Feature #17479: Enable to get "--backtrace-limit" value by "$-B"

• Status: Closed
• Assignee: -
• Target version: -

[neg_hide (Hidenori Negishi)]

Background

There is currently no way to get "--backtrace-limit" value when outputing backtraces from my script, so I have to ignore this value and output backtraces.

In order to be able to output according to "--backtrace-limit" value, I need a API to get this value from the Ruby side.

Proposal

I propose a readonly built-in variable "$-B" to get "--backtrace-limit" value.

# ruby --backtrace-limit=3 script.rb

puts $-B # => 3

[mame (Yusuke Endoh)]

+1 if we can provide -B option too.

[Eregon (Benoit Daloze)]

What's the use case?
I think we should encourage to use Exception#full_message (instead of manually formatting backtraces), and that could default to apply the limit.
Related: https://github.com/ruby/ruby/commit/f6a080047e960e8b1821a97850830286ce9dee1c#commitcomment-45451705

[neg_hide (Hidenori Negishi)]

I am making a library that makes error messages easier to read. Therefore, instead of using Exception#full_message, I need to analyze the Exception and output the error message myself.

For example, suppose I have a script like this:

def foo
  require "typo_library_name"
end

def bar
  foo
end

bar

When I execute ruby --backtrace-limit=1 script.rb in a normal environment, only the backtrace of kernel_require.rb is displayed as shown below, which makes it difficult to understand the true cause of the error. (The same is true for the string obtained from Exception#full_message.)

/home/myname/.rbenv/versions/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb:85:in `require': cannot load such file -- typo_library_name (LoadError)
        from /home/myname/.rbenv/versions/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb:85:in `require'
         ... 3 levels...

When I use my library, it omits the backtraces of kernel_require.rb and outputs them, so the error message looks like this.

ruby.rb:2:in `foo': cannot load such file -- typo_library_name (LoadError)
        from ruby.rb:2:in `foo'
        from ruby.rb:6:in `bar'
        from ruby.rb:9:in `<main>'

This process is impossible for Exception#full_message. So I have to output the error message myself.

However, I want the output to look like this:

ruby.rb:2:in `foo': cannot load such file -- typo_library_name (LoadError)
        from ruby.rb:2:in `foo'
         ... 2 levels...

That's why I need a value for "--backtrace-limit".

[matz (Yukihiro Matsumoto)]

I am OK with introducing a way to get the limit. But I am strongly opposed to $-B. Any other idea?

Matz.

[jeremyevans0 (Jeremy Evans)]

matz (Yukihiro Matsumoto) wrote in #note-4:

I am OK with introducing a way to get the limit. But I am strongly opposed to $-B. Any other idea?

If we are OK with this being a constant and not a variable, RUBY_BACKTRACE_LIMIT.

From the dev meeting log, Thread::Backtrace.limit was decided upon, which sounds better to me.

[Eregon (Benoit Daloze)]

neg_hide (Hidenori Negishi) wrote in #note-3:

I am making a library that makes error messages easier to read. Therefore, instead of using Exception#full_message, I need to analyze the Exception and output the error message myself.

The message is included in Exception#full_message.
And Exception#backtrace is just Strings too anyway.
I think duplicating the code to format an exception is just a high chance to reimplement it incorrectly (e.g., not handling causes, highlighting, etc correctly).

I see your point that Exception#full_message defaults to use the --backtrace-limit.
I think what we need is a backtrace_limit option to Exception#full_message, and a way to get the value (Thread::Backtrace.limit sounds fine).

[neg_hide (Hidenori Negishi)]

I agree with Thread::Backtrace.limit because I think it's easy to understand.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset git|4f47d655e357a7f09943f199969b437a25665ed2.

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Added Thread::Backtrace.limit [Feature #17479]