[ruby-core:100223] [Ruby master Bug#17203] Logger::Formatter won't work with non main Ractor

Issue #17203 has been updated by Eregon (Benoit Daloze).


I tried to make an example why sharing a mutable object is problematic, but I think I just got more confused :D

```
ruby -e '$stdout.instance_variable_set(:@private, Object.new); p $stdout.object_id; p $stdout.instance_variables; Ractor.new { p $stdout.object_id; p $stdout.instance_variables; p $ $stdout.instance_variable_get(:@private) }.take'
60
[:@private]
<internal:ractor>:38: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
80
[]
nil
```

So `$stdout` in each Ractor is a different object?
Might be a good way to still let `p/print/puts` work in Ractors.
The $stdout variable is Ractor-local, so that might make sense.

This sometimes trigger a [BUG]:
```
$ ruby -e 'p $stdout.object_id; 10.times.map { Ractor.new { p $stdout.object_id } }; sleep 1'
60
<internal:ractor>:38: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
80
100
120
140
160
180220200


240
260
-e: [BUG] Object ID seen, but not in mapping table: 0x000055a75f933080 [0    U] T_ZOMBIE 

ruby 3.0.0dev (2020-09-29T15:32:22Z master c38605de6b) [x86_64-linux]

-- Control frame information -----------------------------------------------
c:0001 p:0000 s:0003 E:000b10 (none) [FINISH]


-- C level backtrace information -------------------------------------------
[0x55a75e3b5c6e]
/home/eregon/prefix/ruby-master/bin/ruby(rb_bug+0xe5) [0x55a75e1be426] error.c:723
/home/eregon/prefix/ruby-master/bin/ruby(obj_free_object_id+0x19) [0x55a75e1e9fe8] gc.c:2691
/home/eregon/prefix/ruby-master/bin/ruby(finalize_list) gc.c:2679
[0x55a75e1ee213]
/home/eregon/prefix/ruby-master/bin/ruby(rb_ec_cleanup+0x2e8) [0x55a75e1cf558] eval.c:173
[0x55a75e1cfcc1]
/home/eregon/prefix/ruby-master/bin/ruby(main+0x6f) [0x55a75e1c32ef] ./main.c:50
```

----------------------------------------
Bug #17203: Logger::Formatter won't work with non main Ractor
https://bugs.ruby-lang.org/issues/17203#change-87812

* Author: maciej.mensfeld (Maciej Mensfeld)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* ruby -v: ruby 3.0.0preview1 (2020-09-25 master 0096d2b895) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
``` ruby
class Rogger < Ractor
  def self.new
    super do
      # STDOUT cannot be referenced but $stdout can
      logger = ::Logger.new($stdout)
 
      # Run the requested operations on our logger instance
      while data = recv
        logger.public_send(data[0], *data[1])
      end
    end
  end
  
  # Really cheap logger API :)
  def method_missing(m, *args, &_block)
    self << [m, *args]
  end
end
 
class Rails
  LOGGER = Rogger.new
 
  def self.logger
    LOGGER
  end
end
 
Ractor.new do
  Rails.logger.info "Hello"
end
```

running this ends up with:

```bash
terminated with exception (report_on_exception is true):
ruby/3.0.0/logger/formatter.rb:15:in `call': can not access global variables $$ from non-main Ractors (RuntimeError)
  from ruby/3.0.0/logger.rb:586:in `format_message'
  from ruby/3.0.0/logger.rb:476:in `add'
  from ruby/3.0.0/logger.rb:529:in `info'
  from test.rb:23:in `public_send'
  from test.rb:23:in `block in new'
```

however the same with fixed formatter works.

PR: https://github.com/ruby/ruby/pull/3600


All of the code examples are here: https://mensfeld.pl/2020/09/building-a-ractor-based-logger-that-will-work-with-non-ractor-compatible-code/



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