[ruby-core:110853] [Ruby master Feature#19141] Add thread-owned Monitor to protect thread-local resources

["Eregon (Benoit Daloze)" <noreply@ruby-lang.org>]

Issue #19141 has been updated by Eregon (Benoit Daloze).


It's not only an issue with Fiber scheduler although that is the most obvious way to illustrate.
After all Fiber scheduler is not the only usage of `resume`/`transfer`/`yield`, any gem or app might use those as well and not always in clearly-defined places / in a structured concurrency manner.

Suppose we have a Thread::Monitor, in the example below (with no Fiber scheduler) we end up in two bad cases:
* holding the lock after the .synchronize{} returned
* unlocking the lock preemptively, potentially breaking the resource

How do we address this with Thread::Monitor?
With Fiber-based `Monitor` there is a clear exception as soon as entering `f`.

```ruby
M = Thread::Monitor.new

f = Fiber.new {
  M.synchronize {
    # using protected resource
    Fiber.yield # or other_fiber.transfer
    # using protected resource
  }
}

def transaction(m)
  t = Transaction.new
  M.synchronize {
    yield
  }
ensure
  t.commit_or_abort
end

transaction {
  f.resume
}
# M is still held by f or by main Fiber, that could cause confusing deadlocks

transaction {
  f.resume
  # M is not held here!
  # now another Fiber/Thread could corrupt the resource
  some_more_logic
}
```

----------------------------------------
Feature #19141: Add thread-owned Monitor to protect thread-local resources
https://bugs.ruby-lang.org/issues/19141#change-100211

* Author: wildmaples (Maple Ong)
* Status: Open
* Priority: Normal
----------------------------------------
### Background 

In Ruby v3.0.2, Monitor was modified to be owned by fibers instead of threads [for reasons as described in this issue](https://bugs.ruby-lang.org/issues/17827) and so it is also consistent with Mutex. Before the change to Monitor, Mutex was modified to per-fiber instead of thread ([issue](https://bugs.ruby-lang.org/issues/16792), [PR](https://github.com/ruby/ruby/commit/178c1b0922dc727897d81d7cfe9c97d5ffa97fd9)) which caused some problems (See: [comment](https://bugs.ruby-lang.org/issues/17827#note-1)).  

### Problem

We are now encountering a problem where using Enumerator (implemented transparently using Fiber, so the user is not aware) within a Fiber-owned process, which causes a deadlock. That means any framework using Monitor is incompatible to be used with Enumerator. 

In general, there are many types of thread-local resources (connections for example), so it would make sense to have a thread-owned monitor to protect them. I think few resources are really fiber-owned.

#### Specifics 
* Concurrent Ruby is still designed with per-thread locking, which causes similar incompatibilities. (Read: [issue](https://github.com/ruby-concurrency/concurrent-ruby/issues/962))
* Systems test in Rails implements locking using Monitor, resulting in deadlock in these known cases:
  * when cache clearing (Read: [issue](https://github.com/rails/rails/issues/45994))
  * database transactions when used with Enumerator (Read: [comment](https://github.com/rails/rails/issues/45994#issuecomment-1304306575)) 

### Demo

```ruby
# ruby 2.7.6p219 (2022-04-12 revision c9c2245c0a) [arm64-darwin21]
# Thread #<Thread:0x000000014a8eb228 demo.rb:8 run>, fiber #<Fiber:0x000000014a8eaf80 (resumed)>, locked true, owned true
# Thread #<Thread:0x000000014a8eb228 demo.rb:8 run>, fiber #<Fiber:0x000000014a8eacb0 demo.rb:13 (resumed)>, locked true, owned true

# ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [arm64-darwin21]
# Thread #<Thread:0x0000000102329a08 demo.rb:8 run>, fiber #<Fiber:0x0000000102329828 (resumed)>, locked true, owned true
# Thread #<Thread:0x0000000102329a08 demo.rb:8 run>, fiber #<Fiber:0x00000001023294e0 demo.rb:13 (resumed)>, locked true, owned false

require 'fiber'
require 'monitor'

puts RUBY_DESCRIPTION

# We have a single monitor - we're pretending it protects some thread-local resources
m = Monitor.new

# We'll create an explicit thread
t = Thread.new do
  # Lock to protect our thread-local resource
  m.enter

  puts "Thread #{Thread.current}, fiber #{Fiber.current}, locked #{m.mon_locked?}, owned #{m.mon_owned?}"

  # The Enumerator here creates a fiber, which runs on the same thread, so would want to use the same thread-local resource
  e = Enumerator.new do |y|
    # In 2.7 this is fine, in 3.0 it's not, as the fiber thinks it doesn't have the lock
    puts "Thread #{Thread.current}, fiber #{Fiber.current}, locked #{m.mon_locked?}, owned #{m.mon_owned?}"
    
    # This would deadlock
    # m.enter

    y.yield 1
  end
  e.next
end

t.join
```

### Possible Solutions

* Allow `Monitor` to be per thread or fiber through a flag
* Having `Thread::Monitor` and `Fiber::Monitor` as two separate classes. Leave `Monitor` as it is right now. However, this may not be possible due to the `Thread::Mutex` alias 

These options would give us more flexibility in which type of Monitor to use. 



-- 
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