[ruby-core:113672] [Ruby master Feature#19694] Add Regexp#timeout= setter

Issue #19694 has been updated by byroot (Jean Boussier).


> I do get the point that Regexp.new is less nice than /.../.

The main issue with `Regexp.new` is that it compile the regexp on each invocation, which is slow. So you can't just translate from one syntax to the other, you also have to make sure to keep it in a constant.

> On TruffleRuby all Regexp instances are frozen, notably because they are all globally cached and deduplicated, so this pattern with dup + setter cannot work.

I assume you could have an internal regexp object that is deduplicated and immutable, and the actual `Regexp` object that is exposed could be a tuple of `(internal_regexp, timeout)`.

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Feature #19694: Add Regexp#timeout= setter
https://bugs.ruby-lang.org/issues/19694#change-103317

* Author: aharpole (Aaron Harpole)
* Status: Open
* Priority: Normal
----------------------------------------
# Abstract

In addition to allowing for a Regexp timeout to be set on individual instances by setting a `timeout` argument in `Regexp.new`, I'm proposing that we also allow setting the timeout on Regexp objects with a `#timeout=` setter.

# Background

To be able to roll out a global Regexp timeout for a large application, there are inevitably some individual regexes for which a different timeout is appropriate. While the `timeout` keyword argument was added to `Regexp.new`, this isn't always a viable option.

In the case of regex literal syntax (`/ab*/` or `%r{ab*}`, for instance), it's not possible to set a timeout at all right now without converting to `Regexp.new`, which may be awkward depending on the contents of the regex.

It also is desirable from time to time to be able to set a timeout for a regex object after it's been initialized.

Finally, because we offer a `Regexp#timeout` getter, for consistency it would be nice to also offer a setter.

The introduction of a `Regexp#timeout=` setter was mentioned as a possible way to set individual timeouts in https://bugs.ruby-lang.org/issues/19104#Specification.

# Proposal

I propose that we add the method `Regexp#timeout=`. It works the same way the `timeout` argument works in `Regexp.new`, taking either a float or nil.

This makes it relatively easy to add timeouts to specific regex literals (regex literals are frozen by default so you do have to `dup` them first):

```
emoji_filter_pattern = %r{
  (?<!#{Regexp.quote(ZERO_WIDTH_JOINER)})
  #{EmojiFilter.unicodes_pattern}
  (?!#{Regexp.union(EmojiFilter::MODIFIER_CHAR_MAP.keys.map { |k| Regexp.quote k })})
}x.dup
emoji_filter_pattern.timeout = 1.0
emoji_filter_pattern.freeze
```

# Implementation

This setter has been implemented in https://github.com/ruby/ruby/pull/7847.

# Evaluation

It's just a setter, so pretty straightforward in terms of implementation and use.

# Discussion

It's worth considering other options for overriding `Regexp.timeout`. I'd love to see something like the following for overriding regexp timeouts as well:

```
Regexp.timeout = 1.0

Regexp.with_timeout(5.0) do
  evaluate_slower_regexes
end
```

It's possible to implement something like `Regexp.with_timeout` but it's not thread-safe by default since it would involve overwriting `Regexp.timeout`.

# Summary

Regexp instances have a getter for timeout, and adding a corresponding setter adds consistency and will make it easier for developers to adopt adding a global `Regexp.timeout` by making it simpler to adjust timeouts on a regex by regex basis.

It's a minor change but the added consistency and flexibility help us optimize for developer happiness.



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