introduce Transaction variables (STM support)

[ko1]

Introduce Thread::TVar and Thread.atomicaly to support transactional
variables (STM: Software Transactional Memory). STM is well known
feature to synchronize shared values and has several advantages
compare with traditional locking mechanisms.

TVar also shared with multiple ractors so you can program shared
state between ractors in transactions.

https://bugs.ruby-lang.org/issues/17261

[ko1]

rejected to introduce it into core now.

[jasonwbarnett]

@ko1 Are you planning to implement in a gem instead?

[ko1]

Yes.

[eregon]

@ko1 FYI, I found a bug when running with TVar, not sure if it's a Ractor or TVar bug, but the programs needs TVar to run:
chrisseaton/ruby-stm-lee-demo#1

It reproduces with 4f8d9b0 + this commit.
And chrisseaton/ruby-stm-lee-demo@e9ff5c1

$ time N=10 ruby 7-ractor-tvar-lee.rb inputs/testBoard.txt
<internal:ractor>:38: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
#<Thread:0x000056443c1f9768 run> terminated with exception (report_on_exception is true):
/home/eregon/code/ruby-stm-lee-demo/lib/lee/adjacent.rb:5:in `point_on_board?': undefined method `x' for 0...75:Range (NoMethodError)
	from /home/eregon/code/ruby-stm-lee-demo/lib/lee/adjacent.rb:25:in `block in adjacent'
	from /home/eregon/code/ruby-stm-lee-demo/lib/lee/adjacent.rb:25:in `select'
	from /home/eregon/code/ruby-stm-lee-demo/lib/lee/adjacent.rb:25:in `adjacent'
	from 7-ractor-tvar-lee.rb:42:in `block (2 levels) in expand'
	from 7-ractor-tvar-lee.rb:40:in `each'
	from 7-ractor-tvar-lee.rb:40:in `block in expand'
	from 7-ractor-tvar-lee.rb:37:in `loop'
	from 7-ractor-tvar-lee.rb:37:in `expand'
	from 7-ractor-tvar-lee.rb:153:in `block (4 levels) in <main>'
	from <internal:thread>:25:in `atomically'
	from 7-ractor-tvar-lee.rb:152:in `block (3 levels) in <main>'
	from 7-ractor-tvar-lee.rb:149:in `loop'
	from 7-ractor-tvar-lee.rb:149:in `block (2 levels) in <main>'

It's a bug, the receiver is always a Point, the Range seems to come from another place on the stack/heap.

I would report to https://bugs.ruby-lang.org/issues/17158, but since this is not merged, I'm not sure what's the right place.

[ko1]

Thank you. I need to check it...