[ruby-core:106354] [Ruby master Feature#18367] Stop the interpreter from escaping error messages

Issue #18367 has been updated by mame (Yusuke Endoh).


Dan0042 (Daniel DeLorme) wrote in #note-7:
> I'm not sure it makes sense to allow ANSI escapes that can _only_ result in broken formatting.

Good point. If you want to control attribution, I think you have to reset it by `\e[0m` at the beginning and control everything yourself.

> What about just removing the backslash from the set of escaped characters? For most errors it can be misleading, as in the example of `"\\".no_method` above.

@nobu is against this because it becomes ambiguous.

----------------------------------------
Feature #18367: Stop the interpreter from escaping error messages
https://bugs.ruby-lang.org/issues/18367#change-94992

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
----------------------------------------
## Proposal

At the present time, the Ruby interpreter escapes some characters (*1) in error messages when an uncaught error is printed. I'd like to propose stopping this escaping behavior.

```
class MyError < StandardError
  def message
    "foo\\bar"
  end
end

raise MyError
#=> current:  test.rb:7: in `<main>': foo\\bar (MyError)
#=> excepted: test.rb:7: in `<main>': foo\bar (MyError)
```

*1: Escaped characters are any control characters except `\r` and `\n`, and a backslash `\\`.


## Motivation

This behavior prevents us from adding an attribution (color, underline, etc.) to the error message because it escapes escape sequences. Nowadays, such a rich presentation of terminal output is more and more important.

```
$ ruby -e 'raise "\e[31mRed\x1b[0m error"'
-e:1:in `<main>': \e[31mRed\x1b[0m error (RuntimeError)
```

Also, the behavior in question leads to rather confusing error printing. See the error output of `"\\".no_method`:

```
$ ruby -e '"\\".no_method'
-e:1:in `<main>': undefined method `no_method' for "\\\\":String (NoMethodError)

"\\\\".no_method
    ^^^^^^^^^^
```

The two occurrences of `"\\\\"` must be `"\\"`. Worse, the output of error_highlight `^^^^` points wrong position.

Note that this issue is never specific to error_highlight. The receiver of NoMethodError, `"\\\\":String`, is also wrongly escaped. It must be `"\\":String`.


## Why the escaping behavior was introduced

AFAIK, the behavior was introduced because of a security concern. It is considered harmful for an attacker to be able to print arbitrary escape sequences to victim's terminal. (See [this article](https://marc.info/?l=bugtraq&m=104612710031920&w=2) in detail.)

However, I believe it is rare to see the error logs of an application that may be exposed to attacks (i.e. in production mode) in a terminal, as the error output of the Ruby interpreter.

Even if that is the case, I think such escaping should be done as a responsibility of the application, and not implicitly by the interpreter. I briefly surveyed other major languages than Ruby, and I could find no language that escapes error messages. This is the transcript of Python and Node.js.

```
$ python3 -c 'raise Exception("\x1b[31mRed\x1b[0m error")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
Exception: Red error

$ node -e 'throw("\x1b[31mRed\x1b[0m error")'

[eval]:1
throw("\x1b[31mRed\x1b[0m error")
^
Red error
(Use `node --trace-uncaught ...` to show where the exception was thrown)
```

Just in case, I reported these behaviors to the security contacts of Python and Node.js, and both responded to me that this is not a securty issue. I think their decisions are quite reasonable.

## Migration

It would be a good idea to first make the following behavior as a migration path.

* When an error message does not include a control character, no escaping is applied.
* When an error message does include a control character, "Warning: this error message is currently escaped because it includes a control character(s), but this will not be escaped in Ruby 3.X" is printed, and the escaping is applied.





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