1 (Koichi Sasada)]

Abstract

To enable TracePoint for specific location, I propose new keyword argument target: to TracePoint#enable for specific events (line, call, return, class, end).

Usage:

line_trace = TracePoint.new(:line){|tp|
  p tp
}

def foo
  p 1
end

line_trace.enable(target: method(:foo)) do
  foo
end

In this case, a line trace is enable only on the method foo.

target: keyword accepts Method, UnboundMethod, Proc and RubyVM::InstructionSequence (ISeq for short) . All of objects can pull ISeq objects.

Further more, I propose target_lines: keyword too, which specify effective line(s).

These features can improve "break point" feature and so on.

Background

If we want to insert break point in Ruby's source code, we can use TracePoint (line event, for example). However, it enables all locations. It hurt performance.

This proposal overcomes this performance penalty.

Implementation

Now basic design are completed in my mind :p

Discussion

line or lines

For breakpoint purpose, we only need to specify a line. Specifying lines is needed?

no events on lines

It is not clear that if line is specified (for example: 10) and there are no line event (for example, empty line).

Possible options:

• (1) raise an exception
• (2) adjust before/after effective event
• (3) ignore

I prefer (1).

no events in Proc

Similar to last topic, if we specify call event on Proc object, there are no call event. What happens?

recursive or not

If method foo refers other blocks, I think we need to enable recursively.

how to get File target?

Sometimes we want to specify breakponit to the location specified by "file:line". How to get the lines?

https://github.com/ko1/iseq_collector provides the feature to collect all of ISeqs. debugger can collects all of them and debugger can filter with path name.

Also [Feature #15287] will help to hook not loaded locations.

enable w/ keywords or other method name?

I have no strong opinion, but TracePoint#enable_on(target, lines: ...) is another idea?

reference

Related: https://github.com/ruby/ruby/pull/2000

This implementation is based on trace_ insn.

[shevegen (Robert A. Heiler)]

Sounds nice. Please do not forget documentation when/if approved/added. :)

I have no particular opinion on the API/name(s) as such, but I would recommend
to try to keep it simple and "logical" if possible. A Hash would probably make
the most sense, and the names for the parameters should ideally be the same, e. g.
if "lines:" is used, to also keep the same names whenever possible (both for
TracePoint, but also perhaps elsewhere in ruby if that is necessary; a bit
like how recently :exception was added to Kernel#system and other methods
there, like in the news entry for the upcoming 2.6.x xmas-release.

That is not necessarily an endorsement of the name "lines:" as such, mind
you; just more a general comment.

(I think the name "lines:" may not always be ideal, e. g. in the cases where
a ruby user may only want to add one breakpoint to a single line - but this
is mostly a detail, IMO. I am sure any fitting name can be derived when we
have some examples of how this is used. Ideally a short name if possible.)