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Feature #17930

closed

Add column information into error backtrace

Added by mame (Yusuke Endoh) 6 months ago. Updated 5 months ago.

Status:
Closed
Priority:
Normal
Target version:
-
[ruby-core:104109]

Description

Consider the following code and error.

data["data"].first["field"] #=> undefined method `[]` for nil:NilClass

There are two possibilities; the variable data is nil, or the return value of first is nil. Unfortunately, the error message is less informative to say which.

This proposal allows to help identifying which method call failed.

$ ruby -r ./sample/no_method_error_ext.rb err1.rb
err1.rb:2:in `<main>': undefined method `[]' for nil:NilClass (NoMethodError)

data["data"].first["field"]
                  ^^^^^^^^^

Proposal

I'd like to propose a feature to get column information from each Thread::BacktraceLocation. Maybe it is good to provide the following four methods:

  • Thread::BacktraceLocation#first_lineno
  • Thread::BacktraceLocation#first_column
  • Thread::BacktraceLocation#last_lineno
  • Thread::BacktraceLocation#last_column

These names came from RubyVM::AbstraceSyntaxTree::Node's methods.

Implementation

Here is a proof-of-concept implementation: https://github.com/ruby/ruby/pull/4540

See https://github.com/ruby/ruby/pull/4540/commits/6ff516f4985826e9f9c5606638001c3c420f7cad for an example usage.
(Note that, currently, you need to build ruby with ./configure cflags=-DEXPERIMENTAL_ISEQ_NODE_ID to enable the feature.)

To put it simply, this PR provides only a raw API, Thread::BacktraceLocation#node_id. To get actual column information, you need to manually identify RubyVM::AbstractSyntaxTree::Node that corresponds to Thread::BacktraceLocation#node_id.
But it would be arguable to expose "node_id", so I will wrap it as the above four methods if this is accepted.

Credit: the original implementation was done by yui-knk (Kaneko Yuichiro).

Drawback

To use this feature, we need to enable -DEXPERIMENTAL_ISEQ_NODE_ID to add "node_id" information (a subtree ID of the original abstract syntax tree) into each byte code instruction. If we provide this feature, the option should be enabled by default. However, the option increases memory consumption.

I performed a simple experiment: I created a scaffold app by rails new, and measured the memory usage after rails s. The result was 97 MB without -DEXPERIMENTAL_ISEQ_NODE_ID, and 100 MB with the option enabled.

In my opinion, it is not so large, but requiring more gems will increase the difference. I will appriciate it if anyone could provide the actual memory increase in a more practical Rails app.

Do you think this feature deserves the memory increase?


Files

image.png (73.3 KB) image.png ttanimichi (Tsukuru Tanimichi), 05/31/2021 12:22 PM

Related issues

Is duplicate of Ruby master - Feature #10982: Clarify location of NoMethod errorClosedActions
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