[ruby-core:103160] [Ruby master Bug#12689] Thread isolation of $~ and $_

Issue #12689 has been updated by headius (Charles Nutter).


Also note this experimental PR that eliminates the update of `$~` from String#split, since no specs and no tests check that behavior and it seems unexpected and unpredictable (it updates to the last match during the split loop).

https://github.com/jruby/jruby/pull/6646

And a bug I just filed to eliminate backref updating from `start_with?` which should be a fast boolean check and not create a MatchData or update backref:

https://bugs.ruby-lang.org/issues/17771

----------------------------------------
Bug #12689: Thread isolation of $~ and $_
https://bugs.ruby-lang.org/issues/12689#change-91232

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
We are debating what is correct behavior now, and what should be correct behavior in the future, for the thread-visibility of the special variables `%~` and `$_`

We have several examples from https://github.com/jruby/jruby/issues/3031 that seem to exhibit conflicting behavior...or at least the behavior is unexpected in many cases.

```
$ ruby23 -e 'p = proc { p $~; "foo" =~ /foo/ }; Thread.new {p.call}.join; Thread.new{p.call}.join'
nil
nil

$ ruby23 -e 'def foo; proc { p $~; "foo" =~ /foo/ }; end; p = foo; Thread.new {p.call}.join; Thread.new{p.call}.join'
nil
#<MatchData "foo">

$ ruby23 -e 'p = proc { p $~; "foo" =~ /foo/ }; def foo(p); Thread.new {p.call}.join; Thread.new{p.call}.join; end; foo(p)'
nil
#<MatchData "foo">

$ ruby23 -e 'class Foo; P = proc { p $~; "foo" =~ /foo/ }; def foo; Thread.new {P.call}.join; Thread.new{P.call}.join; end; end; Foo.new.foo'
nil
#<MatchData "foo">

$ ruby23 -e 'def foo; p = proc { p $~; "foo" =~ /foo/ }; Thread.new {p.call}.join; Thread.new{p.call}.join; end; foo'
nil
nil

$ ruby23 -e 'def foo; p = proc { p $~; "foo" =~ /foo/ }; bar(p); end; def bar(p); Thread.new {p.call}.join; Thread.new{p.call}.join; end; foo'
nil
#<MatchData "foo">
```

These cases exhibit some oddities in whether $~ (and presumably $_) are shared across threads.

The immediate thought is that they should be both frame and thread-local...but ko1 points out that such a change would break cases like this:

```
def foo
  /foo/ =~ 'foo'
  Proc.new{
    p $~
  }
end

Thread.new{
  foo.call
}.join
```

So there's a clear conflict here. Users sometimes expect the $~ value to be shared across threads (at least for read, as in ko1's example) and sometimes do not want it shared at all (as in the case of https://github.com/jruby/jruby/issues/3031

Now we discuss.



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